[time-nuts] Re: Oscilloquartz 8663-XS stability problem

[Neville Michie]

The phenomenon that you see may be due to moisture absorbed/adsorbed by one or more of the components. While in storage the unit equilibrates with maybe 60% RH. When the oven is operated the RH is reduced to near zero. (Each degree C reduces RH by about 10%)
It may take weeks to settle.
Cheers
Neville Michie


Sent from my iPhone

> On 28 Oct 2023, at 10:06 am, Brian K via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I recently constructed a homebrew 10MHz distribution amp.
> It has provision for both an external 10MHz source such as a GPSDO or an internal OCXO.
> For the internal OCXO I chose a used Oscilloquartz 8663-XS from China.
> 
> When I began testing the unit I discovered an issue with frequency / temperature stability.
> 
> I plotted a graph that shows the external temperature of the OCXO can as the device warms up.
> It stabilises at 60degC after around 45 minutes.
> 
> I also plotted the frequency error measured using a Racal 1998 with an external 10MHz clock from a GPSDO.
> The frequency also appears to stabilise after around 45 minutes.
> 
> [cid:0acb1b9d-a986-47a3-85de-53b95bb8479e]
> 
> So far so good.
> 
> However I find that if I leave the unit running for a few hours, the frequency gradually drifts upwards by up to 0.5Hz.
> That is well outside the 8663-XS specification. It is supposed to remain within 4x10^-9 from -20 to +70degC (I assume that is external ambient temp).
> The OCXO temperature (as measured on the outer can) remains constant at 60 degC.
> The tuning voltage and the supply voltage (12V) also remain constant.
> The tuning voltage comes from a LH0070 10V precision reference and a 10 turn 10K trimmer.
> I expected the tuning voltage required for 10MHz to be around 5V. The spec indicates a range of 0->10V.
> I found that the tuning voltage needed to be set to just below 10V to get 10MHz.
> Initially I thought that perhaps that's because it's an old used device.
> Subsequent tests however make me think it's related to temperature.
> 
> I find that if I remove the equipment case lid, the frequency drifts slowly downwards.
> When I replace the lid, the frequency error slowly recovers.
> I assume the OCXO temperature drops slightly when the lid is removed despite it having ventilation slots directly above the OCXO.
> I expected the OCXO internal temperature to be maintained to within around +/- 1degC by the heater control circuit such that it would compensate for small external temperature variations.
> 
> I am at a loss to explain what is going on and am running out of ideas.
> It appears that the OCXO internal temperature regulation is not working as well as it should.
> Is it just my particular device that behaves in this way or is it a common issue with this OCXO?
> 
> I don't want to buy another used device from China (around 40 GBP) only to find the same problem.
> 
> I would welcome any thoughts/suggestions from the experts here at time-nuts.
> 
> For the full story, please refer to my recent thread in the eevblog forum:-
> https://www.eevblog.com/forum/projects/oscilloquartz-8663-xs-ocxo/
> 
> 
> [cid:18711b70-068e-474b-8ab5-74e886152aa1]
> 
> Regards,
> 
> Brian.
> 
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