250 BELL SYSTEM TECHNICAL JOURNAL 



in phase upon demodulation. If the delay distortion is not symmetrical 

 about Wo the sidebands will not add in phase upon demodulation, and the 

 demodulated output will suffer both amplitude distortion and some delay 

 distortion which differs from the delay distortion at both wo + Aw and wo 

 — Aw. In the case of frequency modulation dissymmetry introduces 

 harmonics in the demodulated output. A detailed discussion of these 

 effects is beyond the scope of this paper, but we may note that in general 

 a true picture of the delay distortion in carrier circuits is not readily ob- 

 tained by observing the demodulated output and that an unsymmetrical 

 delay distortion is particularly undesirable in carrier circuits. 



Principles of Delay Measurement 



Delay cannot be measured directly on a steady state basis with a single 

 test signal in the simple manner in which amplitude response is measured. 

 Instead, the phase shift through the unknown network must be measured 

 at two adjacent frequencies and the delay, or slope of the phase shift, 

 computed from the relation 



T^f (3) 



Aco 



Figure 3 illustrates the computation of the average delay in the interval 

 Aco from two phase measurements. 



The steady state phase shift of an unknown network can be measured 

 by using the basic circuit shown schematically in Fig. 4.* This is essentially 

 a bridge circuit in which the phase shift in the unknown is balanced by an 

 equivalent calibrated phase shifter. The phase comparator is some kind 

 of device which will give an indication of a known relationship between the 

 phases of the signals arriving over the unknown path and the known refer- 

 ence path. 



The exact form of suitable components and circuit arrangements for 

 applying the basic method of Fig. 4 to a particular delay measuring problem 

 is largely determined by the order of magnitude of the delay to be meas- 

 ured and accuracy desired. In the case of microwave television repeaters 

 we are interested in video bands of the order of 5 mc wide. As a rough 

 estimate we might say that the highest frequency in the band should not 

 be shifted more than one quarter period from its normal phase position. 

 In the case of linear delay distortion or a parabolic phase-frequency char- 

 acteristic, one quarter period for 5 mc is 0.05 microseconds. In a repeater 

 system with 50 repeaters this yields a tolerable systematic delay distortion 

 of 10-* seconds per repeater. Therefore we conclude that in developing 



* Measurement of Phase Distortion, H. Nyquist and S. Brand, B.S.TJ., July 1930, 

 526-527. 



