. The Bell System technical journal . signal. The 60 cps rate is muchhigher than that which can be followed by thermal changes in the TWT. Half ofthe modulated signal serves as input to the TWT under test and half serves as areference phase for a phase detector. The signals at the phase detector input aremaintained equal and at constant level and nominally in phase quadrature. Thedetector is essentially a bridge circuit, the output of which is a dc voltage propor-tional to the phase difference of the two inputs. When operated with inputs inquadrature it is not sensitive to amplitude changes of
. The Bell System technical journal . signal. The 60 cps rate is muchhigher than that which can be followed by thermal changes in the TWT. Half ofthe modulated signal serves as input to the TWT under test and half serves as areference phase for a phase detector. The signals at the phase detector input aremaintained equal and at constant level and nominally in phase quadrature. Thedetector is essentially a bridge circuit, the output of which is a dc voltage propor-tional to the phase difference of the two inputs. When operated with inputs inquadrature it is not sensitive to amplitude changes of as much as two db in eitheror both inputs. Phase modulation introduced by the amplitude modulator appearsat both inputs and thus does not produce an indication. The output of the de-tector is therefore a direct measure of the phase modulation created in the is determined by comparing the percentage amplitude modulationat the input and output crystal monitors. 1320 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 Z gma UJ > 2 q 2 Q. o 5< I - CD O h = Z I- Z < / r (b) GAIN^ • r* ^ \ 1 N ^ 2300 2400 2500 2600 HELIX VOLTAGE IN VOLTS 2700 100 9in LU 75 Q:Q. 50 8 I- 25 5O o5 2800 II- Fig. 25 — Gain, compression and amplitude to phase conversion as a functionof voltage with the output power maintained constant at a level of fivewatts (a) and ten watts (b). there is expansion for some values of power input. Figs. 23 (e) and 23(f)give the AM-to-PM conversion, as functions of input and output powerrespectively. These data indicate that the conversion is very much lessif the tube is operated at lower helix voltages. For example, the con-version at the saturation level of the 2,700-volt curve is about 2^ timesthat for the 2,400-volt curve. A final method of plotting gain, compression, and AM-to-PM con-version data is shown in
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