HP 10544 OCXO

Arnd, There is some really good info on the HP oscillator at

which describes uses for the HP Z3801A GPS Receiver/Time Standard. The page at gives you some really great info on setting the crystal turning point temperature for the best stability. This will help you to understand your questions about the thermistor also. The oven isn't set to a standard temp, but to the best temp that puts the crystal in the middle of its curve. The thermistor is spec'ed at 100K @ 25C and 9.93K @ 80C. You'll have to shop around for a suitable replacement, keeping in mind the physical size of the thermistor. Mouser, Digikey, Allied, etc. are good sources of those. After replacement, you'll have to go through the turning point calibration as described in the above article. The thermistor is part of the oven mass assembly, and isn't separately orderable from Agilent.

The thermal fuse isn't described in the documentation, just given a part number. I don't have the manual for the 10544A, but I do have one for the

10811A/B oscillator, which is very similar.

--
Dave M
MasonDG44 at comcast dot net  (Just subsitute the appropriate characters in 
the address)

Never take a laxative and a sleeping pill at the same time!!
"Arnd Gronenberg"  wrote in message 
news:crsdhn$imb$05$1@news.t-online.com...
> Hi,
>
> the HP 10544A OCXO driving my HP5328A counter failed due to a blown thermo 
> fuse. The underlying problem was a broken Thermistor lead (unfortunately 
> within the sealed hole) which caused the owen to run away (and blow the 
> fuse). I already searched, but didn't find answers to the following 
> questions. In order to find a suitable replacement thermistor, I would 
> like to know:
> -  At what temperature does the OCXO nominally operate?
> -  What are the characteristics of the original thermistor used by
>    HP? (my copy of the 10544 schematic lists the thermistor as
>    "9.93K @80C 0637-0122", but it is difficult to distinguish "6" and
>    "8")
> -  Which temperature specification is required for the thermo fuse?
>
> Thanks in advance!
>
> 73 de Arnd
>
> -- 
>                                Arnd Gronenberg
>                          EMail: arnd@gronenberg.com
>                              HAM: DJ9PZ / AB2QP

Hi,

Arnd-

Are you sure the thermistor is bad? Apparently HP designed this oscillator, as well as the 10811 series, with too low a temperature thermal fuse. It is a very common failure mode. They subsequently replaced the fuse with a higher temperature part.

A few months ago, someone in one of these newsgroups claimed to have been involved in the design of these oscillators, and believed the thermal fuse should be replaced with a piece of wire. He claimed that even if the oven's heater were turned full-on for a long period of time, the resulting temperature would not cause damage. If you choose to replace it, be sure to get the newer part.

In the bridge circuit (of the 10811) there is a resistor hand-picked to match the crystal, that determines temperature. If you are sure the thermistor is bad, then Dave M's suggestion is a good one.

Fred

Hi Dave,

thanks a lot for your reply! And also to Fred for his remarks.

The information regarding the temperature calibration is very helpful. I found a replacement thermistor, which seems to be suitable: 83K @ 25C,

10.24K @ 80C, reaction time 0.7s, size 1.5mm diameter.

As the HP 10544A uses an AT cut crystal I assume that I have to calibrate for a frequency minimum (according to the above mentioned website). As the HP 10544A contains a 10 turn variable resistor instead of a hand-picked resistor, calibration should be quite straight forward.

Thanks again, best regards, 73, Arnd

--
                                Arnd Gronenberg
                          EMail: arnd@gronenberg.com
                              HAM: DJ9PZ / AB2QP

If your oven works at 80 degC, it contains an SC cut crystal and not an AT. AT crystals have their optimum temperature at abt. 28 degC, they are close to useless at 80 degC.

73, Gerhard dk4xp

[...] >

Hmm... [...] > The big difference between the 10544 and 10811 is the change from > AT cut to SC cut crystal. [...] > Rick Karlquist N6RK > HP Santa Clara Division > snipped-for-privacy@scd.hp.com

That's why I assumed that the 10544 contains an AT cut crystal. I would be interested to know which temperature calibration procedure is applicable to the 10544...

73, Arnd DJ9PZ / AB2QP

--
Arnd Gronenberg, DJ9PZ / AB2QP

AT.

This is totally wrong.

In the first place, the 10544 definitely uses an AT cut crystal, not an SC cut. (The 10811 uses an SC cut). I worked at the HP Santa Clara Division where these oscillators were made for 19 years.

Secondly, 28 degrees is the *inflection point* for an AT cut, not the

*turnover* temperature. For NON-oven applications, you want to operate at the inflection point. For oven applications, you want to operate at a turnover temperature. An AT cut can certainly be made with a turnover around 80 degrees if cut at the proper angle.

Definitions:

Turnover: first derivative of frequency w.r.t. temperature = zero Inflection: second derivative of frequency w.r.t. temperature = zero

Rick Karlquist N6RK

While we are talking HP 10544 stuff...

I bought one on eBay recently. It seems to keep time ok, but the output is not a sinewave. Looks like a sine wave flattened near zero volts.

That means something is broken, no? Any ideas if it is worth trying to fix?

Why? It seems like this would just minimize your frequency stability.

Yes, I understand "inflection point." You explained it well, but operating where the second derivitive is zero also automatically puts us where the first derivitive is at its max (pos or neg) so the slope is the steepest here.

-

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

-----------------------------------------------