[time-nuts] GPS Disciplined TCXO

Attila Kinali attila at kinali.ch
Thu Oct 22 04:17:40 EDT 2015

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.

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.

* 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. 

* 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.

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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