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

Bob kb8tq kb8tq at n1k.org
Mon Sep 3 11:09:20 EDT 2018


…. ummm ….. errrr …. cost wise a “pure FPGA” approach may or may not be
best for a GPSDO. Functionally it will do everything you need to do and more. 
Indeed you will need one that is big enough to do the job. That’s getting ever
easier as time marches on. Tools are generally a matter of what you already 
have for a toolchain and your experience with it. There are some terrific ( though
often expensive) tools for doing this and that on almost any device you can
think of.

Functionally, you can go “pure logic” and get things working. It’s maybe not the
way most of us would go. You can get FPGA’s with ARM cores hard coded into
the chip. You can also dump a wide range of MCU’s into the logic of the FPGA. 
Tacking a RAM chip on may well be required in some of these approaches. 

Lots of options !!!!


> On Sep 3, 2018, at 2:49 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 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 
> range.
> 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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