so ive been working on this 40m direct conversion receiver for about two months now, based loosely on the NorCal 40A design but with a few changes i made to the BFO section because i wanted to try using a JFET mixer instead of the diode ring. overall it actually works, which honestly surprised me a little because my PCB layout was kind of a mess — i just went point to point on a piece of copper clad and it looks like something exploded in there.

anyway the issue im having is this weird audio artifact that sounds almost like a low hum mixed with a kind of scratchy distortion when i tune across strong signals. weak signals are actually pretty clean, which is the part thats confusing me. i thought at first it was a grounding issue so i redid all my ground connections and it helped a little but didnt go away. power supply is a 12v regulated bench supply so thats probably not the problem. anyone dealt with this before with a JFET mixer stage? wondering if its an overload thing or maybe something wrong with my audio amp section.

that scratchy distortion on strong signals with a direct conversion design is almost always the mixer getting slammed. JFETs as mixers are great but they have a pretty limited dynamic range compared to a diode ring and if youre not putting any attenuation before the mixer it'll clip pretty badly on anything over maybe S7 or so on 40m which as you know is basically everything during the day. try sticking a 6dB pad right at the antenna input and see if the problem goes away or at least gets better. if it does then you know its upstream of your audio stage and you can start thinking about adding an RF attenuator or maybe a small preselctor with some gain control.

the grounding rabbit hole is a classic time sink, i spent three weeks convinced a receiver problem was grounding once and it turned out to be a cracked ferrite bead i couldnt even see.

what JFET are you using? i built something similar a while back with a 2N3819 and had all kinds of grief until i biased it properly, the drain current makes a huge difference in how the thing handles strong signals. also depending on how your audio amp is configured you might just be running too much gain at the end of the chain which makes any distortion way more obvious. what are you using for the audio stage?