[time-nuts] 1PPS to 32.768 khz

Dave Brown tractorb at ihug.co.nz
Wed Oct 19 05:26:27 EDT 2016

Might be easier to run the GPSDO up at 16,384 kHz, like the E1 area telcos 
used to do (and some still do)- then divide that by 500 when your done with 
the PLL side of things and you have a clean stable 32.768 kHz reference to 
feed into the micro in place of the xtal.
 But first off, I'd be inclined to dig into the micro circuitry in the clock 
and see if and where you might be able to feed in the 1 PPS  instead of that 
derived from the 32 kHz xtal.
DaveB, NZ

----- Original Message ----- 
From: "Chris Albertson" <albertson.chris at gmail.com>
To: "Lee - N2LEE" <lee at n2lee.com>; "Discussion of precise time and frequency 
measurement" <time-nuts at febo.com>
Sent: Wednesday, October 19, 2016 7:26 PM
Subject: Re: [time-nuts] 1PPS to 32.768 khz

> What you want is a GPSDO.  Most people here build these to run at 10MHz.
> But you need one that runs at 32768 kHz.
> Try this:  Divide the crystal oscillator's output by 32768.  This will 
> give
> you one Hz.   Compare the phase of this with the phase of your PPS from 
> the
> GPS.  Push and pull the crystal oscillator so that the phase is locked.
> The first and most critical step is to build a good 32768 Hz oscillator
> that can be fine tuned by a controlling voltage.   Usually a reverse 
> biased
> diode is used because the capacitance of the diode depends on the voltage
> across it.     You'll need a very good quality 32768 Hz voltage controlled
> oscillator (VCO).
> In short, go the other way around, don't try and scale up the 1PPS, scale
> the 32K down to 1Hz
> That said, y you really don't have to divide the 32K signal.   Measure the
> time from the zero crossing of the PPS to the nest zero crossing of the 
> 32K
> signal and control the VCO to keep this time constant.   What you are
> building isa phase locked loop
> Now my questions.
>> 1. Does anyone know of a device that will take a 1PPS GPS timing signal
>> and generate a 32.768 kHz sine wave output ?
>> I have big digital clock that uses an 8 bit micro processor and an
>> external 32.768 crystal. Obviously the external crystal is
>> awful for accuracy.
>> I have searched every where using as many search terms as I can think of
>> and can’t believe there is not a device that performs
>> this function.
>> I have found a couple of Epson RTC chips that might come close that 1pps
>> but I don’t think that corrects the 32khz just the clock time.
>> 2. 10 Mhz Freq Standard
>> I am not in the same league as the majority of the list members and just
>> starting to dabble in GPSDO stuff.
>> I have tried to find a thunderbolt as a starting device but it appears
>> those are either dried up or people want too much money.
>> I wanted to get the opinion of anyone who has tried the Leo Bodnar GPSDO 
>> ?
>> For the money it appears to offer a beginner a lot of features.
>> Thanks and reading the daily digest of what you guys are working on.
>> I have a feeling if I am not careful you guys could cost me a lot of
>> money. hehe
>> Lee
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> -- 
> Chris Albertson
> Redondo Beach, California
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