TD-2 MICROWAVE RADIO RELAY SYSTEM 1067 



C. FM Terminal Test Console 



The terminal test console shown in Fig. 19 is used to measure FM de- 

 viation, Hnearity of the FM transmitter and receiver and for routine moni- 

 toring of wave forms at video frequencies. The equipment includes a con- 

 ventional CW signal generator covering the range of 50 to 90 megacycles, 

 a video '^A" scope, an electronic switch and patching and terminating facili- 

 ties. A rather unique linearity test set described below and an FM terminal 

 receiver are also included. 



1. Deviation Measurement 



For deviation measurements, the IF signal being monitored is patched 

 into one input of the IF electronic switch which switches between inputs at 

 a 1200-cycle rate, and the CW signal generator into the other input. After 

 detection by the FM receiver, the signals are applied to the oscilloscope and a 

 straight line corresponding to the CW generator frequency is displayed 

 superimposed on the video signal. By adjustment of the CW reference 

 frequency, the instantaneous frequency of any signal component may be 

 determined. 



2. FM Receiver Linearity 



For a measurement of linearity of the receiver discriminator, the linearity 

 test set is connected to an FM transmitter which is patched to the receiver 

 under test. The linearity test set supplies a low level 100 kc modulating 

 voltage to the deviation oscillator of the transmitter and a high level 60- 

 cycle voltage to the transmitter beat oscillator. For this test the transmitter 

 AFC circuit is disabled. Under these conditions the signal applied to the 

 receiver discriminator swings over approximately 10 megacycles at a 60- 

 cycle rate and over a small range of less than one megacycle at a 100 kc rate. 

 The 100 kc video component in the receiver output is then proportional to 

 the slope of the discriminator response curve. The envelope of this 100 kc 

 amplitude is recovered in the linearity test set and the a-c. component is 

 applied to the oscilloscope vertical amplifier. The horizontal deflection is 

 synchronized with the 60-cycle deviation. A 30-cycle switch changes the 

 amplitude of the 100 kc signal by a calibrated amount to provide two sepa- 

 rated traces on the screen and make the device self-calibrating. 



3. FM Transmitter Linearity 



For a measurement of transmitter linearity, the same setup used in the 

 receiver test is made use of except that both the 100 kc small signal and 60- 

 cycle large signal are applied to the deviation oscillator of the transmitter 

 under test. The beat oscillator AFC circuit is allowed to operate with a 

 time constant sufficiently rapid to follow the 60-cycle fluctuation of the 

 deviation oscillator, but not the 100 kc component. Thus the 100 kc modula- 

 tion component is applied over a 10-megacycle range of the deviation oscil- 



