so i finally got my first real homebrew superhet working well enough to pull in some stations and im pretty stoked about it. been working on this thing for like 3 months off and on. its a basic AM broadcast band receiver, dual conversion, first IF at 10.7 and second at 455kHz which i know is kind of overkill for broadcast but i had the parts and wanted to learn the topology properly.

anyway the thing works but im getting this weird birdies issue around 1050 kHz give or take. i traced it to what i think is leakage from the first oscillator getting into the second mixer somehow but honestly im not totally sure. the shielding on my first IF strip is pretty janky, just some copper tape i wrapped around a cardboard tube because i was out of proper enclosure material at the time.

also the selectivity on the second IF is not great. im using some surplus 455 kHz ceramic filters i got in a bag of parts from a swap meet years ago. two of them in cascade which should in theory give me decent skirts but it doesnt feel tight enough, stations bleed through more than id expect. anyone dealt with this before or know if matching the filter impedances matters a lot here, because i just kinda threw them in without thinking about that.

the birdie at 1050 is almost certainly what you think it is - LO feedthrough or a mixing product getting back into somewhere it shouldnt. copper tape over cardboard is gonna do basically nothing for RF shielding at 10.7 MHz, you really need a proper tin or copper box. even a cut up sardine can soldered together works better than tape on cardboard. worth spending a weekend fabbing something real before chasing anything else because you wont really know what youre looking at until the shielding is sorted.

on the ceramic filters - yes the impedance matching absolutely matters. most of those surplus 455 filters want to see something like 330 to 2k ohms depending on the type and if youre just dropping them inline without matching networks youre gonna get insertion loss and the passband shape will be all wrong. look up the part number if you can find any markings and see what the datasheet says. murata used to have good docs on their old filters even the discontinued ones. if you cant find specs just try a few different termination resistor values and see what sounds best on a known station, its not super scientific but it gets you in the ballpark.

im also building a superhet right now actually, single conversion 9 MHz IF for 40m SSB, totally different thing but the filter impedance thing bit me too. i just assumed they were drop in and spent like two weeks wondering why everything sounded muddy. turned out i needed matching pads on both sides. once i got that sorted it was night and day difference honestly.

also the cardboard tube thing made me laugh because i did something similar with a biscuit tin and some foam tape in my first build and it was a disaster. real shielding is worth it, just tin plate from a hobby store or even a metal project box you cut up.