GdB + 10log
10
(I
0
) = GdB + 10log
10
(P
IN
)
From which we see that the input power at the point of intersection is, indeed, I
o
.
I
o
is the received signal level that produces 0 dB SNR, that is, signal power equal to noise
power. Do not confuse this with the minimum detectable power P
MDS
which typically is
several dB lower, depending on processor settings. In the above example, a 1.2 dB LOG
detection threshold is shown (horizontal blue line) for the received signal. If RVP900 applies
sucient range and time averaging so that thermal noise alone produces very few false
alarms above 1.2 dB, then P
MDS
are a 5dB lower than I
o
. We would expect a detection rate of
roughly 50% for echoes arriving at this "minimum detectable" level.
Typically a CW test signal is used to generate the test curve shown in the
figure. Follow the
instructions provided by the radar manufacturer for injecting a test signal. During
calibration, the radar should be fully operational, so that all sources of noise are present.
Ideally the transmitter should be turned on during calibration.
Verify with the radar manufacturer that no damage can occur to the signal
generator if the transmitter is running during the calibration.
CAUTION!
1. Raise the antenna up a few degrees to avoid ground thermal noise
2. Insert signals at steps of 5 or 10 dB over the entire range of the system
3. Draw the plot shown in the previous
figure.
You can use fine resolution steps at the ends of the scale to observer the details of the
roll
o.
For IRIS users, you can do this in the Zauto utility.
4. Tune the frequency of the signal generator using the setup command pr, and
displaying the received signal spectrum.
Check the tuning at the end of the calibration to make sure the signal generator and
IFDR have not drifted apart.
Each time that a new signal level is injected, the measured power values are obtained
by
first invoking the SNOISE command and then reading- back the results using the
GPARM command. Use the Log of Measured Noise Level (Word 6) from
GPARM.
This procedure averages many samples together.
5. Turn it all the way down and make one more sample to measure the noise level N.
You can obtain I
o
from the intercept of the horizontal line at N and the straight line
fit
to the linear portion of the curve.
RVP900 User Guide M211322EN-J
216
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