[time-nuts] Simple solution for disciplining OCXO with 1 PPS

[Hal Murray]

BNeubig at t-online.de said:
> Hello Fellow time-nuts, I am looking for a simple solution to discipline my
> 10 MHz reference OCXO in my private lab with an 1 PPS signal from a separate
> GPS receiver. I am curious if there is a solution possible without
> programming a microcontroller, as I am an old-fashioned "analogue" guy ;) 

There are a couple of things you can try.

Use a simple pot and adjust it by hand.  You will have to run some tests to 
see if that is stable enough for your needs.

The basic problem with the normal analog circuit for a PLL is that the time 
constant on the filter is way too long to build out of practical parts.  The 
software in a micro doesn't need to be very complicated.  (But since it's 
free once you have the micro most systems would probably add features beyond 
the bare minimum.)  You can probably build something with no software but 
using a lot of digital logic.

The key part is the DAC.  There is a tradeoff between how many bits you need, 
the range it will cover and the step size.  You can get a reasonable step 
size with non-fancy parts if you are willing to sacrifice range.  The reduced 
range means you will have to readjust the centering occasionally as things 
drift.

I'm not sure that's a practical approach, but if I wanted to play the 
no-micro game, that's the direction I would investigate.

Suppose you have a 12 bit DAC connected to the top bits of a 20 bit up/down 
counter.  The phase comparator says up or down every second.  It takes 256 
counts to bump the bottom bit of the DAC, so that's (handwave) a time 
constant of roughly 256 seconds.

I'd probably try it in a micro before I built anything.  :)




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