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Envelope Distortion in SSB AM

Hams often generate a pseudo AM signal that can be demodulated by an envelope detector, by re-inserting carrier in an SSB transmitter.

If you are using an envelope detector, the case of demodulating SSB-AM is different to DSB-AM.

In the case of DSB-AM with sine wave modulation, the effect of the carrier vector, and the two counter rotating sideband vectors is that the envelope of the wave is directly proportional to the modulating signal (subject to the quality of the modulator of course).

In the case of SSB-AM with sine wave modulation, the effect of the carrier vector, and just one rotating sideband vector is that the envelope of the wave is not directly proportional to the modulating signal, it is distorted.

The graph above shows the modulating waveform and the voltage that would be obtained from an ideal envelope detector for a modulation index (m) of 1. (Modulation index in the SSB AM wave is taken to mean the ratio of the sideband voltage to the carrier voltage.) The extent of the distortion is dependent on the modulation level, and Fourier analysis of the waveform gives THD with an ideal envelope detector as 22%.

Now, envelope detectors suffer a number of problems, but on strong signals, this distortion exceeds the diode non linearity and detector slew rate distortion inherent to envelope detectors.



The graph above shows the lower distortion when modulation index is reduced, in this case to 0.25 which gives THD=6.2%. However, such a signal has a PEP of 56% higher than the carrier power, and sideband power of just 6% of the carrier power. It is not a very effective or efficient solution.


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