so ive been building a direct conversion receiver for 40m, been at it for about three weeks now and finally got something that actually makes noise when i hook up an antenna. problem is theres birdies all over the place, like every 10-15 khz or so i get these tones that dont go away no matter what band im listening to. im using a NE602 as the mixer and a LM386 for audio. built it on ugly style construction on a piece of copper clad.

the oscillator is a colpitts design from that EMRFD chapter on direct conversion stuff. vfo seems pretty stable once it warms up but i honestly dont know if my layout is causing the birdies or if its an IF issue or what. ive got no shielding on anything right now, just the board sitting open on my workbench. tried bypassing the power supply rails harder but didnt seem to help much. anyone dealt with this before with the 602? ive seen people mention that chip can be sensitive to layout but not sure what to actually do about it.

yeah the NE602 will do that if your LO is leaking back into the supply or if theres any coupling between the audio stage and the rf input. the LM386 is a notorious oscillator when you're not careful with decoupling — throw a 0.047uf cap from pin 7 to ground if you havent already, that tames a lot of the weirdness. also make sure your audio ground and your rf ground arent sharing a long trace or wire because at audio frequencies that still causes headaches.

birdies at regular intervals usually means the oscillator harmonics are getting in somewhere. if you can throw even a tin can over the vfo section temporarily, see if they move or go away. also worth checking if the birdies track with the vfo or stay fixed — if they stay fixed its probably a supply issue, if they move youre getting oscillator products mixing with something they shouldnt be.

i had almost the exact same thing happen with my first 40m dc receiver a few years back. ended up being that i had the audio output wire running right next to the input coil and it was feeding back just enough to make a mess. moved the wiring around and half the birdies went away. ugly construction is great but sometimes you gotta be a little intentional about where stuff physically sits relative to each other.

also dont underestimate the wall wart if thats what youre powering it from, switching supplies are awful for this kind of thing. i switched to a 7812 off a small transformer and it cleaned up a lot.