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

[Brian K]

"Brian, Sorry to say I'm pretty sure you have a faulty OCXO.  I have used
several of those and the control voltage to reach 10 MHz is usually
around 5V, which can be easily achieved from the Reference V output,
which is around 8V.  It looks to me like there is a fault in the
oscillator circuit inside the inner oven.  Total control range should be
at least 2 Hz either side of nominal.  I have one in my Shera GPSDO and
over more than two years the control voltage has crept up very slowly
due to ageing and shows a very slight diurnal variation due to room
temperature changes."

Thanks Dan.

On my device, the control voltage for 10MHz sometimes does drop to somewhere between 5.5V and 7.5V.
It seems to depend on the temperature inside the equipment case.
This only becomes fully stable after the unit has been powered on for a few hours.
My guess is that the thermal inertia of the case and the hardware inside is probably quite high so it takes considerable time to stabilise.
The case has plenty of ventilation slots in the lid and the base.
The air temperature in the vicinity of the OCXO is hard to measure but I would guess something like 35->40 degC when stable (I need to recheck that).

You mention that you have an 8663-XS in your GPSDO.
Have you ever tried starting it up from cold with GPS locking disabled?
Ignoring absolute accuracy, how long does it take for the OCXO frequency to stabilise?
I wouldn't be surprised if you find a similar issue.

Brian.

________________________________
From: Dan Rae via time-nuts <time-nuts at lists.febo.com>
Sent: 28 October 2023 02:07
To: Brian K via time-nuts <time-nuts at lists.febo.com>
Cc: Dan Rae <danrae at verizon.net>
Subject: [time-nuts] Re: Oscilloquartz 8663-XS stability problem

On 10/27/2023 7:15 AM, Brian K via time-nuts 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.



And there's no trimmer inside, the solder blob was presumably when they
filled it with whatever inert gas they used at manufacture.

Dan
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________________________________
From: Dan Rae via time-nuts <time-nuts at lists.febo.com>
Sent: 28 October 2023 02:07
To: Brian K via time-nuts <time-nuts at lists.febo.com>
Cc: Dan Rae <danrae at verizon.net>
Subject: [time-nuts] Re: Oscilloquartz 8663-XS stability problem

On 10/27/2023 7:15 AM, Brian K via time-nuts 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.

Brian, Sorry to say I'm pretty sure you have a faulty OCXO.  I have used
several of those and the control voltage to reach 10 MHz is usually
around 5V, which can be easily achieved from the Reference V output,
which is around 8V.  It looks to me like there is a fault in the
oscillator circuit inside the inner oven.  Total control range should be
at least 2 Hz either side of nominal.  I have one in my Shera GPSDO and
over more than two years the control voltage has crept up very slowly
due to ageing and shows a very slight diurnal variation due to room
temperature changes.

And there's no trimmer inside, the solder blob was presumably when they
filled it with whatever inert gas they used at manufacture.

Dan
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