[time-nuts] Choosing a GPS IC for carrier phase measurements

Nicolas Braud-Santoni nicolas at braud-santoni.eu
Sun Aug 19 17:53:52 EDT 2018


On Sat, Aug 18, 2018 at 10:13:49PM +0200, Magnus Danielson wrote:
> >> 2) replace the internal oscillator with one phase locked to your OCXO.
> >> [...]
> > 
> > If you go this route, the close in phase noise of your signal needs to be in the
> > “pretty good TCXO” range. There are indeed some sources out there that
> > are way to noisy ….
> 
> Indeed. You want both pretty good wideband noise and close-in.

Yeap. The TCXO I currently use is meant for GPS applications, but I'm been
switching to a Si549 [0], which boasts jitter as low as 95fs RMS.  :O

Part of the reason for the switch is that, in my current setup, I get enough
noise from my MCU's DAC (for the voltage control of the XO) that it sometimes
creates additional, measurable phase noise.  :(

Obviously, going for entirely-digital control removes the problem: the only
remaining timing-sensitive, analog parts being the RF input for the GNSS chip,
and the clock distribution from the XO to the GNSS IC and to the frequency
synthesis chip (I currently have an on-board Si5334 so I can output any
frequency I need, but I'm planning on using a Si5341 for the new PCB)


[0] https://www.silabs.com/documents/public/data-sheets/si549-datasheet.pdf


Best,

  nicoo
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