[time-nuts] Question for my new GPSDO

[Tobias Pluess]

Hi Jim,

oh yes, I have not shown yet the full schematics because I am not finished yet. But sure, it includes a phase detector. I plan to use a STM32 microcontroller with a timer being used in capture mode, and I plan to add a TDC7200 as external interpolator. (I am still thinking about the additional glue logic needed to interface with the TDC7200, because this IC can only measure up to 8ms, but with 55ps resolution).

So the OCXO will not operate in an open loop. However, I found that the tempco of the resistors I have is 100ppm/K. So, even if the OCXO is operated with a PLL, if the temperature in my lab changes (e.g. door opened, window opened, whatever), the resistors change so much that the resulting change in voltage comes close to 1..2 LSB of my DAC, i.e. the frequency will be slightly off and the PLL will need to correct for that. Sure the PLL will detect the frequency error and correct it, but since the time constant will be some 10min or more, it will take a while until the frequency settles to the new value.

So I wonder whether my assumption about the resistor tempco is correct, and the commercial GPSDOs use expensive low-tempco resistors, or whether I am exaggerating a bit.

BTW, thanks for the tip with the AD5680. I will plan this as an alternative, so that any of DAC8501/8560/AD5680 can be fitted.

Tobias


________________________________________
From: time-nuts [time-nuts-bounces at lists.febo.com] on behalf of Jim Harman [j99harman at gmail.com]
Sent: Tuesday, October 08, 2019 01:11
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Question for my new GPSDO

Tobias,

You have not shown the rest of your GPSDO, but presumably it includes a
phase detector and processor which implement a phase locked loop that
compares the oscillator phase with a 1 pps signal from the GPS.

For open loop VCXO testing, you will certainly need to be concerned with
the tempco and stability of the DAC and associated circuits. But for a
GPSDO, because the DAC and associated circuitry are inside the PLL, you do
not need to be excessively concerned with the temperature stability of this
part of the circuit.

The PLL will generally have a time constant of hundreds to a few thousand
seconds, so the DAC can be very slow. Other designs use a slow but high
resolution PWM DAC because it is inexpensive and guaranteed to be
monotonic. If you want to stick with a hardware DAC you might consider the
18 bit AD5680, which is a 16 bit DAC that interpolates 2 additional bits.



On Mon, Oct 7, 2019 at 6:02 PM Tobias Pluess <tobias.pluess at xwmail.ch>
wrote:

> Hi Guys,
>
> I am planning to make a new version of my own GPSDO. I have attached the
> schematic of the OCXO and DAC. Because the stability of my previous design
> was not yet optimal, I now chose better components; my main criteria was
> the lowest tempco I found.
> As one can see, I plan to use the DAC8560, which is a 16-bit DAC having an
> internal 2ppm/K voltage reference. Alternatively, the DAC8501 could be
> used, which requires an external voltage reference for which I selected the
> ADR441B (typically 1ppm/K).
> I have different OCXOs which I want to test; one of them requires a 0 to
> 5V EFC voltage, and the other has 0 to 10V. By changing the gain of the two
> noninverting amplifiers, both OCXOs can be fitted. The two 1k resistors
> make the tuning range a bit smaller.
> The OpAmp I chose has 2.5 uV/K tempco.
> Besides that I have fitted an additional OpAmp to measure the OCXO
> current. The Oscilloquartz STAR4 GPSDO I have has the same design; I assume
> it measures the OCXO current to determine when the warmup time has elapsed.
> I was now consdering the tempco of the resistors involved. Should I use
> there resistors having an especially low tempco, or are ordinary 1%
> resistors fine?
> One last question; I have further analyzed the STAR4 design and I saw that
> they are using a PWM DAC. Almost all GPSDOs I have ever seen uses PWM, why
> do they do that? what is the advantage over a DAC similar to the one I
> selected?
>
>
> Thanks for your comments,
> best
> Tobias
> HB9FSX
>
>
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