You’ve all seen or heard of PiFM gpio-pin output FM tx software and it’s multimode equiv and other such GPIO pin RF ‘sources’.
I refrain from using TX as a description as, using a default hardware/software raw setup, to even call it a TX is somewhat of an insult to TX design, even calling it a Sig gen is kinda insulting and misleading.
It’s, at very best, an RF source gen - a kind of crude as a random rock used as a cold chisel grade gen and as noisy as ■■■■ spurious emmision, in band and out of band, with more unwanted harmonics (needless and pointless) than you’d want to catalog.
If you want to use a setup as a Sig source, crude as a random rock cold chisel grade, ‘source’ at least build/borrow/source a selectable configurable BPF that’s at least 3 stages (poles), a 7 pole filter is much more where you want to be using.
Yeah, insertion loss will be horrendous, but given that you’ll probably only legitimately using it for Sig injection where a proper Sig gen isn’t available, even the heavily attenuated BPF output is usable (barely) as an injection source. Pretty much anything beyond that, usage wise, is utterly pointless and reputation/license suicide (assuming you have any licenses to start with).
To be blunt, bordering on insulting of intelligence, for experimentation (injection source unless you have license for frequency/band/segment use intended) or as an exiter/oscillator, at the very least consider a known good DDS PLL synthesiser based module - I favour Analog Devices chip based items, but SiS equivalent are still notable to use even though I stick with AD original ref boards and an ADALM-Pluto with upgraded TXCO as a asynchronous (full duplex) capable Sig source and sniffer.
But whatever you use, so a good solid frequency sweep and measure the in-band and out of band harmonic and spurious emissions as even stuff manufactured with some suppressed output by design may still be not so clean.
I know, you’ll probably say why obsess so much over a few mW/■■■■■■ all mV/microVolt output - but quite frankly if your irresponsible enough to willingly emit RF using crude and mostly untamed RF sources with zero consideration and precautions to minimise and localise (at best), you more or less don’t deserve any consideration of any recognition of competence on any level.
Aside from the ADALM Pluto, or a Hack RF One (still with precautions) as a basic crude Sig gen/injection source, the next best item to a Hack RF One as a source & sniffer (simplex usage, not duplex) would maybe be a nanovna v2.2 (the ‘plus’ variant/revision) as at that stage it’s notably a not bad, in crude instrument terms, multi purpose instrument Sig source/sniffer/analyser. I’ve used the most recent version and will be adding one to my portable toolkit as my existing portable VNA is bulky and really showing it’s age and suffering from being ferried around outside of flight case protection. Having limited mobility, I end up needing to use light and compact away from the workshop and do in-depth and precision testing at the workshop only - and with a carry case of mixed attenuators and filters carried over, the nanovna makes sense.
But no matter which of those types of solutions you use, unless it’s a properly designed air-ready item with matched filtering and suppression enabled and verified to be functional, it’s a Sig gen best case or merely an RF/EM source more commonly and any delusional perception of it being more is just that, delusional pure and simple as a cold chisel is solid and simple.
Whatever you use, use it the way it was designed and intended for, within it’s inherent limitations and respect the responsibility you automatically are supposed to undertake putting such things into use.
I mean I could generate an oscillator injectable source from anything upwards of a very aged AVO gen or a more modern HP unit, modulate the Sig source and go pirate on ■■■■ near any frequency and mode with suitable transverter addition to suit, but just because you can and subsequently feed the result into a 30/50/100W or KW or MW rf amp to some kind of driven element/antenna/array doesn’t mean you should with irreverent abandon - hence I won’t, even if being a license holder wasn’t a motivator of caution.
Of course, you’ll do as you see fit - hopefully responsibly.