The Measurement of Delay Distortion in Microwave Repeaters* 



By D. H. RING 



Measuring equipment is described which is capable of measuring delay distor- 

 tion of the order of 10" ^ seconds in a wide band microwave television relay 

 repeater. Two measuring circuits are discussed . The first is a circuit for meas- 

 uring the relative phase shift versus frequency from which the delay distortion 

 may be computed. The second circuit gives the delay directly from a single 

 measuremen t. The measuring equipment is designed to work in the intermediate 

 frequency range from 50 to 80 megacycles, but by applying suitable conversion 

 equipment measurements can be made at microwave frequencies. 



THE successful transmission of broadband television and pulse signals 

 over any communication circuit depends upon the preservation of the 

 complex wave shapes of the original transmitted signals. Fourier analysis 

 tells us that a complex signal wave can be resolved into a spectrum of fre- 

 quencies with certain amplitude and phase relationships. It is well known 

 that the amplitude relationships of all essential frequencies in this spectrum 

 must be substantially preserved. It is equally important that the phase 

 relationships of the essential frequencies should be preserved. The in- 

 stantaneous value of the received signal is the vector sum of the instanta- 

 neous amplitudes of all the component frequencies. Therefore, if the 

 relative phase of some frequency component is changed by 180° the sign of 

 its contribution to the output is reversed, and it is clear that a closer ap- 

 proximation to the original signal could be obtained by suppressing this 

 frequency component rather than permitting it to contribute negatively 

 to the output. 



It can be shown^ that the relative phase relations of the component 

 frequencies in a complex signal wave will be preserved if the phase shift in 

 passing through a circuit is a linear function of the angular frequency. 

 That is 



iS = Toco + WTT (1) 



where To is a constant and n an integer. Distortion of the transmitted 

 signal will occur if Tq is not constant over the essential frequency band of 

 the signal. We shall not be interested in distortion due to n not being an 

 integer^ since this case does not occur in carrier circuits where the phase 

 shift at carrier frequency, rather than the phase shift at zero frequency, is 



* Presented at National Convention of I. R. E., New York City, March 4, 1947. 



1 Phase Distortion and Phase Distortion Correction, S. P. Mead, B.S.TJ., April 1928. 

 199-201. 



2 Phase Distortion in Telephone Apparatus, C. E. Lane, B.S.TJ., July 1930, 494-496. 



247 



