FM880 True FM Modification


The FM880 & the FM828 use phase modulation. Phase modulation is achieved by applying audio to a varicap (variable capacitance) diode which changes the capacitance of the varicap diode. If this varicap diode is placed across a tuned circuit, the tuned circuit is slightly detuned, and this slight detuning produces a small amplitude and phase change in the tune circuit. A phase change is in effect a small frequency change hence we have frequency modulation.

However the amount of phase change (frequency change) is proportional to the amplitude of the applied audio and also the frequency. The more audio the more FM modulation (deviation) and most important, the higher the frequency the more deviation. A 2KHz tone produces twice the deviation of a 1KHz tone. Hence to achieve the correct frequency response in the receiver, from a phase modulated transmitter, de-emphasis (high frequency attenuation) has to be applied in the receiver.

This de-emphasis has beneficial result in improving the overall signal to noise of the received signal. The audio content of a typical voice or music source tends to reduce in amount as the frequency goes higher. Hence the higher frequencies can be increased (emphasised) and not over modulated the transmitter.

The Phase modulator has this built in pre-emphasis which is an advantage as no extra audio circuitry is required to achieve pre-emphasis.

However Phase modulation has 2 bad characteristics. The inherent audio distortion is high, typically 10% and the poor low frequency reproduction (level) becomes an issue if you want to place a CTCSS tone on the transmitter. CTCSS tones are from between 254Hz to as low as 67Hz. If you try to transmit CTCSS tones using a phase modulator, the audio level required to produce even 500Hz deviation is of such a level that the resulting CTCSS tone is distorted. This distortion produces harmonics and these harmonics can be heard in the received audio. 

A direct (true) FM modulator does not have built in pre-emphasis but a flat frequency response hence a 67Hz tone requires the same audio input as a 3KHz tone to produce the same deviation. However pre-emphasis is benificial in improving the overall audio signal to noise (less hiss) hence pre-emphasis is applied to the audio before it is applied to the FM modulator. CTCSS tones go to the varicap via a separate non pre-empasised path.

True FM modulator