HP calibration resistor out of spec

Maybe it got fried.

DVMs are so good nowadays, you hardly need to keep lab-standard resistors around. Our Keithley 2000's are usually within 10 or so PPM of each other.

John

That means they have good internal resistors. Or they all drift together? As for the 4191 reference resistor, it has to be good to 1GHz, so that's not an ordinary resistor. There isn't an easy way to correct for an out-of-spec calibration resistor, is there?

I guess we'd better all go do a multimeter check of our wideband cal resistors.

--
 Thanks,
    - Win

Hello John,

It was never used other than for calibrating that impedance analyzer and it's signal level is quite low. This is actually the resistor from the cal kit for that analyzer.

It's a 1MHz through 1GHz impedance analyzer, not any DC gear. Mostly for transducer measurements. But I am thinking whether I should keep it. This thing is huge and heavy, good old HP stuff. Every time I have to pull it for cal or things like that I have to make sure there is enough Motrin in case a back pain creeps up on me.

Regards, Joerg

Hello Win,

After this discovery I would do that. As to my HP4191A it seems to have another Problem. Got to take it apart, again. A few months ago the back-up NiCd batteries conked out. How could they, being only 20 years old...? Replaced them and now it starts to read erratic, not a good thing. Probably a power supply issue.

Anyway, while you check the cal resistors look at the large gear in your lab and estimate how old it is. Those NiCd backup batteries can keep on humming just fine while they begin to leak. The ones in the HP4191A had made a huge puddle of greenish gunk by the time the unit signaled something was wrong. Only after losing the cal data upon power cycle did I become suspicious. Luckily there is an aluminum panel underneath that caught the stuff that had oozed out.

Every time I lift that thing I figure that there has got to be something much lighter and simpler these days to do impedance analysis up to 1GHz. Or at least to a few hundred MHz.

Regards, Joerg

On Tue, 25 Apr 2006 23:08:58 GMT, Joerg Gave us:

The degree of precision is what determines the width of the band of variance from the declared value it can have.

Actually, yes. If it's fried, it will surely be broadband fried.

Go through the lab and ohm out all your SMA attenuators, and toss any that measure different on opposite ends.

John

Ah, and how many kilowatt pulses did your lab mate dump into it ?

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

It's not a "calibration resistor"- it is a wideband termination specified in terms of broadband VSWR performance to 0.01dB. The DC resistance reading means little to nothing- within reason, and 52.5 is within reason.

Strictly speaking it's not a cal resistor, as zero ohms and open is used for that purpose. But as Joerg said, it's a sanity-check cal resistor, which you use as a last step in cal to be sure everything is working properly. In that sense a 5% error is a *big* problem, these instruments with their 4.5-digit readout normally work to 25x better than that, the datasheet plot says 0.5% typical at 50 ohms.

But I think you're right, it's not a "correct for an out-of-spec calibration resistor" issue, except perhaps to write the measured DC resistance on it with a Sharpie. :-)

--
 Thanks,
    - Win

Um, you got fooled by the "50 ohm system" business.

Many "50 ohm" things are actually 50, 51, 52, 52.5, or 53.5 ohms.

IIRc the reason for that unusual number is something like: ~75 ohms is the impedance of a vertical half-wave antenna, 37.5 ohms is approximately the best impedance for power transfer, so 52.5 was chosen as a happy medium. Which got rounded off to 50 for convenience in many places. But coax comes in 50, 51, 52, 52.5, or 53.5 ohm versions.

I don't know for the 4191, but on the 4195A you can enter specific values for the 0R, 0S, Load cal parameters. So I guess 52... is not much of a pb. Just accurately measure the DC value and set it accordingly. I think we can also expect the reactive part to stay stable.

--
Thanks,
Fred.

No way the DC resistance has any resemblance to the 18GHz impedance where the whole assembly becomes a distributed network with both internal and external impedances and voltages and currents integrals of vector dot sums, especially in a Type N package. Given the extremely flat VSWR, one would suspect advanced engineering concepts similar to that of the ideal distortionless line Zo=srt(L/C) and RC=LG etc, and why not, as long as the thing is expected to be costly anyway. Now if it was a Pasternack termination, then you're talking a badly soldered stock carbon resistor etc...

Sounds like a fun "group project"... I was happy to see that Agilent has the user's manual available for download on their web site, with a sizable section on the theory of operation.

It seems that there are a lot of device out there that implement some of the

4191A's features... so-called "antenna analyzers" that read output input impedance over a wide frequency range, or even just a regular network analyzer measuring S11... which strike me as probably having a good chunk of the hardware necessary for building a 4191A, but are lacking in the appropriate software to make them as "useful."

Joerg wrote in news:e_x3g.51899$ snipped-for-privacy@newssvr14.news.prodigy.com:

That's why all standards get calibrated;to VERIFY that they are what they say they are.

--
Jim Yanik
jyanik
at
kua.net

So both of your Keithleys are drifting out of spec. at the same rate? ;-)

--
Michael

Is it possible, that even though it is called a "50 ohm" resistor it really is supposed to be 52 ohms because the standard (matching sold by HP) instrumentation cabling is 52 ohm?

RG-8/U is actually 52 ohms to 52.5 ohms.

Other "50 ohm" coaxes are technically 53.5 or so (RG-58/U).

Tim.

Hello Fred,

The 4191 requires a third 50ohm cal run and I believe it enters into the calcs. Tried it: When using that 52.5ohm cal resistors from then on it displays 52.5ohms as 50ohms. 5% error right there, not a good thing.

This instrument only goes to 1GHz and it should be quite easy to make a good 50ohm terminator up there. When I was a teenager I made one for a kilowatt that was VSWR flat to 300MHz. The ref needle wouldn't even lift. I didn't ever need it above 144MHz, other folks got theirs flat to much higher bands. The trick is to strictly adhere to a cone shape architecture. I did not calculate the sloping back then but got it out of a book. Wish I still had that resistor array but I threw it out a year ago. It was done "economy style" inside an old wild honey bucket and now it started to leak oil. And I must have used it almost full bore without checking the oil level back then since the top resistors were a bit charred.

Regards, Joerg

Hello Rene,

No lab mate here and I bought it used from a company where the cal kit was always locked away. It's in a little suitcase. Plus it's not a standard N-connector so it would not fit on antenna cabling.

Regards, Joerg

Hello Ancient Hacker,

Nah, this is HP stuff. If it says 50ohms on there it should be 50ohms. I've got other gear from them and that is all 50.0 ohms, like it should be.

Regards, Joerg