This tutorial section on radar basics covers following sub topics:
Main Page Terminology Measurements Doppler Radar FMCW RADAR BANDs RADAR SCOPE Weather Radar Ground radar RANGE and RESOLUTION Range calculator
This page describes radar measurements which is very useful during radar maintanance and calibration. We will cover both the transmitter part as well as receiver part measurements. The common tools used for radar maintenance and measurements are multimeter, spectrum analyzer, oscilloscope, power meter, frequency counter, signal generator, crystal detector, attenuator etc.
The radar transmitter measurements include transmitted Power and Stability, PW, PRF, Duty Cycle/ Factor, Klystron Pulse, Klystron Current, Klystron RF Input Level, Transmitted frequency, Occupied BW and VSWR. Let us examine these measurement parameters one by one.
The transmitted power is measured with the help of calibrated power meter. The value is calculated taking into consideration loss of cable as well as duty cycle factor.
Peak Power (dBm) = Average power (dBm) - Duty Factor(dB) + Loss(dB)
Where, Duty Factor = 10 log(Pulse Width(sec) * PRF(Hz))
Loss will be path loss between output flange of klystron and rf transmitter monitor connector.
With Pulse Width of 1 µsec and PRF of 250Hz.
Acceptable level for power stability is about +/-0.5 dB.
For example, if power output varies from -14.95dBm to -15.05dBm in a half an hour period. Difference in variation of power here is 14.95 - (-15.05) = 0.1 dB. This is less than acceptable level.
PW,PRF,Duty cycle/duty factor
This measurement is carried out using oscilloscope. If oscilloscope is not capable of measuring high frequency than arrangement as shown in the figure is done. step attenuator and crystal detector is used for this purpose.
To measure PW we need 3dB attenuation, as it is the width at half power points. similarly PRF and duty factor is measured using oscilloscope.
Klystron Pulse, Klystron current
Klystron pulse waveform is measured at the output of transmitter front panel using oscilloscope. Klystron current is calculated by multiplying oscilloscope value with 10. For example, oscilloscope reading for max. peak is 1.2V than current is 12Amp.
Klystron RF input level
In this test, rf drive power level and rf frequency is measured to check whether they are within normal range or not. Frequency should be about 5625MHz. The same is measured using either spectrum analyzer or frequency counter.
Occupied BW is the width of band over which emitted mean power is about 99% of the total mean power. Difference between lower and upper frequency points is referred as occupied BW.
It is the measure of how much power is transmitted and how much is reflected.
Return loss in general= Forward power - Reflected power,
Refer RL vs VSWR for more.
Radar receiver measurements
Radar receiver is very critical part in radar system as it has to detect and amplify the received weak signal from antenna. The radar receiver measurements include STALO level measurement, COHO level measurement, receiver Gain, Minimum Detectable Signal(MDS), Dynamic range, intensity check, velocity check etc.
STALO Level Measurement
Radar usually will have different STALO frequencies and power levels. The main aim is to determine weak signal receiver power. The digital power meter is used for this purpose.
COHO Level Measurement
COHO stands for COHerent Oscillator. It has frequency of about 30MHz.
Radar transmitter is kept off and signal generator frequency is set equal to
radar receiver for this test.
Gain(dB) = output power (dB) - input power(dB).
Similarly dynamic range is calculated by finding MDS and saturated input points. Refer MDS vs SFDR for more.
Tx IF Out and Exciter RF
Transmitter IF OUT is pulse modulated COHO output which is up converted to RF level. Exciter RF signal is passed to the klystron. The spectrum analyzer is used for this purpose.
Intensity Check measurement
It is done to see how well the system performs the processing of the reflected signal after coming into LNA through Signal Processor.
Velocity accuracy check measurement is done to see how well the system calculates the velocity of an echo.