[time-nuts] Question for my new GPSDO

[Jim Harman]

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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-- 

--Jim Harman