Experimental Transversal Equalizer for 

 TD-2 Radio Relay System 



By B. C. BELLOWS and R. S. GRAHAM 



(Manuscript received February 26, 1957) 



To determine the effect of improved equalization on the performance of 

 TD-2 radio relay systems, an experimental adjustable transversal equalizer 

 has been developed. The equalizer is based on the echo principle as used in 

 transversal filters, and operates in the 60- to 80-mc frequency band. Seven 

 pairs of adjustable leading and lagging echo terms provide flexibility for 

 simultaneous gain and delay equalization. Directional couplers are used 

 for tapping and controlling the echo voltages. Field experiments have shown 

 that systejn equalization can be improved appreciably by the use of such 

 equalizers. 



INTRODUCTION 



The TD-2 radio relay system' employs frequency modulation to 

 transmit multichannel telephony or television. The frequency modulated 

 signal requires a transmission system whose gain and envelope delay 

 are constant over the frecjuency band, 20 mc wide, used to transmit 

 the signal. Deviations from constant gain or delay result in non-linear 

 distortion of the demodulated signal. For television this results in dis- 

 torted images, and for multichannel telephone transmission this intro- 

 duces cross modulation among the voice channels. 



Basic equalization is provided in each repeater. In addition, certain 

 fixed equalizers ha\'e been used on a mop-up basis. However, there 

 remains some residual distortion of random shape. This paper discusses 

 the design of an adjustable equalizer to compensate for this distortion. 



I. BASIC EQUALIZATION 



The transmission path through a TD-2 repeater consists of an inter- 

 mediate frequency portion covering the 60- to 80-mc band and radio 

 frequency channels 2C-mc wide in the 3,700- to 4,200-mc band. At each 



1429 



