so ive been slowly building a direct conversion receiver for 40m over the past few months, nothing fancy, just a NE602 front end and a simple audio stage, mostly following a circuit ive seen in a few places with some mods i added myself. works okay for SSB but the selectivity is pretty terrible and every loud station on the band just bleeds into whatever im trying to listen to.

someone in my club mentioned i should build a crystal ladder filter and put it before the product detector, which makes sense in theory, but ive never done one and from what i can tell you really need matched crystals for it to work right. i have a bunch of 7.2 MHz surplus crystals from an old box i bought years ago but no idea if theyre even close enough in frequency to be useful. i have a frequency counter but not a proper crystal measuring jig.

is it worth building a jig just to measure and sort these, or should i just order a set of matched crystals from one of the usual places and be done with it. part of me wants to do it the hard way because thats kinda the point of building your own stuff but i also dont want to spend three weekends on something that might not even work that well

yeah building the measuring jig is honestly half the fun and its not that complicated. theres a design thats been floating around for years, basically a simple series circuit with a resistor and you measure the frequency where the crystal resonates by looking for the dip on a scope or even just listening for the null. G3UUR method i think its called. i sorted a batch of 10MHz crystals that way a while back and got four that were within about 50Hz of each other which was good enough for a 4-pole filter.

your 7.2 MHz crystals might be fine or might be all over the place depending on where they came from. surplus crystals are a crapshoot. but even if you only end up with 4 usable ones out of a bag of 20 thats still a win. the filter math isnt too bad once you get into it, W7ZOI has written a lot about this and his stuff holds up. just be patient with the termination impedance because getting that wrong is usually why homebrew filters sound weird or have a weird passband shape.

honestly for a DC receiver i wonder if a good active audio filter might get you further for less pain. ive got a little state variable filter i built from a TL072 running off the audio output and the difference is pretty dramatic, you can tune the Q and center frequency and for casual listening its hard to complain. not saying the crystal filter isnt worth doing, it probably is, just that the audio approach is way simpler to get working on a first try and you could have it done in an afternoon.