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

Attila Kinali attila at kinali.ch
Sun Aug 19 19:22:43 EDT 2018


On Sun, 19 Aug 2018 23:53:52 +0200
Nicolas Braud-Santoni <nicolas at braud-santoni.eu> wrote:

> > 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

You do not really want to use a programable XO for GPS applications.
While it is true that they can get to pretty low jitter for integration
bandwidths of 12kHz to 20MHz, they have pretty awful close in phase
noise due to the fractional-N PLL they use. To give you an idea what
you are dealing with: the minimum integration time for GPS receivers
is 20ms (=50Hz), most high performance GPS receivers use a multiple
of those 20ms as integration time. Which means any spur you have in
the reference oscillator will lead to an unwanted (and wrong)
correlation peak inside the integration bandwidth of the GPS receiver.
Rather stick to a good, classical TCXO.

Same goes, BTW, for MEMS oscillators. as they are generally built
around a single frequency oscillator and a fractional-N synthesis
chain for the desired frequency. (one exception to this rule are
the 32kHz MEMS oscillators, which are generally designed directly
for this frequency and don't use a PLL)

			Attila Kinali

-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.



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