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:
The first way is by varying the G2 bias voltage from 6 volts (most gain) to 0 volts (shut off). Replace the 100k resistor from G2 to ground with variable one, and then set the injection level. This variable resistor can be mounted on the front panel of the receiver so the operator can set the level for varying band conditions.
The second is by changing the coupling capacitor to the product detector. A .01 capacitor is the highest injection level, where values in the 10 to 300 pf range will set lower levels.
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