[time-nuts] Question for my new GPSDO

Tobias Pluess tobias.pluess at xwmail.ch
Mon Oct 7 21:06:37 UTC 2019

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,

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