so ive been lurking here for a while and finally decided to stop just reading about homebrew stuff and actually build something. picked a simple direct conversion receiver for 40m as my first project, mostly following the NorCal 40A schematic but making a few changes because i didnt have all the parts on hand.

the main thing im worried about is the LO. im using a colpitts oscillator with a 7.040 MHz crystal i pulled from an old board, and im getting signal out of it but when i scope it the waveform looks kind of dirty — like theres a little shoulder on the rising edge that i cant figure out. not sure if thats a problem or if direct conversion receivers are just forgiving enough that it wont matter much for basic listening.

also im using a SA602 as the mixer which i know some people love and some people hate, but i had a bunch of them and the datasheet made it seem straightforward. my audio chain after that is just a basic LM386 amp stage for now, nothing fancy.

anyone built something similar and run into the waveform thing? or is that just something i should stop overthinking and just see if the thing works when i button it up?

the shoulder on the rising edge is almost certainly not gonna kill your receive performance on a direct conversion setup, especially if youre just using it for casual listening. the SA602 is pretty tolerant as long as your LO level is in the right ballpark — datasheet says around 200-300mV for decent conversion but ive run them a bit outside that and they still work fine. the classic thing people get hung up on with colpitts oscillators is the loading effect when you connect the next stage, so if you havent already stick a buffer between the oscillator and the mixer input, even just an emitter follower with a 2N3904 is enough. that can also clean up the waveform a bit because youre not letting the mixer pull on the tank circuit.

the LM386 is fine for getting started but fair warning, it picks up every bit of RF that looks at it funny. keep your leads short from the mixer output and put a cap across the supply pin, the datasheet recommends 10uF and a 0.047 in parallel. ive had setups with the 386 that sounded awful until i sorted out the bypassing and then they were totally usable.

i built almost the exact same thing last year, SA602 and LM386, and honestly the hardest part for me was just figuring out why there was so much hum. turned out i was grounding the audio side and the RF side at different points on my ugly-style board and there was enough of a ground loop to make it sound like a bad power supply even though the supply was fine. one ground point fixed it immediately which felt kind of embarasing after how long i spent poking at it.

dont stress too much about the waveform before you even know if the thing receives anything. plug in a wire antenna and see what you hear first, then troubleshoot from there. 40m in the evening you should be hearing something almost right away if the circuit is basically working.