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

Bob Camp kb8tq at n1k.org
Sun Nov 20 22:21:29 EST 2016


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.
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