[time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?

[Azelio Boriani]

A comparison between AT and SC cuts' temperature stability can be found here:

<http://www.conwin.com/pdfs/at_or_sc_for_ocxo.pdf>

On Mon, Nov 21, 2016 at 4:21 AM, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>
> If the frequency drops as the oven warms up, you have an AT cut crystal. If
> the frequency goes up as the oven warms up, you have a BT cut crystal. With an
> AT or a BT, the frequency change between room and “hot” will depend a lot on
> the details of the proper oven temperature. A frequency shift of 20 to 40 ppm is not
> at all unusual as the oven warms up. The oscillator will only tune on frequency
> once the oven is hot.
>
> http://www.aextal.com/tutorial-frequency-stability.htm
>
> Regardless of which cut you have (an AT is the best guess). The oven temperature
> would be adjusted to put the crystal at it’s minimum sensitivity point. For an AT that
> is the lowest frequency. If the oven is not heating to the correct temperature, you will
> probably be unable to get the oscillator on frequency.
>
> If there is a circuit problem, the most likely culprit are the inductors. They form tuned
> traps that put the circuit onto the correct overtone. This increases the circuit’s sensitivity
> to changes in inductance. 50 year old inductors may have been made with core materials
> that aged more than just a little bit. I have empirical data on this :)
>
> As others have suggested, check the oven heater along with the oscillator circuit. The
> issues you see might come from either one.
>
> Bob
>
>> On Nov 20, 2016, at 6:49 PM, Michael Ulbrich <mul at rentapacs.de> wrote:
>>
>> Hi there,
>>
>> I'm new to this list and have some interest in quartz crystal and
>> rubidium oscillators - GPS, NTP, PPS and "clock watching" in general ;-)
>>
>> I snatched an R&S XSD frequency standard accompanied by an XKE frequency
>> controller. Found the manuals on KO4BB's site (Thanks a lot for that!
>> BTW there's a slight mix-up of pages in that some of the XKE pages have
>> found their way into the XSD manual).
>>
>> My initial hope of a quick and easy restoration project faded a bit when
>> I looked at the XSD output frequency after heating up the oven. It was
>> off by about -1 * 10-5 and after cranking the fine tuning for some time
>> and having another look at the specs I realized that the frequency was
>> too far off to be dialed in by the fine tuning which only covers about
>> +/- 2 * 10-7.
>>
>> Next step was to take apart the oven and check series capacitor,
>> oscillator and the crystal itself. I found that even the coarse
>> adjustment range of the cylindrical series capacitor (40 - 110 pF) would
>> not allow to pull the crystal to it's specified frequency. When
>> replacing the series cap by a ceramic and lowering the value to just
>> before the oscillation breaks down (about 10 pF) I managed to set the
>> oscillator frequency offset to  +2.5 * 10-6 at room temperature. But
>> even this will not suffice when taking into account that the frequency
>> will drop by a few parts in 10-5 (cf. XSD manual) when the oven heats up
>> to its operating temperature. I also checked some of the components on
>> the oscillator PCB which might have an influence on frequency but could
>> not find any fault.
>>
>> The crystal itself is a disk of about 30 mm diameter mounted in a sealed
>> glass cylinder of about 38 mm dia. and 43 mm height. There is no socket
>> just 2 bare wires.. It does not show any visual signs of damage.
>> According to a reference given in the R&S XSD manual the crystal's
>> construction follows a publication from A.W. Warner "Design and
>> Performance of Ultraprecise 2.5-mc Quartz Crystal Units" in Vol 39,
>> Issue 5 of Bell Labs Technical Journal (Sept. 1960). According to that
>> it is an AT-cut 5th overtone design.
>>
>> Now my questions:
>>
>> a) Considering that this gear is about 50 years old, a "crystal gone
>> bad" situation shouldn't be that much of a surprise, right?  But could
>> there be any other cause of the "huge" frequency offset besides a bad
>> crystal? I would very much appreciate any idea that I could try to get
>> this baby back on spec.
>>
>> b) if nothing else helps: Could any of you give me a hint about who
>> might be able to supply a spare crystal? I tried my directly reachable
>> contacts but unfortunately to no avail so far. Please consider that
>> similar crystals might also have found their way into other
>> manufacturer's constructions from that era - Sulzer, Racal, HP ... you
>> name it ...
>>
>> Many thanks in advance!
>>
>> Best regards ... Michael U.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.