1430 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



repeater the RF channels are separated by wave guide filters and each 

 is demodulated to the IF frequency for amplification and ecjualization. 

 The outgoing signal is then modulated back to an RF channel, at a 

 different frecjuency from the incoming signal to reduce interference. 

 The RF and IF portions of the repeater were designed so that the com- 

 bination would have flat gain to within the closest practicable limits 

 over the 20-mc band, and a number of adjustments are provided in the 

 IF amplifier to maintain this flatness under field conditions. The unequa- 

 lized repeater, however, has an envelope delay distortion characteristic 

 shown in Fig. 1 which is approximately parabolic. To minimize this 

 distortion, each repeater contains a 315A equalizer, which has approxi- 

 mately the inverse of the delay distortion of a typical repeater. 



II. SUPPLEMENTARY EQUALIZATION 



In a TD-2 system consisting of many repeaters in tandem, both gain 

 and delay distortions may accumulate to the point where additional 

 equalization is necessary. If the pass band of the repeaters shifts slightly 

 in frequency, due to changes in temperature or adjustment, the differ- 

 ence between the repeater delay and the delay of its equalizer will result 

 in delay distortion which has approximately a linear slope with fre- 

 quency. This may be corrected at main repeater stations by combina- 



60 62 64 66 68 70 72 74 76 78 

 FREQUENCY IN MEGACYCLES PER SECOND 



80 



Fig. 1 — Over-all delaj^ distortion of a typical microwave repeater, TD-2 

 system. 



