so i've been messing around with a direct conversion receiver for 40m for about the last three months, based loosely on the EMRFD designs but i kind of went off script with the LO section because i had a bunch of parts on hand and didn't want to order anything. been running a VXO using a 7.030 crystal i pulled out of an old QRP kit i never finished.

anyway it's working, i can hear stations, SSB is pretty intelligible if i tune carefully. the problem is the audio has this low frequency rumble that i cant seem to track down. it's not 60hz hum exactly, more like a 20-30hz kind of thump or beat that comes and goes. doesn't seem to be related to any signal i'm receiving. tried running it off a battery to rule out the power supply and it's still there.

also the image rejection is pretty bad which i kind of expected with a direct conversion but it's worse than i thought it would be. wondering if anyone has dealt with this and whether its worth adding a simple bandpass filter ahead of the mixer or if i should just accept it for what it is on 40m where there's so much going on anyway.

the mixer is a diode ring, four 1n4148s matched as close as i could with my DMM. LO is running at half frequency so the crystal is at 3.515 which is kind of annoying for 40m SSB but it was what i had. open to suggestions on fixing any of this without rebuilding the whole thing.

the low freq rumble is almost certainly microphonics or mechanical coupling somewhere, especially if your LO isn't real stable. with a VXO and those little crystals you can get some weird low rate pulling if anything is vibrating near it. try pressing on different parts of the board while it's running and see if the rumble changes. i traced a similar thing in a receiver i built years ago to a cap that was barely touching the chassis.

on the image rejection thing yeah that's just life with a direct conversion, especially without a proper roofing filter. a simple LC bandpass for 40m ahead of the mixer would help a lot, doesn't have to be fancy. even a double-tuned circuit would knock down a lot of the garbage. the 1n4148 ring mixers are fine but they need a decent LO drive level, like 7dBm or so, and if you're running the crystal at half freq through a doubler you might not be hitting that. what's your LO output level looking like?

im kinda in the same boat built a receiver last winter that had all kinds of weirdness and most of it turned out to be ground loops on the board. do you have a single point ground or just kind of connected things together as you went? i ask because my rumble problem ended up being exactly that, two ground paths creating a loop and picking up something. once i redid the grounding it got a lot quieter. not saying thats definitely your issue but worth checking before you tear into the RF side of things