[time-nuts] Question for my new GPSDO

Wed Oct 16 16:30:19 UTC 2019

Hi Attila,
I have read all your messages, but for the sake of simplicity, I only reply to your last one.
First, thanks for sending the schematics. I have a look at them. To see the components you selected is indeed very helpful.
I attached my own schematics of the EFC control circuit. I think with this circuit, it should be possible to achieve a similar resolution as you mentioned in your thoughts.

D3 is a 16bit resistor string DAC and therefore inherently linear. With a OpAmp gain of 4, 0 to 10 Volts can be produced at the output of N2A. This is sufficient for the UCT-108663 oscillator (which seems to be the same as the Oscilloquartz 8663). For the AXIOM75 oscillator, only 0 to 5 volts would be required, so the OpAmp's gain would be reduced to 2 in that case.
Further, the OpAmp N2B adds an offset to the DAC voltage. For the UCT oven, one would choose a gain of 2 for this OpAmp, such that it produces 5 volts; for the AXIOM oven, the gain would be 1.
Thus, at C5, the voltage is the average of the output voltages of N2A and N2B which effectively lowers the tuning range of the OCXO, and the full 16 bits of the DAC can be used for this narrower tuning range.
On the lower part of the schematic, I have added a circuit which is very similar to that one I reverse-engineered from a commercial GPSDO. N4 is an ordinary analog switch which has a very low on-resistance (2.5 Ohms). A PWM signal toggles the analog switch. N5A and N5B are configured as a Sallen-Key lowpass filter with a cutoff frequency around approx. 1 Hz. If a 16bit PWM would be used, the PWM frequency would be around 1.2 kHz; I measured a similar frequency in the commercial GPSDO so I assume that should be sufficient. The Sallen-Key lowpass filters the PWM very effectively.
My first idea was to have the PWM DAC as an alternative to the resistor string - on my PCB, one could populate either the upper part or the lower part and use one of the two DACs. However, I recently found an application note from Jim Williams where he used two DACs and combined their output voltages such that he could effectively achieve a 20bit DAC. And when I read your email again, you suggest also something around 20 bits, so basically I could to the same thing: I could use only a, say, 8 bit PWM (and therefore a quite high switching frequency) and let the Sallen-Key filter produce a variable offset voltage, while the DAC is used only for the fine-tuning. I have not made any sophisticated calculations so far about this, but from a first gutt feeling I'd say that this configuration would also allow for something around 20 bits, no?
In this case, one would populate resistors R3 and R16 and remove R8. The Sallen-Key then should have a gain of 4 for the UCT oven or 2 for the AXIOM oven, and with a 8 bit PWM could do a coarse adjustment of the OCXO voltage while the resistor string DAC would do only the fine tuning. I wonder whether this circuit would even allow 24 bits, but I guess no (at 24 bits of resolution one would possibly take into account nasty things like building of thermocouples on solder joints or mechanical stress of components due to soldering and other stuff I perhaps have never heard of nor I have even thought about ;-) so this is another rabbit hole I'd like to avoid for now).

As a side note, I have foreseen as an option to populate a better voltage reference; the DAC I planned to use has its own internal reference having 2ppm/K. The external reference is N3, having only 1ppm/K.

Further, I have considered another OpAmp than you suggested; mainly because I have already used that OpAmp in another application where I needed 16 bit ADCs (which worked fine) but also because I have lots of them (and I have not yet known the LTC2057). The AD8626 I use has an offset voltage of around 1mV having 2.5uV/K drift. Both specs seem to be acceptable. The main difference is that your LTC2057 has zero drift - but otherwise, "my" OpAmp seems to have quite similar specs compared to your LTC one, so I guess my choice was at least in the right direction :-) but a zero drift amp would definitely be interesting. However, if the temperature is stable enough, the drift should also not matter too much since it is so low.

OK when the EFC circuit is somewhat sorted, I still have to think about the frequency measurement with the aid of the TDC7200 as interpolator. This point is still open.

One further note about the long time constant of 1000s or even more:
I read somewhere that the more modern OCXOs no more use those long time constants; 1000s or more is more appropriate for old ovens like the HP10811A and similar because these ones seem to use a different type of crystal cut - which is bigger and more stable (?) but I don't know too much about this. >From what I know from Ulrich Bangert's famous paper I'd also say that around 1000s is appropriate, but tests will be necessary for the particular oven to confirm the correct value. At least for the AXIOM75 oscillator 1000s is not appropriatem because this crystal has really good phase noise but not so good ADEV and therefore some 100s or so would be better for that. For the UCT-108663 I don't know yet.

Best
Tobias
HB9FSX

________________________________________
From: time-nuts [time-nuts-bounces at lists.febo.com] on behalf of Attila Kinali [attila at kinali.ch]
Sent: Wednesday, October 16, 2019 10:17
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO

On Tue, 15 Oct 2019 18:43:29 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> You can get a few bits by reducing the tuning range.  Suppose you follow the
> DAC with an OpAmp with a gain of 1/8 while the reference is adjustable.  That
> adds 3 bits of resolution by decreasing the output range.  The downside is
> that you have to manually set the adjustment.

Unless you use fixed, metal foil resistors, then the temperature
coefficient and the 1/f noise of the resistors will dominate the
system. Using a pot is even worse, as these have quite high 1/f noise.
If you go through the math, it turns out that using two DACs with
metal foil resistors is both going to be lower in noise/tempco
and also giving you the ability to tune the "midrange point" in software.

> That adds the op-amp to the temperature consideration list.

The opamp should be on this list anyways. The input offset tempco of
most opamps is high enough that it beats those of the DAC.
Hence, either precision bipolar or auto-zero/chopper opamps
have to be used.

> > While temperature coefficient is important, it is not as important as you
> > think. Unless you want to operate your GPSDO outside or where you expect
> > large temperature swings. Instead, what you do is give your whole GPSDO
> > enough thermal mass, such that you don't see a significant temperature change
> > at time scales shorter than the loop time constant.  ...
>
> How big a box of whatever do I need to get the thermal time constant large
> relative to 100 or 1000 seconds?

Let us do a quick back-of-the envelope calculation:
Let us assume we have to keep the temperature within 0.1°C within 1000s
Let us further assume we have a 1°C temperature jump.
First thing we need is an estimate of the thermal resistance of the
system. Something in the order of 10K/W to 100K/W are probably sane
values. Let's go with 10K/W for a worst-case scenario.
Now we go from a temperature difference of 1K to 0.9K over 1000s.
This is low enough that we can assume the temperature changes linearly,
which gives us an average temperature difference of 0.95K. Over 1000s
with 10K/W that means we are transfering 0.95/10*1000=95W·s. Ie we need
to provide 95J over a temperature difference of 0.1K, which means we need
a heat capacity of 950J/K. Going to your physics book, you see that
aluminium has 900J/(K·kg) of heat capacity. Which means we need about 1kg
of aluminium.

                                Attila Kinali

--
<JaberWorky>    The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.

_______________________________________________
time-nuts mailing list -- time-nuts at lists.febo.com
To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Schematic Prints.pdf
Type: application/pdf
Size: 21065 bytes
Desc: Schematic Prints.pdf
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20191016/655a77d7/attachment.pdf>