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

KA2WEU at aol.com KA2WEU at aol.com
Sun Nov 20 19:44:30 EST 2016

I am surprised that this is the 2.5 MHz  (XSD)and not the new 5 Mhz  
crystal. XSD2 ) Bernd Neubig , on distribution always has a wealth of  information 
and part. 
There was a later R&S ( XSD 2 )  5 MHz frequency standard,  interchangeable 
with the XSRM Rb standard..
I looked at the Internet but did not find a surplus one . Sorry , Ulrich  
Rohde  N1UL
In a message dated 11/20/2016 7:05:11 P.M. Eastern Standard Time,  
mul at rentapacs.de writes:

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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