so ive been building this double sideband transmitter for 40 meters off and on for about three months now, mostly following the basic balanced modulator design from EMRFD with a couple of changes i made because i couldnt source the exact diodes they specified. ended up using 1N4148s matched with a DMM on the diode test setting, got four of them within about 1mV of each other which i thought would be good enough.

anyway the thing actually transmits and my buddy down the road can hear me, but when i key up without audio the carrier is still pretty audible on his end. he says its maybe 20dB down from when im talking which honestly doesnt seem great. ive got the trimmer pot trying to null it out and i can get it a bit better if i fiddle for a while but it drifts around as it warms up and never really stays null for long.

not sure if its the diodes not being matched well enough or something else in the circuit. the carrier source is a 7.2MHz colpitts oscillator that i built on a separate board. anyone dealt with this kind of drift issue before, is there something im missing about keeping these things stable once they warm up

yeah the thermal drift on a diode ring is a real pain and 20dB suppression is pretty much where you land when things arent quite right. the 1N4148 matching you did with a DMM is okay but its a static measurement and those diodes have slightly different dynamic characteristics that only show up under RF drive. what a lot of guys do is actually match them under RF conditions using a little test jig with a signal generator and a scope, you're looking for the best null by swapping diodes around until you cant get any better.

the other thing is your carrier oscillator board being separate -- any coupling or ground loop between the two boards can inject carrier around the modulator rather than through it, and that stuff obviously cant be balanced out. try putting some shielding between them if you havent already, even just a piece of copper clad acting as a wall can make a noticeable differnce. also check that your pot wiper isnt oxidized, those little trimmers get flakey fast and a dirty wiper will absolutely cause the null to wander as temperature changes.

i built something similar last winter and had basically the same problem. for me it turned out to be the transformer balance more than the diodes -- i wound my own trifilar on a binocular core and i guess one of the windings was slightly off in terms of turns ratio which prevented a real deep null no matter what i did with the pot. rewound it being really careful about keeping the wire twist consistent and it got a lot better, ended up around 45dB suppression which i was happy with. might be worth checking that if you wound your own transformers.