Lab L12: Doppler and CWFM Ranging Radar
In the lab, you will put characterize a Continuous-Wave Frequency- Modulation (CW-FM) radar
at X-band, and put together a Doppler radar using WR-90 waveguide components. This simple
Doppler radar is very similar in operation to the ones used by the police to measure vehicles
speeding. We will use robust waveguide components, and measure the velocity of a moving
target in the lab. CW-FM radar utilizes FM ranging and Doppler shifts to determine the distance
to and speed of a detected object. FM ranging uses a frequency difference between transmitted
and received waves to determine a round trip time, and hence distance, to a detected object.
Part I: CW-FM Ranging Radar
Q1: Explain the functionality of all waveguide components used in the setup showed in Fig.
L12.1a. What is the cutoff frequency of the WR-90 waveguide? What is the gain and far field
distance of the horn antennas at f = 10 GHz?
Q2: Will the frequency range and the period of the VCO sweep waveform affect the output
waveform of the diode detector for a given distance? How?
Figure L12.1. (a) Setup for CW-FM radar experiment. The RF signal is generated with a
HP8350B (or equivalent) sweeper in internal controlled sweep mode (Sweep Trigger - INT,
Sweep - INT). Use a sweep period of 0.020 seconds, a start frequency of 8 GHz and a stop
frequency of 9 GHz. Waveguide components include: two horn antennas, 10-dB directional
coupler, ferrite isolator, 3-port Tee, adapters and a diode detector (mixer). Use an oscilloscope
for viewing the output of the mixer, trigger from the function generator. (b) CW-FM radar
signals displaying simultaneous FM ranging and Doppler shift measurements?
Q3: Using a start frequency 8 GHz, a stop frequency of 9 GHz and a sweep period of 0.020
seconds on the VCO, what is the time rate of change of the transmitted frequency (df=dt)? What
will the expected frequency difference between transmitted and received waves be for an object
detected 50cm meters away (1 meter round trip)?
CW-FM Radar Calibration
Assemble the CW-FM radar setup shown in Fig. L12.1a. Sweep the transmitter signal frequency
over the range from 8 to 9 GHz at a period of 0.020 seconds and a power of 20 dBm; monitor the
diode output. It is best to store the output waveform on the oscilloscope, and then take a
measurement over several periods to average the frequency.
Q4: Face the horn antennas toward each other slightly outside the far-field distance. Draw a
period of the diode output signal. Explain the origin of the frequency components of this signal?
Which frequency component represents the distance to an object? How do you know?
Q5: What frequency difference do you expect at the diode output if the transmit and receive horn
antennas are pointed toward each other and touching (i.e. PT = PR)? The difference in frequency
between the received and transmitted signals with zero distance between the two horn antennas is
a calibration constant. This value corresponds to the difference between the received and
transmitted signals' path lengths within the waveguide components and 3.5-mm cables.
Q6: Measure the calibration constant discussed above. What value do you get? Which signal
travels further within the waveguide and by how much?
CW-FM Radar: Stationary Target Range Determination
Next, hold a flat copper plate 40-cm from the radar. Point the transmitter and receiver horn
beams at the plane; making sure each is the same distance from the target. In this part of the lab,
only the position (distance) of the detected object will be measured.
Q7: Measure the frequency di®erence between the transmitted and received signals. Calculate
the distance that the value you measured for the frequency di®erence corresponds to? Remember
to use the measured value of the calibration constant.
Q8: Repeat this measurement at 5 cm increments to a distance of 1 m; plot frequency vs.
distance in your lab report. What do you expect the plot to look like? Explain any discrepancies.
CW-FM Radar: Moving Target Measurements
In the last part of the lab both the distance and speed of a detected object will be measured using
the CW-FM radar setup, as illustrated in Fig. L12.2. Using analog to digital converters and
digital signal processing both of these measurements can be processed simultaneously. However,
using an oscilloscope to view the output visually does not allow this type of functionality due to
the Doppler shift depending on the instantaneous frequency. Therefore, you will measure speed
first, then distance to the moving object.
Q9: Configure the sweeper to output a constant radio frequency between 8 and 9 GHz. Measure
the Doppler shift of the copper plane as you move it toward the radar. Does the calibration
constant you found earlier apply to this measurement? Why or why not? How fast is the plane
moving?
Part I. Doppler Radar
Q10: The block diagram of the setup is shown in Fig. L12.2a. Assemble a Doppler radar, and
tune the tuner for maximum signal response by moving a piece of metal with your hand in front
of the antenna. How much difference in the signal amplitude can you get for different tuner
positions? What is the tuner function and how does it change the level of the received signal?
Figure L12.2. (a) Setup for Doppler radar experiment. Use an X-band sweeper, waveguide
isolator, waveguide 3-port Tee, waveguide shorted stub tuner, waveguide mounted diode
detector (mixer), horn antenna and oscilloscope for detecting the Doppler frequency shift. (b)
Inclined plane and cylindrical target. What is the velocity of the target at the foot of the “hill”?
Q11: Next, use a target made of a heavy cylinder (coffee can filled with rocks, e.g.) that you let
roll down an inclined plane, as shown in Fig.12.2b. Measure the Doppler frequency. Calculate
the velocity based on the Doppler measurement. Compare it to a calculation based on the
inclined plane for several starting positions of the target.
Q12: Measure the velocity of a different target that the instructor or TA will give you with a 8GHz and 12-GHz (at edges of X-band) Doppler radar. What can you conclude?