Si5351 Phase Noise and THD Tests using an Agilent E4402B Spectrum Analyzer

There has been some discussion on various forums, and the comments sections of a number of Ham Blogs and on-air QSOs, concerning the Phase Noise of the Si5351 chip being offered in the form of breakout boards.

I recently commented on a post that Bill Meara, N2CQR, put on his SolderSmoke Daily News Blog. I was thinking about both sides of the argument and since I had an SA with a Phase Noise personality, I figured I’d do the measurements.

14 MHz, 300 Hz RBW

Not a bad raw view at a RBW of 300 Hz. The baseline is at -80 dBm with phase noise seen near the carrier at -60 dBm.

Next is the Phase Noise measurement trace. As you can see, the average SSB phase noise came out to -96.95 dBc/Hz.

Si5351 Phase Noise

Lastly, and probably more telling, was the THD measurement of the carrier and 9 harmonics.

THD and Harmonics

As can be seen above, the notable issue is the THD being 40.67%. However, any low-pass function prior to mixing stages will adequately block most of this harmonic content above 14 Mhz and is a frequent feature of modern superheterodyne architectures.

So while the Phase Noise measurement was not bad at all, the real concern when using what was intended to be a multi-clock generator chip for use in digital systems, is to get the THD down coming into a mixer stage (and after of course) so that the result is closer to a more pure sine wave.


2 Responses to “Si5351 Phase Noise and THD Tests using an Agilent E4402B Spectrum Analyzer”

  1. 1 Vincent
    October 23, 2016 at 7:59 am

    With a square wave output I would have thought that it is ideal for the LO in any TCVR project. After all, the LO is used to switch the mixer diodes (transistors in active mixers), so even if you filter the LO to be a pure sine-wave, the switching action of the mixers converts it back to a square wave anyway.

    On the RF port it is another matter, but for the LO it is my belief that a square wave is ideal.

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