[time-nuts] Question for my new GPSDO

Tue Oct 8 19:52:59 UTC 2019

Jim mentions the LM34 for sensitivity and LM34 or LM35 for cost compared
with the DS1620. But also look at the BME280 : it has digital measurement
of temperature with 0.01C resolution (50 times better than the DS1620),
costs £5 on digikey but less mounted on a breakout from ebay, and also
measures pressure. The very similar BME280 also measures humidity.

On Tue, Oct 8, 2019 at 5:01 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> What is the temperature stability of the OCXO you plan to use? In the case
> of
> open windows, what is the *dynamic* temperature stability of the OCXO?
> What is the tuning range of the EFC on the OCXO?
>
> The wider the EFC range on the OCXO, the more the tempco of the control
> system
> impacts the output. With a small enough range, you may not need a very
> good
> DAC at all. With a super big EFC range then the requirements go up.
>
> As long as the EFC stability is better than the OCXO’s temperature
> stability, there
> really is no benefit to improving it.
>
> One example of all this is not an OCXO, but it’s the same sort of idea. A
> Telecom Rb
> might have a tuning range of +/- 2 ppb. It also might have a tempco of 0.1
> ppb over
> a 50 degree range. That comes out to 0.002 ppb / K. It also comes out to
> 1,000 ppm/K
> in terms of the tuning range. (2/0.002). 100 ppm resistors / Dac’s /
> references would
> be overkill in this case.
>
> None of that gets into the issue of dynamic change. Pretty much every
> component
> you will find is rated for a gradual rather than fast change. The impact
> of a fast change
> can be orders of magnitude worse than a slower change. ( so indeed, don’t
> open the
> window :) )
>
> =======
>
> If you want to go a bit nuts (this being time nuts):
>
> A change in the temperature changes the current through the OCXO heater.
> That
> current flows through a ground pin. The ground pin has a resistance. On
> pretty much
> all OCXO’s that ground pin is also in the EFC ( = it is the EFC return).
> Thus ground
> current “tunes” your OCXO.
>
> However you go about measuring the tempco of your OCXO, be careful that
> the ground
> current is handled the same in your test as in your final circuit …..
>
> Bob
>
> > On Oct 8, 2019, at 2:21 AM, Tobias Pluess <tobias.pluess at 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-bounces at lists.febo.com] on behalf of Bruce
> Griffiths [bruce.griffiths at 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.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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