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

Alan Melia alan.melia at btinternet.com
Sun Nov 20 20:00:48 EST 2016

Michael a couple of wild thoughts, make sure there is no 500kHz there (this 
is the crystal fundamental and the maintaining circuit should be 
degenerative there. (Is it a Meecham Bridge?)

That sounds like a baseless IO GT valve (tube in US ) enclosure which was 
probably originally evacuated....has the vacuum gone soft in 50 years?....I 
dont know how you would check, but I think that would cause a lowering of 
the frequency.


----- Original Message ----- 
From: "Michael Ulbrich" <mul at rentapacs.de>
To: "Time-Nuts" <time-nuts at febo.com>
Sent: Sunday, November 20, 2016 11:49 PM
Subject: [time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?

> 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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