AM
The gate-to-channel diode provides detection and the JFET is working as a square-law detector. Thus demodulation distortion will be a function of signal level and bias on the JFET since the square-law operation is only good over a certain range. The JFET is biased near to cutoff.
Normally, I run Regen#5 with the switch in high current mode for it offers maximum sensitivity. In certain cases such as when tuning strong AM signals, switching to the low current detector mode offers less AF distortion. Often, the switches’ effects on recovered audio distortion sounds subtle.
I determined the 22K source resistor experimentally during listening tests. The goal = to find a source resistor that gives the best audio fidelity.
CW/ SSB
Ideally, we want our detector to operate as a mixer — in this case, we’re running a direct conversion receiver. In SSB/CW reception, you run greater Q-multiplier signal amplitude than while detecting AM signals and we’re probably get some square law detection within the Q multiplier.
1 theory is that rectification results in DC that may be enough to actually drive the FET toward pinch off which kills the gain. Switching in higher JFET source current will help keep the RF detected from the Q multiplier from pinching the JFET off.
This winter, I built some different, very low distortion AM detectors. In 1 design, I ran a low current pair of BJTs with heavy feedback. While remarkable for AM, they really sucked for CW/SSB detection. So the direct-coupled hybrid cascode detector shown is a compromise circuit that works OK for both but better for SSB than AM in terms of AF distortion via listening tests.
This detector begs for further experiments. For example, what happens when it is DC coupled to the Q-M tank and a high ohm gate resistor is added? Should the 8K2 shunt resistor be replaced with a pot, or perhaps a switch to allow 3 or more different JFET source currents?
