I'm putting together a 2m NBFM transmitter/receiver combo using rpitx and rtl-sdr. During testing my low-pass filter I discovered that rpitx is generating spurious signals within and around the band (see screenshot below). They're only about -30dBc so I need to remove them before transmitting. Removing them with a band-pass filter would be difficult and restrict the frequency range. What could be causing these spurs, and can anything be done to clean up the signal?
I've overlayed two screenshots, one with no signal (green), and one with the rpitx signal at 146MHz (red), taken from GQRX+RTLSDR which show some of the spurs.
The spurs are frequency-dependent and generally span +/-5MHz or so. I've ruled out the rtl-sdr as the problem as I can see them on my scope too. The signal is generated with
rpitx/tune -f 146000000 and is on the BCM4 GPIO. My Pi is a 3B+.