Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

388 FREQUENCY MODULATION, Volume Ichange in primary voltage as the secondary is varied from a tuned toa detuned condition. With all circuit conditions normal, and the signalapplied at the center frequency, the primary voltage is noted. Withthe same input signal level, the secondary is detuned so that theprimary voltage rises. The per cent critical coupling can then beexpressed in terms of the ratio between the primary voltage with thesecondary tuned and detuned. For example, if the signal at the platerises 25 per cent when the secondary is detuned, the coupling is 50per cent of critical.Ratio S/PThe ratio between the secondary and tertiary voltages can be measuredindirectly in terms of the rectified output voltage which isobtained (a) with the secondary tuned and (b) with the secondarydetuned. In both cases the input signal is adjusted so that the primaryvoltage remains constant. If the ratio between the voltages read in(a) and (b) is r, that is, ifRectified voltage (secondary tuned) V P 2 + S— = 2= rRectified voltage (secondary detuned) PSthen — = V r 2 — 1PSignal GeneratorIn checking the performance of ratio detectors it is convenient tohave available a signal generator which is capable of being simultaneouslyfrequency and amplitude modulated. The advantage of using thistype of modulation is that it provides graphically a complete picture(Figure 12, for example)of the f-m response of the detector and atthe same time shows the a-m rejection under dynamic conditions atevery frequency within the range of the detector.It is important that the signal generator used to check ratio detectorsbe free of spurious modulation. In particular, it is essential thatwhen the signal generator is amplitude modulated, no frequency modulationbe produced as a result of the amplitude modulation.If frequencymodulation takes place when the generator is amplitude modulated,the resultant output will be construed as being due to faulty a-mrejection whereas it is actually the normal f-m response of the detector.