[time-nuts] PLL/GPSDO/etc learning resources for mere mortals

Hal Murray hmurray at megapathdsl.net
Mon Sep 3 02:49:38 EDT 2018

lists at packetflux.com said:
> I'm trying to fill in some gaps in my knowledge about PLL's, GPSDO's, etc.,
> with the goal to eventually implement some of these either in a
> microcontroller or fpga or some combination thereof.

An FPGA is unlikely to be the way to go for a GPSDO.  There is lots of time to 
do it in software and the tools for micros are generally easier to work with 
than FPGA tools.  (But if you like FPGAs, don't let me scare you away.)

One thing to keep in mind for GPSDOs is that the time constants for filters 
are very long relative to what is reasonable to build with Rs and Cs that are 
readily available.  The usual way to go is a D/A connected to a micro.  That 
moves the filter time constant into software.  Thus you will see lots of 
discussion on this list about which D/A to use.  Generally, you would like 
more bits than you can get.  For a one-off project, you can trade a reduced 
tuning range for better resolution if you are willing to use a pot (or 
soldering iron) for the coarse adjustment, aka the high bits on the tuning 

Another thing to add to your list is hanging bridges and sawtooth correction.

Another magic term associated with PLLs is PID controller - Proportional, 
Integral, Differential.  You may find some web articles that tell you enough 
to be helpful without using complicated math.

These are my opinions.  I hate spam.

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