Si5351 at 16 MHz
There is still a lot of talk about the supposedly horrible phase noise of the Si5351 chip. In a recent episode of a popular (and very good!) podcast about homebrewing, the podcasters talked about this in the context of some megawatt AM shortwave broadcast stations that had oscillator phase noise problems and were wiping out large portions of the HF spectrum. I don't think those stations were running Si5351s, but the listener was left with the impression that these handy little chips are very noisy with lots of spurs and will inevitably produce horrible dirty, spectrally impure signals.
This has not been our experience. Following Pete's lead, several of us are using the Si5351 to generate both VFO and BFO signals in our transceivers, with good results. The receivers sound very good and we have not heard complaints of "broad" or "noisy" transmitted signals.
I decided to dig into this a bit. This was also an excuse for me to use the FFT and screen capture features on my Rigol 'scope.
I now have THREE BITX transceivers in the shack. My BITX17 uses a VXO at round 23 MHz (IF at 5 MHz)/ My BITX20 uses a classic LC VFO running around 3.5 MHz (IF at 11 MHz). Finally, my BITX40 (DIGI-TIA) uses the dreaded and much reviled Si5351 running at around 16 MHz (IF at 9 MHz). I thought that these three rigs would provide a good opportunity to test the scurrilous claims about the Si5351.
As a simple first test, I put my Rigol scope in FFT mode and just put the probe at the VFO Mixer's LO input. The screenshot above is the FFT for the Si5351. It looks pretty clean to me. The 'scope is looking at 15 Mhz above and below the VFO signal.
VFO at 3.5 MHz
Next I measured the output of the BITX20 VFO at the same point (input to the VFO mixer). (I had to change the vertical range, but the horizontal was unchanged.) Here you can see the second harmonic (just because at this low freq it is within the freq range setting of the 'scope). It doesn't look much different than the Si5351.
VXO at 23 MHz
Finally, here is the BITX17 VXO at 23 MHz, again at the input to the VFO mixer. It looks remarkably similar to the Si5351, don't you think?
More on this to come. The ARRL Handbook (2002) has a good discussion on phase noise. I am digging into this and hope to do some more tests. For now, I think we should reserve judgment on the utility (for us) of the Si5351.
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