[time-nuts] GPS Disciplined TCXO

Nick Sayer nsayer at kfu.com
Fri Oct 23 10:30:10 EDT 2015

> On Oct 23, 2015, at 2:25 AM, Attila Kinali <attila at kinali.ch> wrote:
> 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
> either.

I’ve run them with the same LDO and they work. The LDO gets warm, of course, but then, so does the oscillator. When I compare them to the same circuit, but powered with the SC189Z switcher I use for the OH300 variant, their performance is identical. Which is to say, a half order of magnitude worse than the DOT050V at tau 1s.

The data sheets of both claim a short term stability of 1e-9 at 1s. The DOC is exceeding that, but the DOT is *slaughtering* it. I routinely see 9e-11 instead. I asked Connor Winfield about that and they said that they should change the data sheet, as my observations are more in line with reality. Meanwhile, the OH300 (I’ve only tried one of them so far, however) is at around 3e-11 at 1s. So that’s what $100 buys you. :)

The other thing I saw with the DOC020V that I have not seen with either the DOT or OH300 variants is a weird oscillation on the VC pin. I posed the question to Stack Exchange and they suggested that the VC pin internally goes to a varactor which adds too much capacitance for the output of the OP amp. I asked Connor Winfield about this and they didn’t have an immediate answer. The latest boards have the footprints for another RC LPF on the output to counter this, but since I gave up on the DOC part, the cap is no-stuff and the resistor is 0 ohms when I build them out (with the other parts). There’s also a footprint for a cap in parallel to the feedback resistor on the op amp in case further bandwidth limiting is ever desired, but so far adding capacitance there hasn’t had any impact on the output, so that too is no-stuff.

>>> 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).

I do see that now. If you look at the footnote, figure 27 which it references shows that the Vdd headroom with a VRef of 3.3 volts is more like 20 mV or so. I would posit that the headroom issue is more likely to result in non-linearity at high DAC values, perhaps because the output amp may not be capable of full rail-to-rail operation. In practice, I expect the actual DAC values to be constrained within a relatively narrow range clustered around midpoint. Now, 15 years of aging may change that, I suppose. But at the moment, out of the dozen or so I’ve built so far and played with, I’ve not seen a DAC value (in normal operation) outside of ±0x1000 from midpoint.

>>> * 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 latest boards do have a designated spot where you can sever the analog section supply. I (or you) could rather conveniently separately regulate the analog section that way to see if it changes anything. That’s not a separate Vref, though. That would be less than trivial without revising the board. You’d need to lift the VRef pin from the DAC and wire it to something in the air.

In practice, the VC output of the analog section is quite stable (more stable than the analog supply rail), however, because of the LPF on the DAC's output. Remember that the DAC was designed with a maximum clock speed of 30 MHz and we’re using it effectively at 10 mHz. So the LPF trades bandwidth and slew rate (neither of which are interesting in this application) for stability. And to some extent you’d be throwing away the pains to which you went to come up with a super-accurate Vref because you then feed the DAC output into an op amp that’s powered by the analog supply rail, so its own noise and non-linearity would likely have a greater impact (at least, I would expect so).

>>> * 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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