Audio "Intermodulation" and Automatic Gain Control

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Audio "Intermodulation" and Automatic Gain Control

Today on 28MHz I noticed that my WSPR display showed what looked like a classic textbook picture of third-order intermodulation products. At the time there were just two incoming signals, both of which were near the middle of the display. The distortion products appeared above and below these signals offset by the same spacing as the offset between the two main signals. It looked just like an intermodulation effect because the spectrum width of each distortion product was exactly the same, and greater than that of the original signals.

Although the signals were strong by WSPR standards (S5-S7 on the S-meter) I was puzzled because that is not enough to cause true intermodulation in my receiver. I then found that switching the AGC from fast to slow removed the unwanted distortion traces. And then it became obvious what was happening. Although each WSPR signal has constant envelope their sum has peaks and troughs at the difference-frequency. The AGC is driven by an envelope-detector which responds to that difference-frequency and so modulates the IF gain at the difference-frequency. That IF gain modulation generates a set of sidebands that look exactly like classic third-order intermodulation.

Switching the AGC to slow meant that the IF gain was not modulated by the difference-frequency and the distortion disappeared. I just thought I would mention it because the cure was very easy but not immediately obvious.

- Nick.