[Synchrodyne]

When a modulated signal is passed through an amplifier with a curved input-output characteristic, that curvature will add harmonic distortion to the modulation. This addition to the modulation will have the effect of increasing the modulation depth. The increase in modulation depth, known as “modulation rise”, is a convenient way of quantifying the added distortion.

Generally, modulation rise distortion is not a major issue, but it can become one when a final (or sole) IF amplifier stage is in receipt of a relatively large signal, and it has a very curved transfer characteristic. Thus, the signal swing traverse will cover a range with a significant change in slope. The very curved transfer characteristic is representative of a valve that is back-biased by a large AGC bias to be operating at the tail end of its curve.

One solution, as used by Armstrong, is to insert a non-AGC’d gain stage between this IF amplifier and the demodulator/AGC rectifier. As this will be operating in the centre, relatively straight part of its characteristic, it will not suffer from modulation rise. Then as, for a given signal level into the demodulator/AGC rectifier, the main IF amplifier is dealing with a lower signal input level (reduced by whatever gain is provided by the non-AGC’d stage, it will also be operating clear of the modulation rise region.


Cheers,