ATMRPP-WG/WHL/x-IP/
FSMP-WG/4 IP/07
International Civil Aviation Organization
.././06
2017-03-29
INFORMATION PAPER
FREQUENCY SPECTRUM MANAGEMENT PANEL (FSMP)
Fourth Working Group meeting
Bangkok, Thailand, 29 March – 7 April 2017
Agenda Item 6: RF Handbook Volume II (Doc 9718, Vol. II), Frequency Assignment Planning
RADIO TESTING TO OPTIMIZE VHF COORDINATION
(Presented by Andrew Roy, ASRI)
SUMMARY
In attempting to optimize VHF assignments in the US, testing of VHF voice
radios is ongoing to identify the current performance of the mosdt common
VHF ground radios. This includes Adjacent Channel Rejection (ACR),
intermodulation attenuation, and Out of Band Emissions (OOBE) masks. This
paper provides an update of the current work, the current results, and future
possible testing.
1.
INTRODUCTION
1.1 The US VHF spectrum is becoming increasing congested for VHF Datalink (VDL) and airline voice
communications. With both the Aircraft Communications and Reporting System (ACARS) and VHF
Datalink Mode 2 (VDLM2) multi-frequency networks operating concurrently, it has become a challenge to
manage and assign voice users in congested areas such as the North East of the USA.
1.2 As the channel manager for the Aeronautical Enroute Service (AES)1, Aviation Spectrum Resources Inc
(ASRI) has been attempting to identify more efficient means of planning VHF assignments by testing a new
frequency assignment tool. This includes possible changes to the assignment process to fit voice ground
stations closer together with regards to co-site interference, and how much improvement would be seen from
an 8.33k voice conversion. However, plans to do this has been limited by lack of a standardization of radio
RF performance (outside of VDLM2). Radio manufacturers normally only specify the first adjacent channel
for both emissions and adjacent channel rejection in compliance with ETSI standards2, but emissions beyond
this are not normally revealed in any detail, if at all.
1
The AES spectrum is the USA regulatory designation for spectrum primarily used for airline company communications
for voice and data, but also include ATS, AOC and AAC communications.
2
ETSI EN 300 676-1
(11 pages)
Document1
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1.3 To address this, ASRI contracted a communications provider with suitable experience to analyse radio
performance. The goal was to analyse of voice radios that represent various types typically in use across
commercial and private aviation use, from legacy-type to the most modern Software Defined Radios (SDRs).
Through this initial testing and future work, it is intended to fully assess and understand, Adjacent Channel
Rejection (ACR) values, intermodulation attenuation values, and Out of Band Emissions (OOBE) masks. The
initial tests included both 25k and 8.33k modulation, and at different power levels (when supported by the
radios).
1.4 With this and future testing, ASRI intends to develop generic radio performance models that can be
used to more accurately represent GS radio performance. VDL radios were not tested during this initial phase,
as a realistic datalink test signal could not easily be sourced at this time. With these initial results, additional
testing will be conducted to narrow down the data into suitable models to allow for appropriate use in
assignment models.
2.
DISCUSSION
3.
Radio Testing Setup
3.1 To gain a representative picture of existing radio performance, the selection of the most commonly used
ground radios in the USA were tested. The units used were fully operational radios ready for deployment as
ground stations, without any known faults or issues at the time of testing. As the radios were not directly from
the manufacturer, the contractor attempted to use two of each type to ensure individual unit performance was
identified (though this was not always possible). The results provided will be anonymized to prevent
identification of specific radio types.
3.2 The following radios were tested. All radio models are 25 kHz capable, except the JOTRON and Park
Air model radios that are both 8.33 and 25 kHz capable. Powers listed are the different power outputs tested.
Manufacturer
ICOM
ICOM
ICOM
JOTRON
ParkAir
ParkAir
TiL
TiL
TiL
TiL
TiL
WULFSBERG
Model Type
A110
A120
A6
TR-7750
T6TR
T6-TRV SAPPHIRE
92-SC
TBS-100
TBS-200
TBS-300
TRM-420
WT100/WR100
Power Levels Tested (W)
10
10
1
5, 25
5, 25
5, 25
7, 15, 25
7, 15
7, 15, 25
15, 25
15
20
3.3 To ensure the test rig was appropriately setup to the appropriate test standards, all radio manufacturers
were contacted to verify the procedure. Of the several that responded, it was confirmed that the methodology
and testing setup was correct for stated tests. In the testing lab, sources of environmental or test noise were
removed (e.g. non-radiating loads, etc.), and the test rig was calibrated before testing to ensure all components
were operating as expected.
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3.4 Two interesting discrepancies were noted between the ETSI standards and published radio performance
specifications:
3.4.1
Some manufacturer receiver SINAD data was produced with modulation depth of 30%, while
ETSI specifies 60%.
3.4.2
Some manufacturer IM data is tested at 500 kHz or further away from carrier frequency, while
ETSI specifies 150-200 kHz from carrier.
4.
TEST #1 – RECEIVER PERFORMANCE
4.1 The receiver mask test measured the Adjacent Channel Rejection (ACR) rejection in several adjacent
channels to the desired center frequency. Adjacent Channel Rejection (ACR) is the ability of a radio to reject
a signal in an adjacent band while still receiving and decoding the desired signal it is tuned to. The receiver
mask testing was measured in accordance with the procedures in ETSI EN 300 676-1, Chapter 8.6, with a
centre frequency of 127.500 MHz. Testing was accomplished at both 8.33 kHz and 25 kHz dependant on
radio type, noting that most manufacturers state that their radios should meet a minimum of 60 dB ACR for
the first adjacent channel.
4.2 Measurements made were cross-verified a number of times and in different ways to ensure highest
possible accuracy was achieved. For example, the use of additional directional couplers inline allowed for
real-time verification of signal levels and additional isolation. Attenuators were placed in each signal path to
ensure there was no interaction between signal generators. The positions and function of each of the
generators were swapped to verify the same results could be attained. Different signal generators and
spectrum analysers were brought into mix and yielded similar results within +/- 1 dB. Proper cable lengths of
very high-quality silver/Teflon cables were employed, and two different types of signal combiners were used
to further validate collected data.
4.3 The suite of equipment used for this test consisted of Aeroflex 3550, Aeroflex 3920, Anritsu S412E,
IFR FM/AM 1500 analysers, Mini-Circuits ZFRSC-123-S+, Mini-Circuits ZFSC-2-1 15542 signal combiners,
Weinschel 40-20-43, Weinschel 40-6-34. Arra N4425, Bird 15A-MPN-03, 10A-MPN-03 attenuators,
CommScope C-20-N, Narda 3000-10, BIB 1000030788 directional couplers, Bird 43-N wattmeters, and
multiple custom cable assemblies.
4.4 To test, the on-channel reference generator was modulated with a 1 kHz tone at 60% modulation depth,
and set to the appropriate level to achieve a 12 dB SINAD reading at the audio analyser input with appropriate
filtering. The level to achieve these readings overall closely correlated with the manufacturer's published data,
at or near approximately -108 dBm, with some radios requiring slightly higher or lower input levels to achieve
the 12 dB reading.
4.5 The offset signal generator was initially tuned the same way as above. With the reference generator
disabled, the offset generator is first set on-channel to verify the signal path through the combiner, any path
loss differences are noted, then was later set to the required offset channels to produce the interference signal,
using a 400 Hz tone at 60% modulation depth.
4.6 To determine the receiver's signal rejection capability from the adjacent channels, the upper adjacent
channel was measured, then the lower adjacent channel. Each time, the Offset generator output level was set
to the appropriate value to achieve a 6 dB signal reduction at the audio analyser. This level was recorded for
each channel, and subtracted from the initial 12 dB SINAD signal value obtained. The difference was the
actual dB value of the radio's receiver rejection capability. Radios (where controls are available) were set to
RF ACG OFF, Audio AGC OFF, open squelch (squelch defeat).
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5.
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TEST #2 - INTERMODULATION ATTENUATION
5.1 The intermodulation attenuation test calculates the capability of a transmitter to avoid the generation of
signals in the non-linear elements caused by the presence of the carrier and an interfering signal entering the
transmitter via the antenna. Intermodulation attenuation is typically specified as the ratio in dBc, of the
power level of the third order (highest level) intermodulation product to the carrier power level (a minimum of
40 dB is expected for all radios). The intermodulation attenuation test was measured in accordance with the
procedures in ETSI EN 300 676-1, Chapter 7.8, using a 127.5 MHz centre frequency with a +/- 175 kHz offset
channel.
5.2 The transmitter being tested was connected to the directional coupler via a fixed or variable attenuator
and was run at full desired power, or in this case 25 Watts RF, consistent with normal VHF radio licensing, or
less where the radio supported lower-power. The other side of the directional coupler was connected to the
interference source and a variable attenuator.
5.3 The interfering source may be a suitable transmitter of similar power level, or preferably a linear
amplifier capable of producing the same power level as the DUT. To minimize all possible spurious products,
an Empower RF Systems BBM2C4AEL solid state Class AB linear amplifier was used with approx. 50 dB of
signal gain, and was capable of 25W output power from 10-1000 MHz. This low distortion MOSFET
amplifier employs broadband matching networks and EMI/RFI filtering for best performance. Both the DUT
and the interference source were placed physically as far apart as possible to minimize any possible effects
from RFI.
5.4 First, the DUT carrier was turned on unmodulated, and the power level is verified. Next, the interfering
source was also turned on, and the signal level was adjusted to match the carrier power level of the DUT. The
IM components were observed on the analyser display, and the third order components were presented below
in the associated plots. The IM Attenuation value recorded was the difference between the carrier power and
the largest 3 order IM product measured. There were two plots per DUT, one for each offset channel above
and below the carrier.
6.
TEST #3 – TRANSMITTER OUT OF BAND EMISSIONS
6.1 The OOBE testing measured each Adjacent Channel Power (ACP) out to a set distance from the center
frequency, either in 25 kHz or 8.33 kHz channels. The ACP is the total of the mean power produced by the
modulation, hum and noise within the transmitter. The OOBE testing measured in accordance with the
procedures in ETSI EN 300 676-1, Chapter 7.5, with a centre frequency of 127.500 MHz. It was decided to
measure all channels out to +/- 200 kHz from the center frequency. Although in the spurious domain, this
frequency range is the approximate distance that co-site is planned for in the USA VDLM2 channel plan, and
was considered a good starting profile to analyse.
6.2 For both 8.33 kHz and 25 kHz channels, the transmitter was modulated with a 1 kHz tone at 85%
modulation depth and adjacent channel power ratio is recorded. All transmitters were expected to be at least
60 dB below the centre frequency power in accordance with the ETSI standard for the first adjacent channel.
6.3 The testbed consisted of a calibrated Anritsu S412E, good quality double-shielded silver/Teflon ½
wavelength coax lengths, and a high-performance directional coupler terminated in a 250W non-inductive 50ohm load. The coupling ratio was 50 dB, so it was possible to run very high power radios through this
coupler, and monitor directly on the spectrum analyser. The required calculations for power ratio were
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accomplished within the S412E ACP ratio test routine, using these settings to best present the measured data
in a consistent format
6.4 The main channel BW and adjacent channel BW were chosen at 8.33 kHz and 25 kHz for best possible
measurement accuracy. Prior to proceeding, these results were compared with data taken using the narrower
channel BW settings of 4.83 kHz and 17 kHz, which would typically represent the actual bandwidth used of a
properly modulated signal on-channel. There was no discernible difference in measurement results found.
Therefore, the full-channel BW settings were used throughout the testing process to ensure all energy in the
allocated channel would be measured.
7.
RESULTS
7.1 Given the limited sample size for some radios (and some unusual results in this first test), the results
presented below in the relevant annexes have been anonymized as best as possible. Due to this and the
different test setups, please note that radio numbering for each set of results is different (i.e. radio 1 in the test
#1 is not radio 1 in test #2).
8.
RESULTS – RECEIVER PERFORMANCE
8.1 Annex A list of all results of the adjacent channel rejection values (with 8.33k capable radios including
the additional tuning steps). Some overall observations from the results:
8.1.1
No 25k radio fully meets the 60 dB minimum isolation required on the first adjacent channel.
8.1.2
The 8.33k radios show almost no isolation on the first adjacent channel, with the second
adjacent channel seeing approx. 50 dB.
8.1.3
The 8.33k radios had almost the same performance as the 25k radios at a 25 kHz frequency
offset.
8.2 The 25k radio results appear near the expected 60 dB standards performance for the adjacent channel
rejection, however several newer radios are not meeting their stated higher level of performance (70dB). The
almost minimal ACR for the first adjacent channel on the 8.33k radios also suggests these radios are simply
encoding at the 8.33k modulation for transmission, but the receiver front end is minimally changed from the
25k receiver function.
8.3 Although not formally tested this time, a few check measurements were completed for 200+ kHz offsets
for several 25k radios, and the ACR values appeared to continue increasing to nearly 70 dB of isolation.
Several improvements have already been planned to further develop the results, including a greater frequency
range for testing (up to 1 MHz away), interactions between 8.33k and 25k radio emissions, and recording of
sample audio to assess the real-world effects of the ETSI interference levels compared to human perception.
These tests will also recheck the original test results to confirm the values provided.
9.
RESULTS – INTERMODULATION ATTENUATION
9.1 Annex B list of all results of the IM attenuation testing. Some overall observations from the results
with an example provided at Fig 1):
9.1.1
Results values varied from 37.5 to 66 dB of IM attenuation, with most radios performing
around the 55-60 dB point.
9.1.2
Different units of the same radio type had variance up to 6 dB.
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Figure 1 - Example intermod attenuation
results (+175 kHz offset)
9.2 The testing results met and mostly exceeded the minimum performance expected (40 dB) of the
available radios, with multiple radios exceeding the minimum specification (though not always to the
manufacturers stated amount).
10.
RESULTS – OUT OF BAND EMISSIONS
10.1 The OOBE testing measured each Adjacent Channel Power (ACP) out to a set distance from the center
frequency, either in 25 kHz or 8.33 kHz channels. The ACP is the total of the mean power produced by the
modulation, hum and noise within the transmitter. The OOBE testing measured in accordance with the
procedures in ETSI EN 300 676-1, Chapter 7.5, with a centre frequency of 127.500 MHz and a 1kHz tone
modulated onto the carier. It was decided to measure all channels out to +/- 200 kHz from the center
frequency. Although in the spurious domain, this frequency range is the approximate distance that co-site is
planned for in the USA VDLM2 channel plan, and was considered a good starting profile to analyse.
10.2 Unfortunately, the transmitter results produced were not as expected, with an unknown anomaly
suspected within the test rig that created an intermod of the signal at 1 kHz intervals centred on test frequency
(see Fig. 2).
Figure 2 - Unexpected intermod in tranmission test (new
radio model left, older radio right)
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FSMP-WG/4 IP/07
10.3 The signal roll of was also very limited, reduced by only 10 dB from the first adjacent channel out to
the 200 kHz offset point (see Fig. 3). This was seen with both the modulated signal (with its intermod
mentioned above), and with the carrier signal by itself.
Figure 3 - Expanded 500 kHz view radio tranmission
(same radio as Fig. 2 new radio model)
10.4 Both of these combined gave an emission profile that was within approx. 1 dB for all radios at each
offset channel. Therefore, a follow up test is being planned to go deeper into the results of a subset of radios
to confirm performance and any testing issues that may exist. A couple of observations from the existing test
that may be still relevant now until more information is available:
10.4.1 One radio type had a significant deviation away from the rest of the results at the 100 kHz
point in both 25k and 8.33k operation (see Fig 3). Surprisingly, this radio type was expected to see an
improved response given additional filtering, and a second radio of the same type was unavailable to
test to verify the performance. It was subsequently revealed by the manufacturer after testing that the
radio model was having known problems across the product line and was being issued several firmware
fixes. 3
3
Note, this radio also saw receiver performance issues in the ACR testing. See Radio 3 results in Annex A.
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10.4.2 In the 8.33k radios, signal roll off seems to plateau around -70 dBc, and even regress until the
6th channel (50 kHz), and then begins rolling off again (Fig. 4)
10.5 The OOBE transmitter test results are not considered conclusive at this current time until a root cause
can be found or eliminated for the unusual results. Additional testing is expected before the next FSMP to
clarify the transmission performance of a sub-set of the tested radios (such as older vs. newer models), and
provide a more in depth assessment with more complex modulation signals.
11.
CONCLUSION
11.1 The ACR and intermodulation attenuation results are near expected and should hopefully provide useful
information to organizations employing VHF voice ground stations. Given the limited frequency range of the
results, an expanded ACR test has been commissioned to assess values out to 1 MHz offset from the tuned
frequency (this will also reverify the current results, and how both 25k and 8.33k radios co-exist).
11.2 A more detailed transmissions test for a sub set of the radios tested is being considered, with potential
options for VDL tests if suitable test equipment and data becomes available. Other organizations are
encouraged to conduct their own testing to ensure a greater understanding of radio performance in the VHF
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environment. With suitable data, frequency assignments models can be made more accurate, and therefore
more efficient.
11.3 Actions for the meeting
11.4 Note the results included for ACR and intermod attenuation.
11.5 ASRI will conduct additional testing (more in depth emissions, and an expanded range for
ACR). Results will be presented at the next FSMP.
— END —
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ANNEX A – Table of Adjacent Channel Rejection Results
All values in dB
15
16
17
18
-0.0250
51.8
53.2
51.5
53.1
56.1
53.2
52.2
53.3
51.8
53.8
53.7
53.1
52.6
54.6
52.8
52.2
53.3
55.8
Mean
SD
53.2
1.2
Radio
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Tuning step offset from center freq (kHz)
-0.0167 -0.0084 0.0083 0.0166
48.1
1.1
1.1
48.6
48.3
1.3
1.1
47.2
5.6
0.3
0.3
5.3
48.2
0.4
0.1
49.7
37.6
18.4
0.8
0.4
0.6
0.5
37.7
18.7
0.0250
52.1
54
51.2
53.7
55.1
53.1
52.1
53.1
51.8
53.8
54.3
53.1
53.6
53.8
52.8
52
53.7
56
53.3
1.2
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ANNEX B – Intermod Attenuation Results Table
Radio
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Ratio dBc
58.05
54.36
37.49
55.68
55.98
55.86
56.02
57.46
60.59
41.48
53.2
42.9
60.06
66.08
63.24
59.99
56.41
46.4