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

Hal Murray hmurray at megapathdsl.net
Tue Sep 30 01:05:36 EDT 2014

haunma at keteu.org said:
> 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.

One bit D/As need a filter.  The stuff that gets through the filter turns 
into spurs.  Are you OK with that?

16 bits at 10 MHz is 6553.6 microseconds or 152 Hz.  (and harmonics)

If you use the classic PWM with X 0s followed by N-X 1s (where N is a power 
of 2) you get one level of spectrum out.  If you have something like a FPGA, 
you can use DDS like logic to interlace the 0s and 1s and move some of the 
energy to higher frequencies where it is easier to filter.


There are 2 reasons why you need lots of bits on a D/A for a GPSDO.  You need 
low bits to get small adjustment steps.  You need high bits to cover a wide 
adjustment range.

You can avoid some of the high bits if you are willing to do a manual 
adjustment.  (Use a pot for the coarse adjustment and the D/A for the fine 
adjustment.)  How many bits you can save depends on your application and/or 
how often you are willing to do the manual adjustment.

At a given clock rate, fewer bits in the D/A means the spectrum will be 
higher frequency and easier to filter out.

These are my opinions.  I hate spam.

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