# Can these in-band spurs from rpitx be eliminated somehow?

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+.

• So the spurs move with the signal... They could still be caused by the receiver. One way to tell is to attenuate the signal and see if the spurs also drop, and whether they drop by the amount of attenuation, or more than that. IMD should drop by 3 dB for each dB of attenuation. But it wouldn't surprise me that the Pi also generates spurs. – tomnexus May 11 '20 at 10:51
• I've played with attenuation (and preamp gain) but the spurs drop the same amount as the fundamental. Plus I've verified them with a second "receiver" (the FFT function on my oscilloscope), so I'm pretty confident they're from the Pi. – user9103 May 12 '20 at 0:54