Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

464 FREQUENCY MODULATION, Volume Ioccur.Two flexible leads from these plates and another lead from thecutter mechanism are connected to the oscillator discriminator unitmounted on the carriage and located as close to the cutter head asfeasible. The oscillator-discriminator unit is of the same type as describedin Section 1. The variation of the capacitance, between the twoplates and the stylus, because of its motion, changes the oscillatorfrequency and varies the tuning of the discriminator in accordancewith the method of operation described in Section 2. The audio outputof the oscillator-discriminator unit, since it also contains a diode, istransferred to another unit containing an amplifier and a power supply.The output from this amplifier is then either measured with the aid ofFig. 12—The PM calibrator in operation.a tube voltmeter, or is further amplified for listening or other purposesby using a suitable transformer located between the amplifier tube,which has a high-impedance output, and a conventional amplifier of500 ohms input impedance.The push-pull FM plates are mounted on the bottom side of thecutter in such a way that incombination with the cutting stylus theyform a small push-pull condenser. As shown in Figure 10, this assembledbakelite plate on which the two FM plates are mounted fitsthe under side of the cutter and replaces the cover plate.onThis assemblyis again shown separately in Figure 11. The FM plates are, of course,adjustable so as to provide any width of gap to accommodate anycutting styli or for unusual circumstances where a wide gap is necessary.Normally, however, the FM plates are spaced 0.01 in. away fromthe stylus for frequency response measurements and 0.015 in. fordistortion measurements at the lower frequencies. This spacing isample for any modulation and yet provides more than ample output.