[time-nuts] GPS Disciplined TCXO
Attila Kinali
attila at kinali.ch
Thu Oct 22 08:17:40 UTC 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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