[time-nuts] Question for my new GPSDO

[Bruce Griffiths]

Detecting the steep drop in oven heater current when the temperature regulator loop assumes control shouldn't require an extremely low tempco sensing resistor.

In principle PWM DACs can offer high resolution at relatively low cost, one tradeoff being the settling time of the low pass filter between the PWM output and the OCXO EFC input.

PWM DACs are inherently monotonic even when they have very high resolution.

Bruce 

> On 08 October 2019 at 10:06 Tobias Pluess <tobias.pluess at xwmail.ch mailto: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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