[time-nuts] Another "atomic" clock question

Jim Miller jim at jtmiller.com
Fri Mar 7 16:39:38 EST 2014

Didn't mean to cause a firestorm. Just used the term simple to describe a
Lead/Lag D FF phase comparator. I view it as simple compared to a high
speed counter. My GPSDO will have just such a FF whose state will be read
by the micro which will implement a PI filter in software and drive a 20bit
TI sigma delta DAC to apply corrections to the OCXO. Micro and all other
clocks on the design are driven from the OCXO. I may switch the GPS module
at a later date to one which provides sawtooth info if I really feel the
need and add a delay line. Frankly I think I'll never get around to it.

I'll publish a schematic, code and test results once I have something


Jim ab3cv

Date: Fri, 7 Mar 2014 09:23:54 -0800
From: Chris Albertson <albertson.chris at gmail.com>
To: Discussion of precise time and frequency measurement
        <time-nuts at febo.com>
Subject: Re: [time-nuts] Another "atomic" clock question
        <CABbxVHuc41UQMhgWNyCXdW=iChDg6taXeOKa+zDV0Hrrmo1uRg at mail.gmail.com>
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On Fri, Mar 7, 2014 at 5:25 AM, Jim Miller <jim at jtmiller.com> wrote:
> I think the hardware delay line approach is the only solution for a simple
> D FF lead/lag phase comparator. It would be placed ahead of the FF.

Simple?  You are going to need a micro controller and software to (1)
tall the GPS to output the sawtooth function, they don't typically
output it untell you tell it to. then (2) recover the sawtooth
function from the serial data.  Then(3) convert it to the "counts"
that units used in the delay line.  Finally (4) you need to interface
the delay line to the processor and send the current sawtooth function
value over that interface once per second.   Also when I do stuff like
this I always want some kind of LCD display or at least blink LEDS so
I know what's going on inside and then it is at least running.

Your simple analog devices no longer a simple analog device.   Do a
full up parts count for both designes.  I think the digital correction
comes in lower.  Both solutions need the same micro controller and
it's support circuitry.

As to which GPSes send sawtooth.  It's a common feature but typically
you need to enable it, the same way you'd enable a self-survey or set
a minimum elevation angle or whatever.

Chris Albertson
Redondo Beach, California

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