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

Bob Camp kb8tq at n1k.org
Mon May 23 11:46:10 UTC 2016


Hi

Is a DAC in our out? 

I can (with some effort) build the entire computing engine needed for a GPSDO out
of 7400 series logic. It’s actually easier than you might think. Since that’s “small 
scale logic” is it in bounds? 

To me at least, analog is just that, no gates, no 1’s no 0’s, just a good old analog
computer. They went out of style a while back, but they still work …Once upon a 
time in a land far away, control theory labs made students set up loops on them and
do stuff like run motors :)

Bob

> On May 22, 2016, at 6:52 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
> If you can build with small scale TTL gates you can save the need for
> a huge precision capacitor by storing the current EFC value in a
> 16-bit register/counter.  This 16-bit register drives a DAC.   Then
> you build a phase detector that can detect if the XO leads or lags the
> GPS.  If it leads you increment the counter, if it lags you decrement
> the counter.  Every second you either adjust the register or do
> nothing.   The DAC is nothing more than 16 precision resisters each
> one twice the value of the next one, each connected to one bit of the
> register by a transistor switch.
> 
> I would only implement this as exercise to show how cool 1970's
> technology was.   It would be far more impressive if you limited
> yourself to 1960's technology.  I say this because one of my other
> hobbies is building tube based audio gear.  The last thing I built was
> a spring reverb, using a steel spring as the delay.    I already have
> the software version but the physical spring is more fun to use. I
> think a 60's or 70's vintage GPSDXO would be unique if strictly
> limited to period correct technology
> 
> On Sun, May 22, 2016 at 2:55 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> Hi
>> 
>> Of course it’s possible, you simply need enough high quality film capacitors to make up the control loop integrator.
>> You probably also would use a pile of the same capacitors to slow down the PPS jitter a bit before it went into the loop.
>> The setup process would probably involve a bunch of toggle switches or relays to walk the loop into the long time constant
>> mode. You would be wise to put the caps all in some sort of oven arrangement. That way odd drafts hating the caps would
>> not drive the GPSDO nuts.
>> 
>> Bob
>> 
>>> On May 22, 2016, at 1:41 PM, Bernd Neubig <BNeubig at t-online.de> wrote:
>>> 
>>> 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 ;)
>>> I am well aware, that such a solution would have a lot of disadvantages, as it cannot effectively compensate for short-term variations. However I would be happy if such a KISS solution could achieve  a stability (STS) of better 1E-10 over an hour. I know this is a damned long integration time for an analogue integrator...
>>> If that sounds too weird, I am open to receive advises for a microcontroller based solution.
>>> 
>>> Thanks a lot for your comments to come.
>>> BTW: you need not to teach me about basics of short-term stability. I just want to evaluate the limits of a possible analogue solution. For sure, that's not real disciplining, but more like a long-tau integration PLL
>>> 
>>> Best regards
>>> Bernd DK1AG
>>> 
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> 
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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