Some OCXOs have built in temperature outputs, sometimes its just a pir of wires connected to the ends of an internal thermistor.
Bruce > On 08 October 2019 at 20:21 Tobias Pluess <tobias.plu...@xwmail.ch> wrote: > > > Hi Bruce, > > oh yes I know, for sure no special resistor will be used for the oven heater! > By the way, many GPSDOs I have tested (e.g. Trimble Thunderbolt, STAR4) show > info about the OCXO temperature when connected via Lady Heather. Do you know > how they estimate the temperature? I checked almost every IC on the STAR4 PCB > and was unable to find a temperature sensor. Is it possible to estimate the > oven temperature from the current drawn? (for sure it is, but most likely it > will be strongly dependent on the particular OCXO used, I think). > > OK, so using a PWM DAC is more a matter of cost than something else. In this > case I think I am fine with a DAC8501 or DAC8560, which are both based on a > resistor string, and therefore are also monotonic by design. > > Tobias > > ________________________________________ > From: time-nuts [time-nuts-boun...@lists.febo.com] on behalf of Bruce > Griffiths [bruce.griffi...@xtra.co.nz] > Sent: Tuesday, October 08, 2019 00:19 > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Question for my new GPSDO > > 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.plu...@xwmail.ch > > mailto:tobias.plu...@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 > > > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com > > mailto:time-nuts@lists.febo.com > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
