Larry Benko wrote:
> Mike,
>
> This is getting away from the topic even farther but a few people
> might find it interesting. When doing a cal on the 8753 family of
> VNAs from HP you have to pick a cal type (7mm, N female, n male,
> 3.5mm). The HP cal standards are not perfect (at really high
> frequencies) and the opens and shorts actually have different
> reference planes aside from being offset from the connector plane.
> After calibration the analyzer subtracts these offsets to get back to
> a reference plane that is at the connector which HP talks about in
> several documents. In my case, chapter 5 and particular page 5-9 of
> the 8753 analyzer book details the issues clearly.
> After doing a cal. and then measuring S11 of either the short or open
> standard gives crummy results. This bothered my for a long time till
> an HP rep explained the problem. However the 3.5mm standards have
> almost exactly the same offset for both the open and short. By
> telling the analyzer you are using 3.5mm standards and using them or
> discrete opens and shorts and including 33ps of electrical delay after
> the cal fixes the majority of the problems when trying to make
> impedance measurements far away from 50+j0.
>
> I may not have been super careful when I cal'd the analyzer and 0.5pF
> seems about right from the numbers I posted. Everyone is correct
> about 1pF being deal at 30Mhz when trying to measure high impedances.
>
> Does this make sense?
>
> 73, Larry, W0QE
>
Hi Larry,
Yes, I've also noticed the offset between the open/short location and
the cal reference plane. I use primarly 2.4, 2.92, and 2.4mm standards
in my work. I don't recall specifically checking the 2.92 or 2.4mm, but
I do recall observing that the 3.5mm standards seemed to have similar
cal plane offsets. Offhand, I don't recall the offset that I observed,
but 33ps sounds about right.
One lesson I learned the hard way is that after making lots of microwave
measurements it is easy to get seduced into thinking that expensive cal
kits and fancy 12 term correction will guarantee a good measurement at
HF frequencies. As this thread aptly points out, even at 30 MHz a
little stray C can cause large errors when the impedances are very high
relative to 50 ohms even when you are using a $30,000 analyzer.
73, Mike W4EF.........................
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