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