[time-nuts] GPS Disciplined TCXO

Attila Kinali attila at kinali.ch
Fri Oct 23 05:25:40 EDT 2015

On Thu, 22 Oct 2015 21:07:38 -0700
Nick Sayer via time-nuts <time-nuts at febo.com> wrote:

> > 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.

BTW: Tom van Baak mentioned in a private mail, that the DOC supply current
is much higher than i thought it was. So it isn't exactly a drop in replacement

> > 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.

In the AD5061 datasheet (Rev C) on page 3, at the bottom,
section "Reference Input/Output", item "V_REF Input Range".
Yes, it will not fry the device, but it will not work up to spec either
(unfortunately, manufacturers never mention what part of the spec gets
degraded if you violate something).

> > 
> > * 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.

Variations in Vdd will change the oscillator frequency slightly (20ppb for 5%).
Changes in Vref will have a higher impact on the frequency. Also, the control
voltage of an oscillator is usually referenced against ground, ie its "processing"
is more or less independent of Vdd.

> > 
> > * 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.

Oops.. right. Missed that the resistors arent of equal size.

			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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