so ive been working on this little 40m CW transmitter for probably three months now, mostly evenings after work, and i finally got a signal out of it last night. based loosely on the old ARRL handbook crystal oscillator design, running a 7.040 xtal through a buffer stage and then into a single IRF510 final.

problem is the output looks kind of rough on the scope. i was expecting something close to a sine but what im seeing has this weird shoulder on the rising edge, almost like a little pre-ringing thing before the main waveshape settles. power output seems ok, im getting around 4-5 watts into a dummy load, but something about that waveform bugs me. ran it into a spectrum analyzer app on my phone (yeah i know, not ideal) and there seem to be some harmonics that are a bit high.

the lowpass filter i built uses the values from the ARRL book for 40m, toroids wound on T50-2 cores. i double checked the turns counts but maybe i screwed something up there. anyone dealt with this kind of thing before?

that shoulder on the rising edge is almost certainly the IRF510 doing its thing before the gate fully turns on — those MOSFETs are not super linear at the transition point and without proper bias they can do weird stuff. what are you running for gate bias voltage? a lot of people just ground the gate through a resistor and call it day but that leads to exactly what youre describing.

also worth checking your lowpass filter toroids with an LC meter if you have access to one. winding on T50-2 is fine but if the cores got warm during winding or you miscounted a turn or two the cutoff frequency shifts and you get harmonics bleeding through. i built a 40m rig a couple years back and spent two weekends chasing a third harmonic that turned out to be one turn short on the last inductor in the filter.

honestly 4-5 watts out of an IRF510 on 40m is pretty decent for a first build. dont get too hung up on what the phone spectrum analyzer says, those things are not exactly calibrated test equipment and can show artifacts that arent really there. i would trust the scope more and even then your probe ground lead being too long can introduce ringing artifacts that look like they're part of the signal.

try shorting your scope probe ground lead as short as physically possible and see if that shoulder changes. i spent like a whole weekend diagnosing a problem that was just my probe clip flopping around.