[time-nuts] GPS Disciplined TCXO

Nick Sayer nsayer at kfu.com
Fri Oct 23 00:07:38 EDT 2015

> On Oct 22, 2015, at 1:17 AM, Attila Kinali <attila at kinali.ch> wrote:
> On Tue, 20 Oct 2015 13:53:54 -0700
> Bryan _ <bpl521 at outlook.com> wrote:
>> Saw this on the Hackaday site if anyone is interested.
>> https://hackaday.io/project/6872-gps-disciplined-tcxo
> Oh.. that one looks nice.
> I like the choice of the TXCO. Which allows you to replace it
> by the pin compatible DOC050V and have an OCXO instead.

That actually is a downgrade. I’ve tried both, and the DOC020V’s short term stability is much worse. I’m guessing that the OCXO’s selling point is it’s medium to long term stability, which is mooted by the GPS discipline here.

> But I have three things to critisize:
> * The AD506x DAC family has a peculiar restriction: the maximum
>  specced Vref is 50mV below VDD. Ie connecting Vref to VDD means
>  that the DAC is used outside of specs and thus might or might
>  not get the speced accuracy/precision.

I don’t see that anywhere in the datasheet. Can you give me a reference for that? The only thing I see is an absolute maximum of Vdd + 0.3V, but of course “absolute maximum” ratings aren’t necessarily what you’re supposed to design to.

> * Using VDD as reference voltage, when VDD is generated by an LM1117
>  is kind of iffy. Neither the internal reference of the LM1117 nor
>  the control loop are very stable. There ar LDOs out there, that are
>  speced for use as ADC/DAC references, but this one isn’t. 

I’ve considered a separate regulator for the reference, but my thinking is/was that it would be bad for the supply voltage of the oscillator to be regulated separately from the reference. The two regulators might drift apart. Connor Winfield’s OCXO application note actually says this too. It’s too bad that these oscillators don’t have a reference voltage output, but I guess that’s part of what makes them cheaper.

> * The DOT050V has an upper limit for the control voltage of 3.0V,
>  (and a lower 0.3V) but the circuit could potentially supply 3.3V there
>  losing 10% (or 20%) of precision.

It shouldn’t be able to do that. The buffer amplifier is in a less-than-unity gain configuration with a Vdd/2 virtual ground. It’s designed to reduce the swing to 51% for the DOT050V variant or 82% for the OH300 variant.

> Taking these things into account, I would have used one of the cheap
> 3.0V refernces like a REF3030 (or even a 2.5V reference) to supply Vref
> to the DAC and changed the opamp circuit to slightly shift the 0V based
> voltage up so the lower limit becomes 0.3V and the upper limit 3.0V.
> 			Attila Kinali
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
>                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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