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

Discussion:

Michael Ulbrich

2016-11-20 23:49:54 UTC

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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KA2WEU--- via time-nuts

2016-11-21 00:44:30 UTC

Permalink

http://www.classicbroadcast.de/downloads/rohde_XSD.pdf

Greetings,

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,
***@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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Alan Melia

2016-11-21 01:00:48 UTC

Permalink

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.

Alan
G3NYK

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

Post by Michael Ulbrich
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.
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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https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

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Graham / KE9H

2016-11-21 02:05:49 UTC

Permalink

Is your oven doing what it is supposed to be doing?

Sounds more like an oven problem than a crystal problem.

--- Graham

==

Post by Alan Melia
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.
Alan
G3NYK
Sent: Sunday, November 20, 2016 11:49 PM
Subject: [time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?
Hi there,

Post by Michael Ulbrich
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.
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.
_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/m
ailman/listinfo/time-nuts
and follow the instructions there.

_______________________________________________
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ailman/listinfo/time-nuts
and follow the instructions there.

Alexander Pummer

2016-11-21 03:09:53 UTC

Permalink

"that should cause lowering the frequency" and it will lower the Q of
the crystal too, therefore it will need more drive!

73

KJ6UHN

Alex

Post by Alan Melia
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.
Alan
G3NYK
Sent: Sunday, November 20, 2016 11:49 PM
Subject: [time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?

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No virus found in this message.
Checked by AVG - www.avg.com
11/20/16

Bob Camp

2016-11-21 03:21:29 UTC

Permalink

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

Post by Michael Ulbrich
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.
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.
_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Attila Kinali

2016-11-21 14:23:54 UTC

Permalink

Hoi Bob,

On Sun, 20 Nov 2016 22:21:29 -0500

Post by Bob Camp
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 :)

Do you have that data available somewhere? I would be very much
interested in it.

Attila Kinali

--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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Bob Camp

2016-11-21 16:14:52 UTC

Permalink

Post by Attila Kinali
Hoi Bob,
On Sun, 20 Nov 2016 22:21:29 -0500

Post by Bob Camp
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 :)

Do you have that data available somewhere? I would be very much
interested in it.

We had a few thousand oscillators that had to be replaced due to coil issues
in the 1970’s. We were not the only ones with the problem. Other outfits had very
similar experiences. Eventually the coil guys took a look at the materials and
“changed something”. After that the coils did not have as much aging when
run at 100C forever and ever.

Bob

Post by Attila Kinali
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
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Graham / KE9H

2016-11-21 17:20:10 UTC

Permalink

The core of the inductors are commonly powdered iron (of various alloys),
held together with a plastic binder.
The early plastics shrank and changed shape with time, changing the
inductance over decades.
Later binders are much more stable.
Not much you can do about it, other than change the inductors.

--- Graham

==

Post by Bob Camp
Hi

Post by Attila Kinali
Hoi Bob,
On Sun, 20 Nov 2016 22:21:29 -0500

Post by Bob Camp
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 :)

Do you have that data available somewhere? I would be very much
interested in it.

mailman/listinfo/time-nuts

Post by Attila Kinali
and follow the instructions there.

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mailman/listinfo/time-nuts
and follow the instructions there.

Michael Ulbrich

2016-11-21 23:31:09 UTC

Permalink

Hi all,

thanks a lot for your helpful suggestions and pointers to sources of
additional information.

I also got a few replies off list and one list member may actually have
a spare crystal!

Will report to the list as this project (hopefully) progresses.

BTW what is the "attachment policy" of this list? I will try to attach 2
- small - images of the crystal (side and top view) and see if they get
through somehow.

Thanks again ... Michael

Bob Camp

2016-11-22 00:24:24 UTC

Permalink

Hi

Thanks for the pictures !!!

Another possible thing to watch for: It may take a *long* time for the oven to
stabilize at the correct temperature.

Bob

Post by Michael Ulbrich
Hi all,
thanks a lot for your helpful suggestions and pointers to sources of
additional information.
I also got a few replies off list and one list member may actually have
a spare crystal!
Will report to the list as this project (hopefully) progresses.
BTW what is the "attachment policy" of this list? I will try to attach 2
- small - images of the crystal (side and top view) and see if they get
through somehow.
Thanks again ... Michael
<XSD_2.5_side.jpg><XSD_2.5_top.jpg>_______________________________________________
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and follow the instructions there.

Azelio Boriani

2016-11-22 00:00:56 UTC

Permalink

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

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

Post by Bob Camp
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

Post by Michael Ulbrich
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.
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.
_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

_______________________________________________
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Bob Camp

2016-11-22 01:06:58 UTC

Permalink

Hi

If the part is 50 years old that pre-dates the invention of the SC cut. AFIK, all of the
QK glass bottle crystals date to the “pre SC” era. The BT is about the only other
alternative from that era. HP made it their “goto cut” for OCXO’s back then. Others
may have followed along ….

Bob

Post by Azelio Boriani
<http://www.conwin.com/pdfs/at_or_sc_for_ocxo.pdf>

Post by Michael Ulbrich
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.
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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