[time-nuts] Question for my new GPSDO

[Bob kb8tq]

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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