Spurious/Sideband Troubleshooting with the RTL-SDR

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majolsurfmajolsurf 06/05/2014 at 07:140 Comments

Previously I wrote about the bring-up of the MRF49XA Shield. Recall that instead of the standard FSK spectrum I was met with an asymmetric comb about the carrier.

the problem

Decreasing the span we see are able to measure the comb spacing, about 80kHz.

SDRsharp Analysis

When viewed in this way, we see more of a traditional harmonic series from the carrier. I'll assume for now that the series is not an odd function; the waveform is not square or compressed. Typically symmetric sidebands that reside xdBc from the carrier are indicative of A.M. distortion. I'll also chalk up the asymmetry seen in at wider bandwidths is due to heterodyning.

The first place I like to look when experiencing undesired A.M. spurs is the power supply.


Setting the DMM to AC mode and turning on the frequency counter, I find a dominant 78kHz AC component on my 3V3 supply.


That's close enough for me to be a smoking gun. A bypass capacitor is placed near the MRF49XA supply...


And the major sidebands are gone. We can still see spurs about 35dBc at 200kHz; for now I'll assume they are Frac-N PLL spurs and move forward.

SDRsharp Analysis cap installation