Checking the Transmitter

Author: J.B. Hoag

In order to check the linearity of the f.m. transmitter, a highly selective receiver which has an internal oscillator is tuned to the carrier frequency. If frequency multiplication is used in the transmitter, the receiver can be tuned to any one of the frequencies of the successive stages. An audio tone source of constant frequency is then placed in front of the microphone and increased in intensity until the carrier frequency is found to change. This will be noted by a change in the loudness of the output of the receiver. In using the transmitter, the sound intensity should always be kept below this level.

In order to check the frequency deviation, repeat as above, leaving the audio level just below the point where non-linearity begins, but this time slowly change the audio frequency from zero to higher and higher pitches until there is no sound output from the receiver. The audio frequency at this point, multiplied by 2.4, is the frequency deviation. If measurements are made in front of the output stage of the transmitter, be sure to multiply by the frequency multiplication, in order to get the total frequency deviation.

An unchanged antenna current, with or without audio input, indicates that the transmitter is free from amplitude modulation.