[TangerineSDR] GPSDO Thoughts

[John Ackermann N8UR]

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



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