Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

152 FREQUENCY MODULATION, Volume Iapproximately 0.02 per cent distortion in the audio output obtainedfrom the detector.Tests were also made to determine the susceptibility of the detectorto amplitude modulation in the input signal. With 25 per cent amplitudemodulation (and a mean value of input signal of 20 volts peak topeak) the output contains an undesired signal corresponding to lessthan 0.05 per cent of the audio voltage produced by 100-kc deviation.The sensitivity of the detector is approximately one volt peak-topeak(audio) for 100-kc deviation in the input signal (and 20 voltspeak-to-peak applied signal). Therefore, if a single frequency signalof 100 kc is applied to the detector the d-c output should be approximately0.5 volt. Knowledge of this sensitivity makes possible theadjustment of the heterodyne oscillator frequency (in Figure 3) to avalue which causes the beat frequency to liein the band for which thedetector linearity is optimum. It is necessary merely to measure thed-c output of the detector as the heterodyne oscillator frequency isvaried, and to adjust the latter to produce a d-c output voltage correspondingto 150-kc carrier input.ApplicationsThe monitor detector as thus constituted is an excellent instrumentfor studying the effects of the pass characteristics of a frequencymodulationreceiver's r-f and i-f circuits on distortion, for checkingthe performance of frequency-modulation signal generators, for use asa monitor and peak deviation indicator at a frequency-modulationtransmitter, and for many other applications.Part II—Low Distortion Frequency-Modulation GeneratorIntroductionFrequency-modulation signals can be derived from systems basedon either frequency or phase modulation principles, provided that theaudio potentials (or program material) be properly integrated beforeapplication to the phase modulator.In a frequency-modulation system of the type employing a reactancetube modulator, the frequency deviation produced by a certainaudio voltage is independent of the frequency of that audio voltage,and is determined only by the sensitivity of the reactance tube (kcdeviation per audio volt) , and by the subsequent frequency multiplication.Assuming that the circuits in the generator are sufficiently flatover the desired deviation band, the harmonic content of the output ofa frequency-modulation generator of the aforementioned type is deter-