[TangerineSDR] GPSDO Thoughts

[Lyle Johnson]

We used the Si570 in the K3S and KX3 synthesizers.  Excellent noise characteristics but can’t be slaved to an external reference.  We use a SiLabs synth in the KX2 that can use an external reference but noise performance is much worse.

FWIW,  Lyle KK7P

Sent from my iPhone

> On Sep 29, 2019, at 2:39 PM, John Ackermann N8UR via TangerineSDR <tangerinesdr at lists.tapr.org> wrote:
> 
> To the TangerineSDR list --
> 
> I've spent the week since DCC thinking about GPSDO questions and getting
> things in place to do some experiments.  I was going to put together a
> starting-point paper and send it to you and a few time/gps-nuts but
> thought it was better to get some data first.
> 
> In quick summary:
> 
> A GPSDO is nothing more than a crystal oscillator ("XO") with an EFC
> input that is steered to frequency by reference to the precise time
> available from a GPS receiver, usually in the form of a pulse-per-second
> signal.  The crystal oscillator can have excellent short-term stability,
> but will drift (age) over time and is subject to environmental
> variables, particularly temperature.  The GPS system has excellent long
> term stability and accuracy, ultimately tracking USNO(UTC) but short
> term usually requires long averaging times to reach that performance.
> 
> The task of the GPS designer, given the known performance of the XO and
> the GPS constellation, is to optimize the control loop to extract the
> best of both.
> 
> A better XO allows the time constant of the control loop to be longer.
> A quieter GPS implementation allows the time constant of the control
> loop to be shorter.  What's of interest to me is that a shorter control
> loop implies lower performance requirements on the XO, and that might
> result in a way to lower overall GPSDO cost.
> 
> This wouldn't be very interesting except that there are now some low
> cost GPS receiver modules available that might -- maybe, perhaps,
> possibly -- provide a lower-noise GPS time reference.  u-Blox
> (https://www.u-Blox.com) has released a bewildering variety of
> navigation and timing modules with varying capabilities.  I've attached
> a table that I put together by extracting data from the u-Blox web site.
> 
> I'll soon have my hands on five of these modules with different
> capabilities (and price points).  Once I've had a chance to take some
> initial measurements, and verify some specs that aren't clear from the
> documentation, I'll provide an updated report that might serve as the
> basis for some design discussions.
> 
> I'm also reaching out to a few friends in the time-nuts world to get
> some recommendations for readily available 10 MHz XOs at a couple of
> different price/performance points that we can at least use to provide
> cost information.
> 
> Finally, I'm thinking about whether the SiLabs frequency synthesizer
> chip is the most cost-effective way to get the low-phase-noise 122.88
> MHz performance we need.  It might be cheaper, and better, to use the
> Hermes scheme, where a very low jitter 122.88 VCXO is locked to the 10
> MHz reference -- provided we fix the known problem with the Hermes
> implementation.  The synthesizer offers flexibility, but I think its
> cost (with required supporting components) will be greater than the
> Crystek 122.88 oscillator module, and its phase noise performance not
> quite as good.
> 
> Anyway, more to come.
> 
> 73,
> John
> 
> 
> 
> <uBlox_GPS_Comparison_v2.pdf>
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