The "DX Receiver for the hf bands", by Ovi Florea, Ham Radio, December 1976, Page 10, used a variable injection control at the product detector. I quote from the article, Page 16, "Theory says that, for minimum distortion a product detector requires carrier injection at least 10 times the level of the incoming signal at the input port. What would happen upon lowering the injection level? Decreasing the level caused strong perturbing signals to become unreadable, while weak DX signals were still crystal clear. One helpful finding was that the audio output versus bfo level dropped about 10 db faster for the stronger signal compared to the weak one."

A .01 capacitor is used to couple to the product detector. You can fine tune your BFO injection level a couple of ways:

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