[time-nuts] Re: pulling some crystals

[Ed Marciniak]

Perhaps someone far wiser than I can chime in, but here’s my experience with crystal aging:

I ordered some 100.00000 MHz TO-5 crystals specified for frequency west type phase locked brick oscillators from international crystal. They were also ordered with forced aging.

I put one in the brick, and attached an FRS-C to my frequency counter. I then over a month or more, plotted the frequency versus time.

The resultant curve was an exact parabola with no jumps. I recall there being a slight diurnal variation as a result of the room temperature changing by a few degrees. I don’t have the graph handy, but I’d say the ballpark aging rate was around 20-25 parts per billion per month at the SC cut turnover temperature. The diurnal component was in the vicinity of 1-3 parts per billion. In fairness, if that sounds too good to be true, it’s possible that the linear power supply and brick circuitry could have partially cancelled or even enhanced the frequency change.

All that said, that was a sample size of two. We also have no way of knowing what exactly forced aging corresponds. It could be that there’s shelf stock that’s been sitting for a year. It could be that it’s run in a fixture at higher temperatures, or higher drive power or both. It could even be that rhe crystals were stored in an elevated temperature. It could be a mixture of all three or something else I’ve failed to account for. Bottom line, unless there’s a specific callout or specification you don’t really know where you might fall on what might happen to be a parabola.

What we do know is that if the drift were linear that if it drifted for 120 months at the first month drift rate, you’d likely have no chance of havaing enough adjustment range. What’s also not clear is what would happen if you took an old crystal and mated it to new components or even components run for a few months. My guess is that part of the aging is component aging other than the crystal.

It would be interesting to take some ocxo modules apart and replace the transistors, diodes and op-amps and see if devices past their pulling range could be brought back into range, and what the aging rate looks like.

In summary, based on my personal experience, I’d bet money on the aging being parabolic excluding grossly poor components other than the crystal. While you don’t know where on the parabola you are, you can estimate.

P.S. this was done on a bench that was stable enough to see interferometer fringes frozen for long periods of time unless an 18 wheeler drove by.
________________________________
From: Bob kb8tq via time-nuts <time-nuts at lists.febo.com>
Sent: Thursday, December 7, 2023 3:38:38 PM
To: glenlist at cortexrf.com.au <glenlist at cortexrf.com.au>; Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Bob kb8tq <kb8tq at n1k.org>
Subject: [time-nuts] Re: pulling some crystals

Hi

Their idea that 3 days at 105 is the same as 1 year is …. errrr …. ummm ….. questionable. Even more so in an oven application that will have the crystal running at something well above room temperature (our starting point for the thread was heating it to 60C ….).

Simple example:

A typical OCXO crystal can easily be 20C above the max temp spec on the device. If the OCXO is rated to go to 70C, that would be 90. In the case of 85C, you are up at 105C. If indeed 3 days is a year, those 85C OCXO’s would have an insane daily aging rate. In practice, that’s not what folks observe.

Bob

> On Dec 7, 2023, at 3:07 PM, glen english LIST via time-nuts <time-nuts at lists.febo.com> wrote:
>
> thanks Bob and Rick for your fine input, Rick thanks for the treatment , I will follow your recipe and report back. Yes these days I have troubles with AVAILABLE SMT RF transistors, they all have Fts 4-10 GHz and I generally resort to common base with ballast R. 2N5179 I used to put a bead on the emitter lead as described in "ARRL Solid State Design " remember that ?
>
> If I ran out of pull after 10 years  while  I can tell the radio just to go off freq a bit (digital fix in FPGA) , the issue then is the digital sample rate fixed to the oscillator  go off meaning the world runs fast or slow.
>
> From the manufacturer, Krystaly ,  on my batch of 3OT  98.304 crystals : I reproduce word for word here. These guys are very helpful.
>
> "
>
> All our crystals have undergo the process of preaging, also called "accelerated aging" (i.e. 3 days storage at +105 deg.C). This aging is equivalent to one year aging at normal temperature (according standard IEC 60122-1). The crystals produced for you had frequency shift after this preaging below 1.0 ppm, so it guaranties aging below 10 ppm after 10 years. The aging curve shows deceleration (saturation) in time.
> Information to pulling parameter:
> Trimsensitivity at frequency 98.304 MHz, 3.rd  overtone is approx. 35 ppm/pF.
> Trimsensitivity at frequency 32.768 MHz, https://urldefense.proofpoint.com/v2/url?u=http-3A__1.st&d=DwIGaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=JsDsKeR7cZC8wbZhIlxxBQ&m=UZDFnXR4FMcG48jE8ECaIGEclIxlkjzZpe2CGyGeHOqc-zil-duTUiDbA6tKkCCO&s=pg5Qi1TxJfA64zXmLFq-AL4HXlfdXl4I7X4uPUDjVmo&e=  overtone is approx. 400 ppm/pF so ten times higher."
>
> regards
>
> On 8/12/2023 4:19 am, Richard Karlquist wrote:
>>
>> When I was working for Zeta Labs, circa 1978, I was in charge of a huge order for 5th overtone VCXO's.  My boss told me to use the "standard Zeta VCXO design".  It had various problems, and I quickly decided to invent a new design that actually worked right, and quietly put it into production without alerting my boss.  The new design was wildly suc
>>
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