MICROWAVE RADAR TESTING 447 



A major problem in the design of microwave signal generators is the 

 provision of shielding adequate to reduce leakage signals due to unwanted 

 couplings or stray fields well below the minimum signal required for receiver 

 testing. This minimum level may be as low as —70 or —80 dbm, depending 

 on the coupling loss in the test connection. 



A Pulsed and FM Signal Generator 



To illustrate the functioning of a signal generator, there is shown in Fig. 5 

 a block schematic of a design (Army-Navy type TS-35A/AP) which covers 

 a 12% frequency band in the vicinity of 9000 mc. An RF connection to the 

 radar is established with a wave guide flange coupling. The frequency and 

 power of the radar transmitter are measured by means of a coaxial-type 

 frequency meter and thermistor power measuring circuit as described in 

 subsequent sections. The attenuator and pad are adjusted to reduce the 

 incoming average power to about 1 milliwatt, which gives a suitable deflec- 

 tion on the indicating meter. The thermistor is mounted across the wave 

 guide. 



An RF pulse train is employed in many tests. To produce this the RF 

 oscillator output is modulated by a multi-vibrator which pulses continuously 

 except when being synchronized. Synchronizing pulses are derived by 

 crystal rectification of the RF pulses from the radar transmitter. The result 

 is an initial RF pulse of 7 microseconds followed by an off period of about 

 10 microseconds followed by a train of RF pulses each 2 microseconds wide 

 and recurring every 8 microseconds until resynchronization occurs at the 

 next radar pulse. 



Using the pulse train, the radar system components can be tuned for 

 maximum sensitivity by maximizing the signal on the indicator. To check 

 receiver sensitivity the CW power is first adjusted so that a power of 1 

 milliwatt is delivered to the pad and attenuator. Then with the set in the 

 pulsed condition the amplitude of the test signal is adjusted by means of 

 the attenuator and pad until the signal on the radar indicator is barely 

 discernible. It is necessary for this test that the frequency of the test 

 signal be equal to the magnetron frequency. The frequency meter is 

 provided as part of the signal generator for this purpose. 



The receiver recovery, i.e. the time required by the receiver to recover 

 after disablement by the transmitter pulse, determines the minimum range 

 at which a radar can be used. With this test set the receiver recovery 

 characteristic is indicated by the amplitude of the test pulses in the interval 

 immediately following the transmitter pulse. 



The set is also adapted to serve as an FM signal generator. A sawtooth 

 wave applied to the repeller gives a succession of frequency sweeps, each 



