[TangerineSDR] GPSDO Thoughts

[John Ackermann N8UR]

Thanks for that Lyle.  The 570 is a neat device, but as you said is
stand-alone.

We were looking at the SiLabs 543x series clock generator/cleanup/driver
chips which are basically complex synthesizers with virtually arbitrary
input frequency and multiple independent outputs settable to anything
from 1 Hz to over 1 GHz.  The niftiest versions use a 48 MHz crystal
oscillator as a phase noise cleanup and can get to a pretty impressive
noise floor (something around -150 at 144 MHz) but not as good as the
Crystek 122.88 MHz VCXO.

But they're in the $13-18 price range (depending on type), have some
fiddly layout requirements (want a six layer board), and require
ultra-low-noise regulators at significant current, so as much as I love
the idea, I'm not sure it's the right choice, at least for a first-out
version.

For just a 122.88 output, the Crystek with a four-way output buffer is
cheaper and cleaner.  And we've learned what caused the PLL performance
problem in the Hermes-derived boards, so hopefully can avoid that this
time around.

73,
John
----

On 9/29/19 6:27 PM, Lyle Johnson wrote:
> 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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