MICROWAVE REPEATER RESEARCH 



231 



reason, these two transformers largely determine the band pass and delay 

 distortion of the whole amplifier. Typical overall characteristics for a 

 complete IF amplifier are shown in Fig. V-10. 



3.5 



u) 3.0 



2.0 



1.0 



0.5 



10-9 



60 



Q 



Z 

 O 50 



to 

 Z 40 



20 



10 



55 60 65 70 75 80 50 55 60 65 70 7! 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. V-10. — Band pass and delay characteristics of complete IF amplifier. 



Transmitting Converter, or Modulator* 



The transmitting converter problem diflFered from the first converter 

 problem in that high output power was the major consideration, rather than 

 low conversion loss and noise figure. 



There will normally be three frequencies present in the output of a con- 

 verter. These are the desired output sideband frequency, /', the beating 

 oscillator frequency, /' + IF or / — IF, and the unwanted sideband fre- 

 quency, /' + 2 IF or /' — 2 IF. The strongest of these is the beating os- 

 cillator frequency, but fortunately this can be suppressed by 20 to 30 db 

 by balance in a balanced converter. Investigation showed that the input 

 impedance at the IF terminals of the modulator was affected by the load 

 impedance of the modulator at both the wanted and the unwanted sideband 

 frequencies. The load impedance consists of the input impedance of the 

 RF amplifier as seen through the length of transmission line which connects 

 the two components. At the wanted sideband frequency, the RF amplifier 

 presents a good termination and the load impedance is independent of the 

 length of the connecting line. At the unwanted sideband frequency, how- 

 ever, the RF amplifier presents a short circuit and hence the load impedance 

 is a function of the length of the connecting line. A waveguide filter was 

 incorporated in the output circuit of the modulator so as to reflect the un- 



* This section prepared by W. W. Mumford who did the work on the transmitting 

 converters. 



