MICROWAVE RADAR TESTING 449 



about 20 mc wide, and lasting about 6 microseconds. With this frequency 

 modulated signal the width of receiver response may be observed on a 

 Class A oscilloscope (i.e. one showing signal amplitude vs. time). However, 

 with non-adjustable IF strips such measurement is seldom required. Failure 

 of the radar AFC to follow frequency changes due to antenna scanning or 

 other causes is indicated by a change in the indicator presentation. Pulling 

 of the magnetron frequency due to changes in load impedance can be de- 

 tected by turning off the AFC. 



Signal Generator Designs 



Designs of signal generators developed for the military arms during the 

 war are interesting as landmarks of progress. The signal generator of the 

 IE30 test set, deliveries of which began in May 1942, delivered pulsed RF 

 signals in the 10 cm range, using sine wave synchronization. Following 

 only three months later was the signal generator of the Army IE57A and 

 Navy LZ test sets (Fig. 6), which covered a then very broad frequency band 

 of 20% in the vicinity of 10 cm, and was designed to be triggered by the 

 incoming RF pulse from the radar instead of by a separate synchronizing 

 connection. This set and a redesigned version of it have seen wide usage 

 in testing Army, Navy and Marine Corps radars. 



Delivery of a test set for the 3 cm range, designated TS-35/AP, started 

 in the fall of 1943. This set furnished both a train of pulses and a train of 

 FM signals, both of which features have proved valuable. It covered a 9% 

 frequency band with no tuning adjustment except for the oscillator. An 

 improved design known as TS-35A (see Fig. 6) covered a 12% band. 



Progress in reducing the size and weight of the test units is indicated by 

 the fact that the IE30 signal generator weighed 121 lbs., IE57 74 lbs., 

 whereas TS-35 and TS-35A weigh approximately 30 lbs. 



Frequency Measurement 



Usually a radar need not operate at a precise frequency. Accurate 

 measurements are required in the field, howeyer, to keep the operating 

 frequency within limits, to set the local oscillator, to check the measuring 

 frequency, etc. In the laboratory, accurate frequency measurement is 

 fundamental. 



Frequency measurement in the microwave range is ordinarily accom- 

 plished by (1) a resonant coaxial line or (2) a resonant cavity, generally 

 cylindrical. These types are illustrated in Fig. 7. Sometimes a combina- 

 tion of the two, referred to as a hybrid or transition type resonator, is em- 

 ployed. The measurement is actually one of wavelength, with the scale 

 calibrated in frequency or a conversion chart provided. Some specific 

 designs of frequency meters are shown in Fig. 8. 



