CALIFORNIA STATE UNIVERSITY, NORTHRIDGE
REPLACEMENT OF
CO!'-ll·1UN
ICA'I'IONS TERMINAL UN IT
F
""
t
A SYSTEt>lS APPROACH
A project report submitted in partial satisfaction of the
requirements for the degree of Xaster of Science in
Engineering
by
Heral Timothy Singleton
~·
July , 1976
The
th~i.s._,of
He]:1a.l Timothy Singleton ls approved:
Comrlli ttee Chairman
California Sta.te Univet·sity, Northridge
June
, 1976
ABSTRACT
REPLACEHENT OF COt1:,1UtaCATIONS TERHIN,'\L UNIT
A SYSTE:'-1S APPROACH
by
Heral Timothy Singleton
Haster of Science in Engineering
July
~
1976
The large capacity communications terminal uni. t in service om
the Pacific Hissile Range for the past decade has been found to be
unsatisfactory
for
future use. Three alternatives were considered to
provide the required terminal unit: use of smaller capacity terminal
units already on hand, modification of the present unitss or selection
of a new unit. After careful analysis of the requirements and alternatives it was decided thata new unit should be obtained. After a
search of the market it l-.•as determined that a suitable unit could not
be purchased off-the-shelf, so it -v1as decided to design and build the
new units in-house.
The block diagram of the required terminal unit was derived from
the mission requirements and consultation with users of the system.
Several design approaches \-Jere considered for achieving the block
diagram. A modular self·-contained approach t.;ras chosen after an analysis
iii
relating the alternatives to the mission requirements of the system.
Overall objectives and strategies v:ere derived to satisfy the
mission requirements during the detailed circuit design phase. Key
strategies were to minimize the number of front panel controls and
employ human factors engineering in configuring the front panel.
Important specifications of the system \-:ere measured on an
engineering evaluation model constructed to verify operation of the
circuits. These measurements indicated satisfactory performance.
The final configuration of the unit will be decided in cooperation
with the users of the system by employing the Delphi method. The final
production units will be built by the Engineering Applications group
on the base. ··
iv
CONTENTS
liST OF FIGURES
liST OF TABLES
BIBLIOGRAPHY
APPENDIX
Chapter 1
v
vi
63
64
INTRODUCTION
J .1 Gene'ral Introduction to Problem
1.2 Previous Approaches
1.3 Project Goals
Chapter 2
l
5
7
PROBLEH APPROACH
2.1 Problem Definition
2.2 Mission Requirements
2.3 Elements of General Communications System
9
11
13
2.4 Block Diagram of Telecommunications Switching System
2.5 Block Diagram of Terminal Unit
2. 6 Preliminary Decision
2.7 Futhur Definition of Block Diagram
16
Chapter 3
3.3 Circuit Design
3.4 Panel Layout
3.5 Construction
3.6 Options
•
39
42
44
49
51
52
RESULTS
4.1 Parametric Testing
4.2 Operational Testing
4.3 Achievement of Project Goals
Chapter 5
33
PROBLEN SOLUTION
3.1 Terniinal Unit Configuration
3.2 Circuit Design Objectives
Chapter 4
18
22
53
56
56
CONCLUS IOf-!S
5.1 General Conclusions
5.2 Hot-; t·lany Units Should be Built?
5.3 Spare Parts Inventory
v
58
59
61
--
List of Figures
1.1-1 Pt. Hugu System
1.1-2 Terminal Unit - TSS Relation
2e3-l
2.3-2
2.3-3
2.3-4
2.3-5
2.3-6
2.4-1
2.5-1
2.5-2
2.5-3
2.7-1
2.7-2
2.7-3
2.7-4
2.7-5
Transducer
Amplifier Symbols
Switching Network Symbol
Hultiple:x:er Symbols
Signalling Symbol
Hodem Symbol
Teleco~~unications System
Ba.sic System
TransMission Path Equivalent Circuit
Improved Equivalent Circuit
Terminal Unit Block Diagram
Terminal Unit Block Diagram
Terminal Unit and System Block Diagram •
Cost Versus Number of Units
Transdu<;er Combinations
Transducer Parameters
Transducer System Diagram
Signalling Block Diagram
Total System Block Diagram
3$1-1
3.2-1
3.3-1
3.3-2
3.3-3
3.3-4
3.3-5
3.3-6
3.5-1
3.5-2
Utility Curves
Hierarchy of Design Objectives
Hultiple Control
Switching System
Power Supply
Signal Transmit Circuit •
Amplifier Circuit
Voice Operated Lamp
Nodule Construction
Card Cage
2.5~4
2.5-5
2.5~6
2.6~1
4 .. 1-1 Amplifier Bode Plot
4.1-2 .Hodified Audio System
4.1-3 Cross-Talk Heasurement
3
4
13
14
15
15
16
16
17
18
19
19
20
21
21
24
34
35
36
37
38
41
43
45
46
!t6
47
48
48
51.
51
.
5.2-1 Decision Tree
..
53
54
55
60
vi
List of Tables
I. Alternative (1) Costs
II. Alternative (2) Costs
lii Alternative (3) Costs
IV Human Factors Ranking
V Flexibility Values
VI Availability Data
VII Haintainability Data
VIII Utility Table
o
IX Relative Values for Criteria
X Utility Table
XI Front Panel Controls
XII Push-Button Truth Table •
· XIII Production Alternatives
XIV Hodel Results
vii
23
23
.23
25
26
29
29
"
•
•
30
40
40
44
45
59
62
1
Introduction
1
2
1
Introduction
1.1
General Introduction to Problem
The Range Communications Division of the Range Operations Depart-
ment at the Pacific Niss.ile Test Center provides the Communications
·.services required to support the sophisticated test and evaluation
operations conducted on the Pacific Nissile Range. The division supports
the transmission, reception, distribution, and recording of data) video
and voice information.
The Pacific Hissile Range is made up of several instrumented sites
located at distances of up to two hundred miles from the central site
at Point Mugu. InfO.rmation must be transmitted to and from each of
these remote sites and then distributed locally to the appropriate end
' distribution point at the major sites is a large
user. The .central
switchable matrix referred to as the TSS (Telecommunications Switching
System). The TSS is manually operated and monitored by Range Communications personnel.
This project will address the problem of providing the·Pacific
Missile Range with voice communications capabilities. The existing
teleco~munications
system will be used as the transmission medium.
Figure 1.1-1 is a simplified block diagram of the existing telecom.•
munications system at Point Hugu. The intercommunications between the
various sites is accomplished by the telecommunications system.
~To VAFB
'
'
'
'
-- -Santa Cruz
Island
I
I
I
I
I
I
I
I
Island
......
.....
...........
......
.....
San Clemente' '
Island
Figure 1. 1-1
Pt. Hugu Sys tern
4
Basically vJhat is required is a users interface \vith the telecommunications system that will convert a voice input from the user to
a signal that \dll be compatible \-lith the sys-tem and conversly will
convert a signal output from the system into an audio form that can be
understood by the user. Since this unit will be used at the end points
of the system it is referred to as a communications terminal unit.
Figure 1.1-2 is a simplified block diagram of the terminal unit and
telecommuhications system relationship.
TU
TU
TU
TSS
TU
TU
TU
Figure 1.1-2
Terminal Unit - TSS Relation
5
1.2
Previous Approaches
The communications requirewent of the Pacific Nissile Range have
changed dramatically since the range vJas first established in 1946. The
early operations conducted on the Range consisted of attempts to launch
captured German V-2 rockets and then hopefully guiding them to impact
at San Nicholas Island. The few ground sites at Point Hugu that needed
voice communications were connected by a l\iorld
\~ar
II surplus telephone
switchboard. The terminal unit was simply a standard telephone handset
"1ith a hand crank generator for signalling.
As data collection sites were added at Point Mugu the capacity of
the simple switchboard system was quicklyexceeded by requirements. A
standard telephone central office switching system was installed to
improve communications services. The terminal units of this system were
standard telephone sets. Loudspeaker amplifiers could be added to allow
the conversations to be monitored by a group. This system proved to be
satisfactory for voice communications. However data and video signals
were also transmitted through the central office system on the same
cables as used for voice signals. The standard telephone system operates
in a duplex mode, that is a single pair of wires is used for both transmission and reception. This is perfectly satisfactory for voice signals.
Unfortunately however data was considerably degraded due to crosstalk
occuring in the cables. This was due to the fact that at any one time·
some of the wire pairs in the cable 'l.vill be transmitting while others
are receiving data. The solution to this problem is to physically seperate the transmitting and receiving pairs, but in a duplex system this
is impossible. Thus it \vas decided to build a new system using seperate
pairs for both transmission and reception.
6
This type of system was not available off-the-shelf so a contract
to custom build a four wire telecommunications system for Point Mugu
"·'as awarded to \-!estern Electric Company. The resulting system is still
in use today. The terminal unit that was designed for the system was
designated as the v:ECO 2250 communications terminal unit. The 2250 is
capable of selecting any one of twenty inputs to the switching system.
A block diagram of the 2250 terminal unit is _shown in figure 1.2-1.
Each block shown is a functional subunit. Each subunit except for the
operating panel is mounted on two floor to ceiling rack mounts and
requires approximately thirty square feet of floor space for the equipment -and adequate access space. The electronic circuitry for the
terminal unit is collectively referred to as the back-up gear. The
operating panel for the 2250 system is used to remotely control the
back-up gear. A one hundred and fifty wire cable is used to interconnect the operating panel and the
back~up
gear. Figure 1.2-2 is a
representation of a typical terminal unit installation. For convenience
the back-up gear for all the terminal units installed in a particular
building is normally physically located in the same area. Cables are
then routed from the central location to the location of the individual
operating panels throughout the building.
From figure 1.2-1 it can be noted that the 2250 contains circuitry
·for standard duplex telephone operation. This capability was required
during the transition from the duplex to the four wire system, but is
no longer
required~
The 2250 also has the capability to monitor any combination of the
twenty inputs on either headphones or loudspeaker. Each input has an
individual volume control and on-off switch.
7
The 2250 circuitry
~las
designed Hi th relay and transformer logic
which is directly compatible \\'i th the rest of the telecommunications
system. A 48 volt battery supplies all operatlhg power. Vacuum tubes
are used as active elements.
Although the ktCO 2250 terminal unit has performed satisfactorily
in the past two factors \¥ill make it unsatisfactory for future use in
its present form.
The first problem is the space required to house a 2250 unit. As
previously mentioned an average of thirty square feet of floor space
is required for each unit .• Building 53, the Range Operations building,
has a.critical need for additional office space. A large room in the
building is dedicated to housing the 2250 back-up gear for the units in
the building. This room has been pledged by management for other usage.
The 2250 back-up gear must be removed from this room. No alternative
location can be found in the building. However the continued capability
supplied by the 2250s in the building is essential.
A second problem is the operating panel itself. New control
. consoles are presently being designed to be part of the proposed Range
control center that will replace the existing tracking and control
facilities. These control consoles will be heavily packed with electronic equipment and panels will be densly populated with controls and
displays. The 2250 operating panel is very bulky and is physically too·
large to fit in the new consoles.
1.3
Project Goals
This project has three primary goals. The first goal of this
project is to provide the Pacific Missile Range with a satisfactory
s
communications terminal unit for present and future use.
The second goal of this project is to arrive at a satisfactory
solution to the problem of removing the 2250 back-up gear from building
53 without disrupting the voice
co~Junications
capability of the
building.
The third goal is to employ human factors engineering concepts to
redesign the operating panel so that it will be compatible with future
complex control consoles.
2
Problem Approach
2.1
Problem Definition
The first step towards achieving the goals of the project is to
define the mission requirements of a communications terminal unit that
would satisfy the project goals. Before the mission requirements can be
derived, a more complete description of the time frame, operating
environment, state-of-the-art, and problem constraints must be detailed.
Time Frame
The office space problem in building 53 is critical and must be
resolved by the latter part of 1976. The new Range Operations control
centeR.will not become operational for some time and the time constraint
here is not critical. The solution should be a permanent solution
designed to satisfy all anticipated voice requirerr.ents of the Range.
Operating Environment
The terminal units are always housed indoors in protected areas.
The average operating temperature is about twenty five degrees centigrade with little daily or seasonal variation. The humidity is normally
low and little condensed moisture can be expected. The units may be
frequently exposed to dust, dirt, sand, smoke, and other atinospheric
contaminents.
The operating panel will be located in a crowded equipment console.
9
10
The background lighting \vill vary from dim to very bright, ho":ever no
direct sunlight is expected. The background noise in the area where the
consoles will be located may be very high.
The unit will be subjected to rough handling during installation
or maintanence. The controls on the operating panel will be subjected
to very heavy usage.· The units may be required to be operational for
periods of several days or more.
State-of-the-Art
The \-JECO 2250 terminal unit employs transformer and relay logic.
The current state-of-the-art that could be applied to this problem is
the use of medium and large scale integrated circuits vJherever possible.
Techniques such as optical fiber transmission are not fully developed
and use of these technologies would involve considerable risk.
Constraints
The basic constraint imposed on the solution is that the resulting
terminal unit be compatible with the existing telecommunications system.
The terminal unit must not degrade the system in any of its other modes.
The operating panel must be no larger than
inches long. The length shoulq be exactly
19~
7t
inches high and
19~
inches since this is the
standard rack mount size.
Funds for the project are very limited. A six month delay can be
expected on any parts orders. The proper test equipment, working space,
. and tools are not available in the Range Communications Division. These
will have to be found from other sources.
11
2.2
Mission Requirements
One of the goals of this project is to provide the Pacific Missile
Range with a satisfactory communications termfnal unit. Since the unit
will be required to be operational for periods of up to several days
continuously, the unit will need a high reliability in order to be
satisfactory.
The military standard definition for reliability is the probability
that the terminal unit will remain operational for a specified period
of time.
Reliability data has been kept for the wECO 2250 unit by the Range
·communications Division for the past four years. The time frame used
for computing reliability figures has been one month. For convenience
reliability data used for this project will be in terms of the average
number of failures per month.
Another usefull reliability figure is the HTBF (Nean Time 3efore
Failure) of a system. Both the systems rfrBF and reliability per month
will be used as decision criteria for ranking candidate solutions with
respect to the reliability mission requirement.
A second mission requirement for a satisfactory terminal unit is
good maintainability.
The military standard definition for
maintain~bility
is the
probability that the system or equipment, "'hen in an inoperative
condition, will be restored to an operative condition within the
allo\-Jable repair time. The decision criterion to be used for the maintainability mission requirement will be the probability that a
defective terminal unit can be repaired within a single work day.
12
A third and related mission requirement is availability. The
military standard definition for availability is the probability that
the termina 1 unit wi 11 be in an operable andcommittable state v;·hen
required for operational support. The equipment is defined as not
available during scheduled, preventive, or corrective maintenance
actions.
The criterion that will be used to rank the candidate systems
with respect to availability is the probability that the terminal unit
will be ready to support given a two hour advance notice.
Low cost is another important mission requirement. In order to
insure the true costs of each candidate system are considered, life
cycle cost will be used as the decision criterion. Life cycle costs
include all development, procurement, and operating costs associated
with the system throughout its lifetime.
A satisfactory terminal unit must be flexible enough to meet
the unique requirements of particular installations. Ko military
standard definition exists for flexibility, so an arbitrary scale
will be used to rank the candidate systems as follows:
5
Highly Flexible
4
Good Flexibility
3
Fair Flexibility
2
Poor Flexibility
1
Inflexible.
Human factors wust also be considered in the selection of the
best candidate system. The primary considerativns are the placement
of the controls, labelling of the controls, and ease of operation of
,the operating panel.
13
Again there is rio military standard definition for rating human
factors. An arbitrary scale will be used to rank the systems
~ith
respect to human factors as follows:
5
Superior
4
Good
3
Fair
2
Poor
1
Unsatisfactory.
The final fuission requirement is small size for both the operating
panel and the back-up gear. The criteria to be used to evaluate the
·relative size of the various systems \olill be the heigth of the
operating panel and the volume of the combined back-up gear.
2.3
Elements of General Communications System
There are several basic communications building blocks that
CcHl
be
used in the proper combination to make up any communications system.
A basic building block common to any communications system is the
transducer. A transducer is a device that converts a signal from one
form of energy into another form of energy. The symbol that will be
used to represent a transducer is a circle with an inscribed
shown in figure 2.3-1.
Figure 2.3-l
Transducer
T~
as
14
A second basic building block often employed in communications
systems is the amplifier. ~'O types of amplifiers are used, the buffer
amplifier and the distribution amplifier.
The function of the buffer amplifier is twofold. First it serves
as a device to match impedances between blocks. Secondly it can be used
to increase the amplitude of an input signal by a factor of K, · t-Jhere
K is the gain of the amplifier. The symbol for a buffer amplifier is
a triangle with an inscribed K as shown in figure 2.3-2.
The function of the distribution amplifier is to make the output
signal independant of the output load. Typically a distribution
amplifier is used to route a signal to several different locations
while keeping the amplitude at each location constant. The symbol for
a distribution amplifier is a triangle \vith an inscribed D as shown in
figure 2.3-2.
Buffer Amplifier
Figure 2.3-2
Distribution Amplifier
Amplifier Symbols
A third communications building block is the switching network.
The general form of a switching nett-Jork is a matrix where any number of
input signals can be connected to any combination of outputs. The symbol
for a switching network of size N by M is shown in figure 2.3-3.
15
inputs
M
Switch
N
outputs
Figure 2.3-3
Switching Network Symbol
- Th'O more comll-.unications building blocks are the multiplexer and
demultiplexer. Multiplexers can combine several signals into one
composite signal, while a demultiplexer can break a composite signal
into its component parts.·Hultiplexers are used to increase the amount
of information that can be sent through the same medium. The symbols
used for the multiplexer and demultiplexer are shown in figure 2.3-4.
I
I
Hux
Multiplexer
Figure 2.3-4
Demux
l
Demultiplexer
Multiplexer Symbols
Another communications block often used is the signalling unit.
Signalling is usually used as an aid to the operator of the system
16
although signalling is also sometimes used between comp6nents of the
system. The symbol for the signalling unit is shown in figure 2.3-5.
-----..~j. Signal!~-o
Figure 2.3-5
Signalling Symbol
A communications block important in data communications is the
modem. A modem changes infonnation from one format to another format.
Hodems are frequently enrployed to allow computers to communicate
\-Ji th
each other. The symbol for a modem is shown in figure 2.3-6.
------~1 ~d
Figure 2.3-6
Hodem Symbol
The last important consideration in any communications system
is the transmission medium. This medium is usually designed to pass
the desired signal only while screening out external interference.
2.4
Block Diagram of Telecommunications S\·;itchirig System
The object of this section is to derive a block diagram of the
relevant parts of the TSS system. Basically the only part of the system
that will interface.with the terminal unit is the cross·bar matrix.
This can be represented by a switching net\-.'Ork as shown in figure 2. 4-1.
There are nine hundred and sixty inputs to the matrix. These inputs
17
are referred to as std tch trunks. Each switch trunk is a four \d re
cable running from the cross-bar matrix to the end equipment. Thus the
terminal units td 11 directly interface with the
S\d tch
trunks.
!he system outputs are called ll.nks. There are a hundred and
fi~fty
links on the Point Hugu system. A link can be thought of as a
conferencing network made up of some combination of switch trunk inputs.
The dotted lines in figure 2.4-1 represent a typical example of
hm. .• the system might operate. In this case switch trunks A and B are
connected to link 4 'Vlhich essentially means S\vi tch trunk A is connected
to switch trunk B. Switch trunks C,D,and E are connected to link 1
. ,.,hich means trunks C, D, and E are interconnected in a conference
nett-;ork.
Switch
trunks
A
---,
B
··-------!
c
I
I
.,
D
I
E
1
I
I
f
I
I
I
I
I
I
TSS
I
I
I
I
I
I
I
!
1
i
2
3
4
Links
Fi.gure 2.4-1
Telecommunications System
18
2.5
Block Diagram of Terminal Unit
The basic function performed by the terminal unit is to convert
a signal from the user into a form that can be transmitted over the
system andvice versa. The transducer building block is required to
accomplish this. Two different transducers will be required, one for
receive and one for transmit. Figure 2.5-1 shows the most basic form
of the terminal unit and its interface with the TSS.
-,
Unit 1
I
I I
:v
Unit 2
Unit 3
TSS
AI
®
tJ
G)
2
1
3
links
Figure 2.5-1 Basic System
~ith
the simple system sh6wn in figure 2.5-1 a problem will occur
"·'hen more than two terminal units are connected on the same link. The
equivalent circuit for the transmission path in the system \-lith N
number of terminal units switched onto the same link is shown in figure
2~5-2.
Since the impedance at every site is equal, the current, In,
will be equal in each leg of the system. The value of In is dependent
Qn the value of N as follows:
In
= VIZ =
where Zeq
Vt(Zeq/Zeq+Zt)/Z
= Zli/Z21/Z3/IZ4
... 1/Zn.
(I}
19
terminal unit sites
2
3
4
1
n
z
m
Figure 2.5-2
Trensmission Path Equivalent Circuit
Thus as the value of N is increased the value of Zeq decreases
·rapidly which causes a corresponding drop in In. Since the potver
received at each site is proportional to In squared,
site is significantly decreased each time
~n
~he po~er
at each
additional unit is added
to the link.
A distribution amplifier building block will solve the problem.
The equivalent circuit including a distribution amplifier is shown in
figure 2.5-3. Note that in the ideal distribution amplifier the output
impedance is
zero~
which makes the value of Zt
i~
equation 1 equal to
zero, which effectively makes In independent of N.
Zi
= oo
Zo
=0
1
Figure 2.5-3
2
3
4
Improved Equivalent Circuit
n
20
The terminal unit block diagram as it no"' stands is sho"m in
figure 2.5-4.
().,._,..,...
______
Figure 2.5-4
Terminal Unit Block Diagram
The terminal unit as it is shown in figure 2.5-Lf is not very
satisfactory from the users point of view. The only way the user can
select which link he wishes to have his terminal unit connected to is
by having it switched by the TSS operators in the Range Communications
building. A much more desireable method would be to add a switching
net't-;ork to the terminal unit and allow for local selection of the
desired switch trunk. This means that more than one switch trunk must
be dedicated to each terminal unit, the actual number \dll vary
according to the needs of each particular installation. The revised
blockdiagram of the terminal unit is shown in figure 2.5-5.
----.-,..--,
TSS
I
I
I
I
L
__
Figure 2.5-5
I
1 2 3
-
- - - ~- ___j
Terminal Unit Block Diagram
21
The terminal unit as it now stands still lacks one important
capability,
signa~ling.
The terminal units are not monitored contin-
uously but are usually located within easy reach of an operator. Thus
it is necessary to alert the operator when there is an incomming call.
Therefore a signalling building block must be added to the terminal
unit block diagram.
However no additional wires to carry the signal can be added to
the system. Thus the same wire pairs will have to carry both the voice
signal and the signalling signal. To accomplish this a multiplexer
and demultiplexer building block will be required. The final block
diagram of the terminal unit and the TSS is shown in figure 2.5-6.
Sw
TSS
Sw
3
Figure 2.5-6
Terminal Unit and System Block Diagram
22
2.6
Preliminary Decision
Now that the block diagram of the required terminal unit has been
obtained, a decision on how to proceed can be~made. There are three
. feasible el ternatives:
(1)
Remove all
~ECO
2250 terminal units and replace with
existing units of less capacity,
(2)
Hodify the \·:ECO 2250 terminal units so that the project
goals are achieved,
(3)
Buy or build a new terminal unit that will satisfy the
goals of the project.
A zero maximum utility analysis will be made to determine "t>:hich
of the three alternatives should be pursued. The criteria to be used in
the analysis \\'ere defined in section 2.2. Values for the decision
criteria will now be established.
Cost
The Altec 2512 five channel communications terminal unit could be
used in place of the \VECO 2250 unit, but four 2512s would be required
to match the capacity of the 2250. Therefore cost calculations for
alternative (1) will be based on using four 2512
units~
The life cycle cost per unit calculations are shown in tables I,
II, and IlL The follOlving assumptions have been made:
(1)
An average expected lifetime of ten years,
(2)
One half of the required 2512 units would already be in
stock, so only two new units per installation would be
required,
(3)
Engineering costs are fifteen dollars per man-hour,
23
(4)
Installation and maintenance costs are ten dollars per
man-hour,
-,.---
(5)
Only incremental costs will be considered.
Table I
Alternative-(!) Costs
Fixed costs
Engineering time
40 hours @ $15/hour
600
Unit costs
Unit purchase price
Installation time
Yearly maintenance
Lifetime maintenance
Total lifecycle costs
Table
rr
=
5 hours @ $10/hour
.53 faults X 4 hours @ $10/hr
10 years @ $25/year
2,500
2,500
200
25
250
$600 + $5,575/unit
Alternative
(2)
costs
Fixed costs
Engineering time
400
hours @ $15/hour
6,000
Unit costs
Installation time
Yearly maintenance
Lifetime maintenance
Total life cycle costs
Table III
120 hours @ $10/hour
1,200
1.13 faults X 6 hours @ $10/hr
68
10 years @ $68/year
680
= $6,000
+ $1,880/unit
Alternative (3) Costs
Fixed costs
Engineering time
1000 hours @ $15/hour
15,000
Unit costs
Parts purchase (estimate)
Assembly (estimate)
Installation time
4 hours @ $10/hour
Lifetime maintenance costs (estimate)
Total life cycle costs
=
$15,000 + $700/unit
250
300
40
200
24
Figure 2.6-1
Cost versus number of units
25
Figure 2.6-1 is a graph of the cost of the three alternatives
versus the number of units installed. Since at least ten units will
have to be purchased to solve the space problem in building 53, the
cost for ten units will be used in this analysis.
Human Factors
The decision criteria for human factors is a relative weighting
of several considerations as discussed in section 2.2. For these three
alternatives the ranking was determined by consultation '"ith terminal
unit users. The results are presented in table IV.
Table IV
Human Factors Ranking
Alternative 11
Score
l
2
2
4
3
5
Flexibili tr
Again the decision criteria for flexibility is a relative weighting of the flexibility of each alternative as discussed in section 2.2.
The values were determined by surveying the needs of several terminal
unit sites and making a list of all unique requirements. The capabilities of each alternative t-:as compared to the list of requirements to
arrive at the values shot-m in table V. The value for alternative
(3)
was set at the maximum possible because it was assumed that if a new
terminal unit was obtained that it would be possible to buy or build
one with all of the desired capabilities.
26
Table V
Flexibility Values
Alternative it
Value
1
2
2
3
3
5
Size
Two criteria must be considered for the small size mission
requirement, the heighth of the operating panel and the volume of
the back-up gear. However no accurate figures can be obtained for
alter~natives
(2)
and (3), so some estimation will be required. The
size values are estimated as follows:
Alternative
(1)
Panel heighth (each unit)
Total heighth (four units) .•••..... 21"
Volume ...•.••..•.•.•..•...•....••... 2.4 cubic feet
Alternative (2)
Panel heighth (estimate)
Total volume ..••••••..•••••••••••.. 6 cubic feet
Alternative (3)
Panel heighth (estimate)
Total volume .•.•••••..••••••••.•...• 5 cubic foot.
To simplify the analysis a relative weight based on the same scale
as used to rate the human factors criteria will be assigned to each
alternative as follows: alternative(!.) = 2, alternative(2) = 3,
alternative (3) = 5.
27
Reliability
Reliability data has been kept by the Range Communications
Division over a period of several years. This data is used to establish
. normal expected fault indices so that abnormal fault trends can be
identified. The values for the reliability decision criteria are
calculated for each alternative as follows.
Alternative
(1)
The data for the 2512 terminal unit indicates a reliability of
• 97 for a one month period. HO\>Jever four 2512s each fully operational
. will be required to replace one 2250. Thus the system reliability can
be calculated by considering the four units are operating in. series.
The reliability for a system whose components operate in series is
given by equation (2).
Rs
=
(Rl)(R2)(R3) ..• (Rn)
(2)
Applying equation (2) the reliability for alternative (1) can be
found as follows:
Rs
= (.97) 4
=
(3)
.883 •
To calculate the Iv1TBF of the system a distribution of the failure
rate must be assumed. The data indicates that a normal distribution
is most applicable. The formula for the !,ITBF is given by equation (4).
t
R(t)
=
14 2ffs ~exp(-~(x-o/s) 2 )
-
where u
x
dx
(4)
OD
= HTBF,
= time
s
= standard
period, R(t)
deviation,
=
.B83
28
The equation is solved as follows:
R(t)
=
1 - Q(x-u/s)
Q(z)
=
11 zus.~exp(-x 2 /2)
.88
=1
u
=1
(5)
dx
( 6)
- 0(1-u/.25)
+ .25(1.19)
(7)
= 1.3
months
(8)
Alternative (2)
The 2250 back-up gear has a reliability of .82 for a o_ne month
period. Since alternative (2) would involve only repackaging the
present equipment, this figure is valid for this analysis ..Again a
normal reliability distribution will be assumed. The NTBF is calculated
as follows:
R(t)
.82
u
=
=
=
1 - Q(x-u/s)
(5)
1 - 9(1-u/.25)
(9)
1 + .25(.92)
= 1.23 months
(10)
Alternative (3)
It will be assumed that a reliability of at least .95 can be
obtained in a new unit. Any new tern1inal unit would utilize solid state
technology. A valid assumption that can be made when using solid state
components is that the failure rate is constant regardless of the age
of the components. When this assumption can be made the exponential
reliability distribution is used. The
forn~la
for the exponential
failure law is given by equation (11).
R(t)
= exp(-at)
where a
= failure
(11)
rate
A value of .0005 is a standard value for the failure rate of solid
29
state circuits, and will be used for this analysis. The HTBF for alternative (3) is calculated as follows:
= NTBF = 1/a
E(t)
1/a
= 1/.oobs = i,ooo
(12)
hours
2,000 hours/672 hours/month
(13)
=
2.9 months
(14)
Availability
The values for availability can be taken directly from the data
and no calculations need be made. The value for alternative
(3)
"·'ill
be assumed to be equal to the value for alternative (2). The data is
presented in table VI.
Table VI
Availability Data
Alternative il
Availability
1
.98
2
.95
3
.98
Maintainability
Again the values for maintainability can be taken directly from
the data without calculations. The results are presented in table VII.
Table VII
Alternative
Haintainability Data
'IF
Maintainability
1
• 76
2
.64
3
.76
30
The next step in the analysis is to develope utility curves for
each of the decision criteria. The utility curves are used to establish
a common unit for the various criteria so they can be summed for each
alternative. These utility curves are shown on the following page in
figure 2.6-2.
Since the criteria are not all equally important, a relative
weight will be assigned to each criteria as follows:
Reliability
=
.8
HTBF
=
.85
Size
= .8
Flexibility= .7
=
Human Factors
Cost
=
.6
.5
~~intainability
Availability
=
= .4
.4 •
These \'leights were determined by considering the relative contribution ·
of each criteria towards achieving the project goals. Table VIII
contains the utility values taken from figure 2.6-2 for each criteria.
Table VIII
Alt.
1
Utility Table
Rel.
HTBF
.o5
Size
.8
.8
Flex.
.7
H.Fac.
.6
.5
• 25
.1
.1
• 25
Cost
.5
Hain •
Avail.
.4
~4
0
.75
.8
'
2
.15
.2
.2
.5
1.0
.b
• 25
.2
3
1.0
.95
1.0
1.0
1.0
.9
.75
.8
31
1.
1
.75
.75
.5
----;-5
.25
.25
0
0
5
20 25 30 35 40 45
4
3
2
1
Human Fa.ctors
Cost ($K)
1
1
.75
• 75
.5
.25
0
0
5
-··
4
3
2
.95 .9 .85 .B .75
1
Reliability
Flexibility
1
1
• 75
.75
.5
.5
.25
.25
0
o.
1 .99.98.97.96.95
3. 2.5 2. 1.5 1.
MTBF (months)
Avai labi 1 i ty
1
1
.75
.5
.75
.25
• 25
0
0
.5
5
.9 .8 .7 .6 .5
3
Size
Maintainability
Figure 2.6-2
4
Utility Curves
2
1
32
The t6tal utility for each alternative can now be calculated.
For alternative(l) the total utility is found as follows:
U( 1)
=.• B (. 5)
+
+ • 85 (. 25) + • 8 (.1) + • 7 (:f) + • 6 (. 25)
.5(0) + .4(. 75) + .4(.8)
=
(15)
1.53
For alternative (2) the total utility is found as follows:
U(2)
=
.b(.15) + .85(.2) + .8(.2) + .7(.5) + .6(1.0)
+ .5(.8) + .4(.25) + .4(.2)
=
1.98
e
(16)
For alternative (3) the total utility is found as follows:
U(3) = .b(l.C) + .85(.95) + .8(1.0) + .7(1.0) + .6(1.0)
+ .5(.9) + .4(.75) + .lf(.8)
=
3.97
(17)
These results indicate that alternative (3) is far superior to
the other two alternatives. Therefore it is clear that a new terminal
unit will be the best solution to the problem.
The next decision to be made is tvhether to buy a new unit from a
commercial vendor, or build a ne"t-J unit in-house at Point Hugu.
The only decision criteria that will have different values for
these two alternatives is cost. Therefore a fixed effectiveness cost
analysis will be used to choose between the two.
A check with the commercial vendors revealed that a satisfactory
terminal unit v.'as not available off -the-shelf. A custom design would
be required to have the unit commercially produced. The lowest estimate
from the vendors for ten units was approximately fifty thousand
dollars.
The cost estimate arrived at in table Ill \,·as based on building
the ten units in-house. The result was twenty-.two thousand dollars.
Thus on the basis of lower cost, the units will be built in-house.
33
2.7
Futhur Definition of Block Diagram
Since a new terminal unit is to be designed and built in-house,
the first step 1n this process will be to furthur define the terminal
unit block.diagram derived earlier.
Transducers
The key building blocks in the terminal unit are the transducers.
Selection of these blocks is critical since the function of the other
blocks in the system is mearly to support the transducers.
Several transmit transducers could be used. The following is a
list of those that are most suited to this application:
1.
crystal microphone
2.
dynamic microphone
3.
carbon microphone
4.
speaker used as dynamic microphone
5. telephone handset (carbon mike)
6. telephone headset (carbon mike) •
Several receive transducers are also feasible as follows:
1.
loudspeaker
2.
headphones (standard)
3.
headphones (binaural)
4.
telephone handset
5. telephone headset
6.
telephone headset (binaural).
A survey of terminal unit users was made to help determine \,•hich
transducers should be used. The most popular receive transducer was the
loudspeaker. However many terminal units are located in crowded rooms
34
or in noisy environments where use of a loudspeaker is in1practical.
Thus in addition to a loudspeaker, the terminal unit will require the
capability to allow use of either headsets or telephone handsets or
both.
The survey revealed that it was also necessary to have the capability to monitor any of the switch trunk inputs to the unit independently of the switch trunk selected for transmission. This rules out
standard headphones or headsets because the user would not have access
to both the active switch trunks and the monitered switch trunks
simultaneously. A binaural headset will solve this problem since each
earpiece receives a different signal. The binaural telephone headset
also includes an attached carbon microphone, so this combination will
be used for noisy or other environments where headphones are desireable.
The next step is to choose a transmit transducer to go along with
the loudspeaker in quiet environments. The loudspeaker can best be used
as the raoni tor transducer. This means another independent transducer
will be required for the active switch trunk, and the use of either
transducer should not interfere with use of the other. The telephone
handset is ideal for this purpose. The carbon microphone on the handset can be used for the transmit transducer. A block diagram of the
transducers is shown in figure 2.7-1.
Active~
Tra.nsmit .
Active "<$
Receive
ive Receive
Transmit
Nonito~
Normal
Noisy
Figure 2.7-1
.
Transducer Combinations
35
Associated Circuitry
The addition of the switch trunk monitor will require the addition
of some support circuitry. The parameters of the system and those of
the transducers are shown in figure 2.7-2 •
16~
.,....Zo=60o.n.
'
Transmit
""""-
• 5\.Z
~-VDC=-48
Tx8500~~
lK.n.
VAC=35mv
Int.
Rx
1
.n..
. $1W
B.!t ~-'~
2\oi
v---
Figure 2.7-2
Zo=6QQ!'!..
-
....
VDC=-48 VAC=35mv
Receive
Transducer Parameters
The ne\<J circuitry ml,lSt convert the voltage and impedance levels
of the system to values that are compatible with the transducers. This
can be accomplished by using buffer amplifier blocks in the appropriate
places.
Some modification to the switching net,.;ork will also be required.
The monitor output must be selectable independently of the active
channel output. This will essentially require tv1o Sv7itching networks.
Some consideration must also be given to the method of controlling
the volume of the transducers. Two methods are feasible as follows:
(1) Use an automatic volume control circuit to assure
all channels produce an output of equal strength)
(2) Use seperate volume controls on each channel .
On the basis of reducing the complexity of the operating panel, the
36
first alternative v,;ould appear more desireable. Hm.,ever the survey of
the terminal unit users indicated that retention of seperate volume
controls v:as desired for the follov1ing reason. The inclusion of
individual volume controls allot-;s the user to t-ieight channels in order
of importance so that the most important channels being monitored can
be given sufficient volume so that they can not be drowned cut by less
important channels. Therefore individual volume controls will be used.
The block diagram of the transducer system is shown in figure 2.7-3.
S"Y. itch
Control
1
To
__
· ainaural---t--~..,._----------1
Hike
Active
Channel
Switching
Network
Binaural
Headset
Honitor
Switching
Network
To
Binaural
Headset
Figure 2.7-3
Switch
Control
Transducer System Diagram
37
Signalling
Two types of signalling were desired by the terminal unit users.
The terminal unit should be capable of both signalling any specific
switch trunk and any specific link. Signalling a switch trunk t-Jill
signal only one terminal unit while signalling a link may signal
several units.
Both audio and visual signal indication will be required. Some
method of controlling the volume of the audio indicator will be needed.
Selective signalling can be accomplished by using either a standard telephone dial unit or a touch-tone unit. The
teleco~nunications
system at Point Nugu is set up to process standard dial units, so a
telephone dial unit will be used for signalling.
Figure 2.7-4 is a block diagram of the signalling units.
Visual
indicator
Demux
Audio
indicator.
select.
signal
(dial)
Mux.
link
signal
Figure 2.7-4
Signalling Block Diagram
Combining the transducer and signalling block diagrams with the
general diagram obtained earlier, figure 2.7-5 is obtained.
38
Rx
Switch
Trunk
#1
To
Binaural
Headset
Active
Channel
Switch
Monitor
Switch
Switch
Trunk
Volume
Controls
Binaural
Headset
Rx
S\'litch
Trunk
1/n
Demux
Switch
Control
Figure 2.7-5
Switch
Control
Total System Block Diagram
3
Problem Solution
3.1
Terminal Unit Configuration
Now that the terminal unit block diagram is fully defined some
consideration can be given to the final configuration of the unit.
Several approaches are possible as listed belmv.
(1)
Self contained unit (backup gear and panel)
(2)
Seperate back-up and panel
(3)
Nodular self contained
(4)
Modular back-up gear and seperate panel
Again in order to determine v:hich approach should be selected
the technique of zero maximum utility optimization will be employed.
The same criteria as used before will again be employed. The criteria
will be -.,:eighted as they
~.1ere
in section 2.6n
No quantitative data exists to determine accurate values for the
decision criteria. Therefore a relative scale of 2 to 5 ,.,ill be used
to rank the alternatives in relation to each other. An alternative that
is judged clearly superior to most of the other alternatives in
satisfaction of the mission requirement will be assigned a value of 5
for the corresponding criterion. A value of four will.be assigned to
criteria where that alternative does not greatly exceed other alternatives in satisfaction of mission requirements. A value of three means
that.other alternatives are superior while a value of two indicates
only marginal satisfaction of mission requirements.
39
40
The data is presented in table IX •
Table IX
Alt 1r••
Rel
Relative Values for Criteria
Size
Flex
. H.
Fac
Cost
Hain
Avail
1
5
,5
2
5
5
3
4
2
3
2
4
4
3
2
3
3
5
4
5
4
2
5
4
4
3
3
5
4
2
4
...
4
..
····-·-
The next step is to develope utility curves for each criteria for
this range of _relative values. These curves are shown on the follolving
page in figure
3~1-1.
The final utility table is shown in table X below. The values were
taken from the utility curves of figure 3.1-1.
Table X Utility Table
alt 11
Rel
.8
Size
.8
Flex
.7
H.Fac
.6
Cost
Hain
.4
.5
Avail
.4
-.---
------··
1
.8
.8
.15
.6
.5
.12
.32
2
.24
~24
.6
.43
.4
.16
.28
3
.8
~72
.7
.43
.15
.4
.32
4
.24
.56
.7
.43
.15
.36
-
Total Utility:
(1)
= 3.29
Alternative (2)
= 2.35
Alternative {3)
= 3.52
Alternative {4)
= 2.76
Alternative
..
.32
···--
..
41
Reliability
1
~7'j
.5
.. 25
0
:s
4
3
2
F lexi bi l i ty
Human Factors
.7~ t
G~
.. 25
tT
0 -r·--+;.~--,--;-
Cost
Maintainability
.75
.. 5
.2~~!--T-~~--+-~~-5
4
3
2
1
Availability
Figure 3.1-1
Utility Curves
42
Thus alternative (3), a modular self-contained approach, has the
most utility and this approach will be taken.
·The next step is to break down the terminal unit block diagram
into functions that can be modularized. The number of different modules
should be limited to as few as possible to facilitate spare parts
stocking and maintenance.
One module must be reserved for functions that are common to all
of the switch trunk inputs. Only one other basic module will be needed.
This module will contain the circuitry that is unique to each switch
trunk input. For convenience each module will contain the circuitry to
handle four channels. Figure 3.1-2 shows how the block diagram function
..
will be distributed.
Common ;,todule
Loudspeaker
Honitor
Headset Interface
Signalling
Power Supply
3.2
Four Channel Hodule
Switching Network
Distribution Amplifiers
Switch Control
Circuit Design Objectives
The project is now ready to proceed into the detailed circuit
design phase. First some design objectives need to be developed to
insure that the mission requirements are met. The design strategy is
developed from the project goals, mission requirements, and decision
criteria. This relationship is shown in figure 3.2-1 which is a hierarchial chart of the project objectives.
Figure 3.2-1 Hierarchy of Design Objectives
Provide
Satisfactory ,
Goals
T~rminal
Solve ·
Operating
Panel
Problem
Solve bld 53
Space ·
Problem
Unit
Nission
Rqmts.
Reliability
Use High
Reliability
Components
Stratigies
Use Small
Number of
Parts
y
Cost
Size
Hum~n
Factors
Use AS Fet-;
Nodules As
Possible
Reduce
Panel·
Area
Use Clear
Status
Indicators
Hinimize
Number of
Controls
l
Group
Controls
Functionally
Use Human
Factors
Engineering
Clearly
Label All
Controls
+'
w
44
3.3 Circuit Design
One of the key stratigies derived in the previous section was to
minimize the number of controls on the operating panel. Table XI is a
list of- the required controls for each module.
Table
XI Front Panel Controls
Common Nodule
Four Channel Module
Pol<Jer
Nonitor Select (4)
Link Signal
Active Channel Select (4)
Haster Volume Control
Channel Volume Control (4)
Alarm Volume Control
Alarm On-Off
Several possible control function combinations can be considered
for reducing the number of controls on the four channel module. The two
most promising are:
(1)
Combine the monitor and active channel select function
on one dual function switch,
(2)
Combine all three control functions onto a potentiometer wiyh a push-pull switch attachedo
A related consideration is the method of providing some indication
of the status of the controls. To take full advantage of combining
control functions the indicators should be incorporated in the control.
A dual bulb illuminated pushbutton s";itch can be used for the first
alternative. However no standard control exists that would be satisfactory for the second alternative. A suitable control is shown in
figure 3.3-1. This control would have to be custom made exclusively
45
for the terminal unit. The knob would be made of lucite or some other
translucent material. The two light bulbs would be mounted in the back
side of the knob.
figure 3a3-l
Hultiple Control
Although the triple control achieves a significant reduction in
the number of required controls, it will not be used because of its
much higher cost and questionable reliability. Additionally replacement
parts would be difficult to obtain in the future.
A momentary push-button 1111ill be used. The truth table of the
desired switch operation is shown in table XII below. A seperate toggle
switch will be required to select either active or monitor mode.
Table XII
Switch Operation
0000
1000
0100
0010
0010·
0001
0001
0001
1000
0100
0010
0001
1000
Push-Button Truth Table
Act-Hon
Act
II
II
II
.II
II
II
II
Hon
II
II
II
II
Active Chan.
0000
1000
0100
0010
·oooo
0001
0000
0001
0001
0001
0001
0001
0001
Honitor
0000
0000
oooo·
0000
0000
0000
0000
0000
1000
1100
1110
1111
0111
46
The switch function truth table can be obtained using digital
logic. The block diagram of the switching system is shown in figure
3.3•2 shown below.
Honitor
Honi tor
Nonitor
Switch <====:tSelect
Active-Honitor
-r>.,._.., Select
Push-Button
Active
Chan.
Switch
Active
~====::t Se lee t
Figure 3.3-2
Indicator
Switching System
The amplifier system will be built using integrated circuit
operational amplifiers. These amplifiers require both a positive and a ·
negative power supply in the range of twelve to fifteen volts. Both a
twenty-four and a fourty-eight volt battery supply are readily availabrefor use by the terminal unit. The digital logic used in the switching
system will employ CHOS technology which will allow them to operate
over a voltage range of five to fifteen volts.
All the required voltages can be derived from the
twenty~four
volt
battery source by splitting it into a plus and minus twelve volts. The
block diagram of the power supply is shown in figure 3.3-3.
I
24 v
Battery
Voltage
Splitter
Gnd
Circuit
I"
+
+12
volts
-12
volts
-
I
Figure 3.3-3
Po\o;er Supply
47
The signalling circuit will be triggered by a one hundred and
twenty volt alternating current signal arriving over the transmit wire
pair. This input will cause the select indicator for ·that channel to
flash at a two hertz rate. The signal will also trigger an audio
oscillator which \-:ill be fed through the loudspeaker to produce an
audio indication. The block diagram of the signal receive system is
shown in figure 3.3-3.
2HZ
_fi_fL
Active
Indicator
Signal
Circuit
Transmit
In
Osc.
Alarm
~Volu~e~
~~ L_j
~- ~~~~ff
Figure 3.3-3
Signal Receive Circuit
To transmit a signal will require a circuit that will convert
pulse inputs from a telephone dial into a pulsed simplex ground of the
transmit and receive lines. A continuous ground will be interpreted
as a link Signal. The block diagram of the transmit signal section is·
shown in figure 3.3-4.
Selective
Link
Signal
~------~--~
Signa 1
Transmit
Circuit
\.
Figure 3.3-4
·1 Transmit
b
(
1-------'-----J
Signal Transmit Circuit
Receive
48
The amplifier circuit must sum the inputs from the various
channels to achieve the monitor function. This will require two summingamplifiers; one to sum the four channels on each module and one to
sum the outputs of each module. Differential amplifiers will be used at
each input to convert the two line balanced input to a grounded signal.
The amplifier system block diagram is shown in figure 3.3-5.
Channel
1
Summer
Summer
Channel
2
Channel
3
From Other
Modules
Volume
Figure 3.3-5
Amplifier Circuit
The final circuit to be considered is the channel status monitor .•
This circuit will detect audio at the input of any channel in use and
operate a light bulb when audio is present. The block diagram is shown
in figure 3.3-6.
Audio
Input
Channel
Status
Indicator
Voice
Oper.
Lamp
\\\
.Figure 3.3-6
Voice Operated Lamp
49
3.4 Panel layout
The layout of the controls on the front panel is a very important
consideration. The wishes of the terntinal unit user will be placed
first in determining the final layout.
It is felt that user acceptance of the unit will determine the
ultimate success or failure of the unit. In order to promote user
acceptance of the unit the users will be given the opportunity to
participate in the panel design.
An informal interview of many term ina 1 unit users \'las made to
allow them to submit their ideas on how the panel should appear. The
initial results show widely varying views on the subject. In order to
narrow this variance and bring about some consessus of opinion the
Delphi method will be employed.
The Delphi method is essentially an iterative technique where
each person is given the chance to modify his opinion based on the
opinions of others.
The final results will be incorporated into a cardboard mock-up
of the operating panel, and this panel will be used to build a prototype unit. This unit "'ill be circulated amoun& the terminal unit users
to allow them to become familiar with the operation of the unit and
give them a chance to suggest modifications before the design is made
permanent.
Figure 3.4 ... 1 is a drawing of the preliminary panel layout as it
is now envisioned.·
so
...
;---------~-----~---------~--~--- ----------~
.
'
"r.;
"?
·-
0
©
.o~. l± . ·o· ~~
c
0
.
.
.J?
<:(
.
~·//_. ----=----·~~=-···----~
i--------"~· ~----------~-----~
Ir._.. ~ ........-..... - ................ -
~~.---..--·-~-~--,_,_.....,.,.....
I
_____
----~-- ...... .,...,....~- ...- ........ _~·~·'""''.
:
I
0..> - · - - - · - - - -
------~
~
!
--·-----~-
i---
!
lI
li
·------··------·------~-~·~-~
""--·--- ---------
·-··-·-
Figure 3.4-1
Panel Layout
51
3.5 Construction
The circuitry for each module will be mounted on printed circuit
boards. The front panel.for eachmodule will be attached to the board
as shown in figure 3.5-1. The printed circuit board will have a built
in rear connector which can be easily unplugged when required. The
module will be secured to the unit by a thumbscrew at the bottom of
the front panel.
D
D
PC Board
D
D
Figure 3.5-1
Thumb
Module Construction
The unit will be constructed in a standard printed circuit card
cage as shown in figure 3.5-2. The cage contains guides for the printed
circuit boards and a rear mounted connector for each board. The connectors will be wired in parallel so that
modul~s
can be placed in the
cage in any order.
Socket
nn
nn
Figure 3.5-2
no
Card Cage
,MM
52
3.6 Options
Two optional plug-in modules will also be considered.
An important capability required in some installations is simultaneous transmission on multiple switch trunks. This module will be
referred to as a broadcast module.
The broadcast module will be basically very similar to a standard
four channel module. However the switching net\olork must be modified so
that more than one channel can be selected at the same time. Additionally a distribution amplifier \-Jill be required bet,veen the transmit
transducer and the various outputs.
The other possible optional module would contain the circuitry
for standard two wire telephone operation. At
pre~ent
there is no clear
need for this option at Point Hugu, but a future requirement is
possible.
4
Results
4.1 Parametric Testing
Several parameters can be measured and used to predict the overall
performance of the unit. The parameters of particular importance are
associated with the amplifier system.
The
perf~rmance
of an amplifier is specified by its frequency
response. The frequency response is defined as the bandwidth between
the points where the gain of the amplifier is three decibels less than
the maximum gain. The minimum bandwidth for voice frequencies is from
three hundred to three thousand hertz.
The frequency response of the terminal unit amplifier was measured
and the results were plotted in figure 4.1-1. This figure is a bode
plot
of the magnitude of the gain versus frequency on a logarithmic
scale.
0
-3
Mag
(DB)
10
100
Figure 4.1-1
1000
10,000
Frequency (HZ)
Amplifier Bode Plot
53
100,000
54
The signal to noise ratio of the system is defined as the ratio
of the average signal
po~:er
to the average noise power.
The noise present on the Pt. Hugu telecommunications system can_
be represented by gaussian white noise. This essentially means that the
magnitude of the noise is constant across the entire frequency
spectrum.
Measurements of the noise in the telecommunications system
revealed a re_latively high noise level, yielding an unacceptable signal
to noise ratio. To improve the signal to noise ratio an active audio filter
cir~
cuit will be added to the amplifier circuit just in front of the power
amplifier. This filter will have a frequency band,·:idt!t of three hundred
to three thousand hertz which is standard for voice signals.
When the filter is added the noise becomes bandlimited which
greatly reduces the average power of the noise signal. Accordingly the
signal to noise ratio of the system is greatly improved. An additional
advantage is the substantial reduction of sixty cycle hum in the output.
The modified block diagram is shown in figure 4.1-2 belo'"·
Hodule Summer
Inputs
From
Hodules ,__..
Active
Filter
Figure 4.1-2 i'lodified Audio System
55
The cross-talk amoung the inputs can be measured by using the test
set-up shown in figure 4.1-3. Each signal generator is set for a
different frequency and each frequency is set at a non-harmonic of the
other frequencies. With only one input connected to the summer at a
time, the output is checked on a spectrum analyser. If no-cross-talk
is present, the output will show energy present only at tbe frequency
of the generator feeding the input that is connected to the summer.
However some energy at the frequencies of the other generators was also
present. The magnitude of the cross-talk was more than fourty decibels
lower than the magnitude of the input signal which is well within
acceptable limits. Thus the amplifier is satisfactory with respect to
input cross-talk.
Figure 4.1-3
Cross-Talk i'leasurement
The other important parameter of the terminal unit is the loss of
signal strength incurred when the unit is placed in the system. The
insertion loss of the unit was about one decibel, as expected.
56
4.2 Operational Testing
The terminal unit has passed the parametric testing satisfactorily
·but perhaps the most- important testing is actual in-service use.
The only way to verify the reliability, availability, and
maintainability values assumed for the terminal unit is to record the
data over a period long enough to give an accurate forecast. The trial
period for the new terminal unit will be four months. This period must
be limited due to the time constraints of the project.
Two terminal units will be used for the operational testing. These
two units will be located at strategic sites so that a large number of
operators will have access to the units.
All operator comments will be examined and any appropriate
corrective action will be taken.
4.3 Achievement of Project
r~als
The major question to be answered is whether the goals of the
project were met or not.
The terminal unit can not be termed satisfactory until all oper-.
ational testing is completed and the results evaluated. However the
parametric test results and some very preliminary operational test
results give every indication that the unit will be very satisfactory.
The back-up equipment problem in building 53 will be solved by
the new terminal unit since the back-up circuitry is included in a
relatively small case behind the operating panel. Once the uriits are
determined to be satisfactory the change-over to the new units can be
accomplished quickly since only the minimum number of connections
need to be made to each unit.
57
The size of the operating panel will allow the new unit to be
installed in any anticipated control console. In fact the size of the
panel will be ·slightly less than the size of the 2512 five channel unit.
Careful consideration is being given to the layout of controls on the
panel. The final design should be satisfactory from a human factors
viewpoint.
5
Conclusions
5.1 General Conclusions
The new terminal unit will be designated as the RCD-20X. The
RCD-20X will offer significant improvement over the \vECO 2250 terminal
unit by having a greater reliability, maintainability, and availability,
and yet will be much smaller and less expensive. Even more importantly
the RCD-20X will be capable of meeting the future voice communications
requirements of the Pacific Hissile Range, while the WECO 2250 l-:ill not.
The general block diagram of the terminal unit was derived from
requirements only and will remain valid despite changes in technology
as long as the requirements remain the same. It \-.Jas for this reason
that the emphasis of the report was on deriving the blocks rather than
the specific circuits designed to achieve them.
The components used in the terminal unit'are standard off-theshelf types and thus contribute to the low cost and high reliability
of the unit. This will also insure that spare parts can be obtained in
the future as they are needed.
Production of the units v1ill be handled by the Engineering
Applications Directorate on the base.
58
59
5.2
H0\'1
Nany Units Should Be Built ?
The future demand for the new terminal units is not precisely
known. Therefore it is not clear how many terminal units should be
built in the original production run.
A probabilistic decision tree can be used to help make the
decision. The tree \'Jill be evaluated on the basis of minimum expected
additional cost.
The
choi~e
of how many units to produce has been narrowed to four
likely choices as shot-m in table XIII. In addition table XIII gives the
probability that each choice will turn out to be the number that is
actually needed. These probabilities are only guesses based on present
knowledge of future requirements.
Table XIII
Production Alternatives
Number Produced
Probability
10
.1
20
.4
30
.3
50
.2
The values for the additional costs were derived by making the
following assumptions:
(1)
The cost of producing each terminal unit increases
by fifteen percent for each additional production
run,
(2)
The cost of each unit not used is not recoverable.
The decision tree is shown in figure 5.2-1. A circle represents a
decision node while a square is a chance node.
60
-4625
-3382
-14125
-4200
-10500
~-4200
-28000
-17500
0
-12560
Figure 5 • 2-1
. ·on Tree
Dec1S1
61
The decision tree indicates that twenty units should be built in
the first production run.
5.3 Spare Parts Inventory
In order to quickly restore· .an inoperative terminal unit to
operating status it is necessary to have extra modules so that
defective modules can be replaced on the site and then repaired later.
It would be desireable to have some idea how many spare modules should
be kept to insure an adequate supply.
The method that will be used to give some prediction of the number
of spares that will be needed will be a simulation model. The model
will be based _on the estimated reliability of .95 for the terminal
unit. In the preceeding section it was determined that twenty units
would be built. This \-Jould mean a total of at least one hundred and
twenty modules. The reliability of .95 yields an expected failure of
six modules·per month. Thus the probability that a module will fail on
any given day is .2, the probability that t"10 modules will fail on the
same day is .C4, the probability that three modules will fail on the
same day is .0008. The probability of more than three failures on a
single day is very small. The Honte Carlo technique will be used to
produce the number of times each of the three possibilities might
occur in a years time. Since it might take up to a week to repair a
defective module, the number of failures per week will also be noted.
A table of random digits was used to generate the data. A summary of
the results is presented in table XIV on the follo\.Jing page.
62
Table XIV
Nodel Results
. ................
Number of days only one module fails
Number of days tHO modules fail
••••fl••••eoee~~eee
~Jith
no failures
Number of weeks with one failure
9
4
19
•
•
26
................... ,.
17
•
Number of ,,,eeks with .three failures
Number of ,,·eeks with four failures
resu~ts
,.
43
......................
Number of l>.•eeks with two failures
The
.. • • • • • •
....................
Number of days three modules fail
Number of weeks
"
•
•
e
e e e e
•
•
o •
•
e
e e
e •
•
•
e e
•
•
•
e o •
e e
e
e
e
•
•
•
e
•
•
....................
5
4
indicate that the most failures likely to occur in a
week is four. Thus it is reccommended that five spare modules be kept
on hand to insure an adequate supply. Although this is only a rough
guess that may have to be modified after actual experience, it is a
good starting point.
. BIBLIOGRAPHY
Meyer, P., Introductory Probability and Statistical Applications,
Addison Wesley Company, 1972
Bazovsky, I., Reliability Theory and Practice, Prentice-Hall, Inc,
1961
Kreyszig, E., Introductory Mathmatical Statistics, John Wiley &
Sons, Inc., 1970 ·
Wagner, H., Principles of Operations Research, Prentice-Hall, Inc,
1975
Fisher, H~G., Cost Considerations in Systems Analysis, American
Elseiver Publishing Company, Inc., 1975
Quade, E.S., System Analysis and Policy Planning, American Elseive
Publishing Company, Inc., 1975
Dunlap and Associates, Inc., System Analysis Guide, N{\1C TP 5298,
January 1972
Hall, A.D., A Methodology for Systems Engineering, D. Van
Norstrand Co., Inc., 1962
Hillman and Halkias, Integrated Electronics, Me Graw Hill, Inc.,
1972
Western Electric Company, Inc., Consoles, Panels, and Associated
Equipment, PMR CE-395, June 1968
Western Electric Company, Inc., Telephone Sets, Hand Telephone
Sets, Subscriber Seys, Head Telephone Sets, and Handsets, Pi'1R
TE-345, June 1967
63
Apeendix
Circuit Diagrams
64
65
vee
10---
TRIG
~+--'=DpG~--.--J
f.
. i
Honitor
Select
Active
Select
~~~I
~---------1
·.·.· J
.__..__R
Fig 1
Active-Honitor Select Circuit
I .
66
VOL 1
RT2·---.--i
f-,
4/N\.
RT3 ---...----
~
!TI
----r--t) f-]
RT4 .......
RR4
~
L-.Af\A~
d. 1~/\W>
.·*VOL 4
Fig 2 Differential Amplifier Circuit
67
BUZZ
TT2
TR2
l
l
l
m
TT3
TR3
Fig 3 Signal Receive Circuit
68
vee
~
Rl ......-----+----1
Hl-·~4V
24V
,
R2
·--+---------1
24V
vee
~
24V
24V
r
IT7
24V
Rlr--+-------i
24V
Fig 4 Voice Operated Lamp and Honitor Indicator
~v
·~.
vee
l,
"'11/
,3·
T
tn
~I
I
)1
I
WI
i
Haster
FT7Volume
Speaker
~tr=(]
l rh .
~r
m
Fig 5 Summing and Power Amplifier Circuit
0\
\0
72
TRIG5
TRIG6
TRIG7
TRIGB
Honitor
+24VDC
+12VDC
GND
-12VDC
Fig 7 Input Conditioning Logic and Voltage Splitter
Circuit
71
Pl
...!_
· HUTE
~~~~·~--~--~"---Al
Pt
Handset
Receiver
Pj3
l-A2
1 - - - - j l _ . A3
P,4
:....--.....~i~A4
m
Handset
Mike
~-.......---if"01-""-l---4----...
PTT
S
D
Select·----~--~
Active Rx
Transmit
Honitor Rx
Fig 6 Handset and Binaural Headset Adaptor