[time-nuts] GPS-disciplining an ordinary VCXO?

SAIDJACK at aol.com SAIDJACK at aol.com
Mon Sep 29 13:50:58 EDT 2014

Hi Mark,
that really is up to you and your skill-set. I don't use FPGA's in products 
 anymore because they are not field-serviceable generally, expensive, 
usually  require some sort of recurrent registration of the compiler, the ones I 
like  have external program storage so are not easily protected against 
theft, and are  not needed if you have a good Microcontroller and a handful of 
discrete gates. I  do use PLD's from time to time to do simple dividers etc.
But integrating everything into an FPGA for a one-off if you are versed at  
programming them and know how to do IIR and FIR filters etc is a completely 
 different story.
Lastly, there is no need to generate a DC signal with a 1-bit DAC  
(sigma-delta, PWM etc) for a one-off design since there are very good and  low-cost 
12 bit or 16 bit DAC's available. I would use two cheap 12 bit DAC's  (SPI 
or better I2C) cascaded to give me 20+ bits equivalent DAC resolution.  Don't 
forget that the DAC reference is just as important as the DAC, maybe even  
more so for applications that will see larger temp variations.
As I said before we have discussed this subject on numerous occasions  over 
the last decade in very great detail, you may want to search and read  
through the archives - there is great detail on all of the above.
Nowadays you can get a great 10MHz OCXO on Ebay for $10, buy a  
PLD/FPGA/Micro eval board for less than $30, and add a DAC and some low-pass  filtering 
and voltage reference for probably less than $10. So you can do a  
complete, high-end double-oven GPSDO for around $50.. Adding the Crystek VCXO  onto 
an Analog Devices PLL eval board would give you the desired high-frequency,  
low phase noise output.
In a message dated 9/29/2014 10:36:14 Pacific Daylight Time,  
haunma at keteu.org writes:

Said,  would you suggest implementing the dividers and PFD in the FPGA, 
with  the digital filtering?  Or feeding the FPGA with some version of  the
PFD output?  I am trying to avoid an extra A/D step here, but I  have no
experience with it.  Post-filter, I am satisfied that a simple  one-bit D/A
with passive filtering will get me to 16 bits resolution for  the VCXO
control, enough for ppb resolution.

Thanks for the data  point on the vcxo thermal sensitivity; it's very  



Said Jackson [saidjack at aol.com]  wrote:
> Stephane, you will need to replace the analog low-pass filter  that 
> the phase comparator with a digital low pass filter to  get 0.1Hz or lower
> loop bandwidth.  This is what a GPSDO  does.  A simple PID loop is what
> accomplishes this  typically.
> On the thermal sensitivity of that Crystek vcxo:  it is slow enough for
> even a loop with 0.1Hz BW to compensate for it  easily if you shield the
> crystal from  airflow.

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