Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

158 FREQUENCY MODULATION, Volume IExperimental ProcedureAn essentially distortionless audio source is required for measurementsof phase distortion in the frequency-modulation generator. Thishappens because the amplitude of the nth harmonic in the developedfrequency deviation is proportional not only to the amplitude of thenth harmonic in the audio-source voltage, but also to the order of theharmonic, since the frequency deviation isproportional to the rate ofphase deviation. Thus, 1 per cent 3rd harmonic in the audio inputwould produce (in an otherwise distortionless system) 3 per cent harmonicin the resultant frequency deviation, which would then appearas 3 per cent 3rd harmonic in the audio output of the monitor detector.This problem may be circumvented in two ways, i.e., first integratethe audio voltage before application to the modulator, in which casethe device produces frequency-modulated signals, requiring constantaudio input for constant-frequency deviation as the audio frequency isvaried. The source harmonics are then reduced in magnitude by anamount proportional to their order. The alternative method was usedin the experimentalwork ;an audio source of good waveform was usedin conjunction with a series of band-pass filters, each capable of passingan octave of frequencies, starting at 30 cycles and ending at 8000cycles. No integration was employed: hence, the generator producedphase-modulated signals.A General Radio Type 636-A wave analyzer was used for all distortionmeasurements. The harmonic content of the audio source wasfound to be less than 0.05 per cent for all audio frequencies employed.The wave analyzer was then connected permanently across the monitordetectoroutput terminals. In parallel with this a 9-inch oscilloscopewas connected to facilitate preliminary adjustments. In general theaudio input to the modulator was adjusted to produce 100-kc deviation(for all audio frequencies) at the final carrier frequency of 40.5 Mc.The heterodyne oscillator in the detector unit was adjusted to produce150 kc as the intermediate frequency for application to the monitordetector.This had previously been ascertained as lying in the regionof optimum detector linearity.As pointed out in a preceding section, there are two sources ofdistortion, in a generator of this type; the first occurs at low audiofrequencies (less than 400 cycles) and is due to non-linear relation*ships in the modulator, which are accentuated by the large audioswings required.This type of distortion is generally a function of the modulatorbias, which controls the degree of matching between the curves of.£ vs. B (Figure 6) and R pvs. E g of the modulator tube.