Re: si5351a issue spectral output above 90Mhz


Glen Leinweber
 

Hans,
Thank you for adding phase noise measurements for SI5351. Just another
value-added QRP_Labs feature for those of us who build-to-learn. Noise
floor measurements seem prone to mis-interpretation and measurement
errors. Including your noise-measurement setup progression is particularly
enlightening.

Hans didn't test fractional MultiSynth dividers for the very good reason that
QCX uses integer MultiSynth division only. Don't confuse the SI5351
MultiSynth PLL with its MultiSynth divider. I believe that QCX uses
fractional division in the PLL, but integer division in the divider. Seems very
reasonable that QCX spurious response is quite good with this
arrangement: SI5351 configuration (187 internal registers!) can be set
up many different ways, impacting spurious response.

Hans' measurements of phase noise would include spurii, but I'm wondering if all the
noise averaging would hide sharp spurious responses of jittery PLL, mushing them into
the measured noise floor of about -135dBc/Hz? I see that some phase noise is
measured not including spurs, i.e. between spurious peaks.
Perhaps the internal SI5351 fractional PLL that takes 27 MHz up to nearly 900 MHz
is mostly contributing to the -135dBc/Hz measured noise floor? All guesses, since
the SI5351 data sheet is so sketchy. Silicon Labs could learn a thing or two
about documentation from QRP-Labs ;-)

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