[time-nuts] Thunderbolt? (re simple gpsdo.)

Bob Camp lists at rtty.us
Sun Jan 1 20:30:48 UTC 2012


Hi

Actually a minimalist GPSDO *could* be a person looking at a scope and tweaking a pot every so often. It all depends on what the desired result is (how accurate / how automated / how failure resistant). Is simplicity measured by low cost, low parts count, easy to find parts, or easy to work with parts? Do parts I already have (OCXO, Rb, GPS) count or not?  Does time matter or just frequency?  Is there a "that's good enough" level (as in do you need a GPSDO at all)? Does design time count / does it need to be an existing design? 

Without some definition, there really is no way to answer. 

Bob

On Jan 1, 2012, at 6:23 AM, Tom Harris wrote:

> I too have been mulling over a minimal GPSDO.
> 
> Has anyone mentioned a PWM DAC? Even a "toy" microcontroller like the
> Atmel AVRs can generate 16 bit resolution PWM, which sounds like it
> would be heaps. As long as the voltage reference is low noise, it does
> not matter if it drifts slowly, since the control loop adjusting the
> OCXO frequency will adjust it. Of course you will need a long time
> constant filter to smooth the PWM, but a 2 pole filter constructed
> from an opamp will do the job.
> 
> The micro can also capture the 1PPS from the GPS if it is clocked by
> the OCXO, and use the value to close the control loop.
> 
> So a minimal GPSDO looks like a simple microcontroller (I would use
> Arduino since we use these at work whenever we need to build something
> in an afternoon) with a filter for a PWM DAC, so one opamp and a few
> discretes. Of course as I write software for such beasts as a trade I
> am giving the software a zero cost.
> 
> On 31 December 2011 06:46, Stanley <timenuts at n4iqt.com> wrote:
>> 
>> 
>>> 
>>>> "What is the simplest design for a GPSDO that uses only the PPS signal
>>>> from
>>>> a modern GPS?"
>>> 
>>> 
>>> Some sort of oscillator with a voltage control.
>>> CPU with a timer/counter that can capture the PPS.
>>> DAC.
>>> Software.
>>> 
>> 
>> How about MSC1200 : http://www.ti.com/lit/ds/symlink/msc1200y3.pdf
>> 
>> Stanley
>> 
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> 
> 
> 
> -- 
> 
> Tom Harris <celephicus at gmail.com>
> 
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