Re: Capacitance standardizers

Stan or Patricia Griffiths <w7ni@...>
 

Joseph Orgnero wrote:

Could that be the gadget that Fair Radio Sales is selling? They call ot
Input RC Normalizer (PN 067-053-00) . It is in their feature items section
of their web page.
Hope this helps
Joe Orgnero VE7LBI
Hi Guys,

This discussion about scope capacitance standardizers is an interesting but
very old one. It has been going on for at least 40 years that I know of . . .
yes, since at least 1960 when I joined Tektronix in the Factory Calibration
area. Let me confirm a few things I have heard here.

First, Dean is certainly correct that using a probe instead of a real
capacitance standardizer will not give you really enough amplitude at the
highest attenuator ranges. It is also true that the real standardizers used
x2 attenuation and will give you enough amplitude to do the job right at even
the highest attenuator ranges.

Second, Tek DID change over from calling these things "Capacitance
Standardizers" to "Input RC Normalizers" sometime in the mid-60's. The main
reason is that when they were called "Standardizers", cal labs, both military
and commercial, wanted to "standardize" them and have them traceable to the
National Bureau of Standards. This really did not make good technical sense
but that is sometimes the result of calling an item by an improper name . . .
in this case, "Capacitance Standardizer". We have to remember what the goal
is that we are trying to achieve by doing all of this in the first place . . .
We are trying to make sure a probe can be attached to a scope input and the
input attenuator switched to its other positions without the necessity of
readjusting the probe compensation for each attenuation range. For a scope
with a nominal 20pf input, does it really matter that this is done with 19,
20, or 21 pf? NO. If you do it with 17 or 23pf, there is a good chance you
will run out of adjustment range on some of the capacitors used for adjusting
the attenuators. A 20pf standardizer will generally keep you in the center of
the range of most of the adjustment caps so it is best to use one. If you buy
one of these standardizers at a flea market or on eBay, there is no guarentee
it is accurate since it has a variable cap inside of it and it could well have
been misadjusted by anyone who touched it . . . There is a calibration
procedure for Input C Standardizers, too, but that is beyond the scope of this
note.

Now to the second part of the name change from "Capacitance Standardizers" to
"Input RC Normalizers". It turns out that there is two parts to the scope
input that determine proper compensation. If the input R (usually 1 megohm)
is off tolerance significantly, like, say 5%, you probably cannot properly
adjust the input C and the attenuators for optinum squarewave response. This
is because it is the RC timeconstant of the input C AND the input R together
that determines how the adjustments are made. This is the reason that those
devices were later called "Input RC Normalizers".

So what do you do if you have an "Input RC Normalizer" with a UHF connector
installed on it and your scope is equipped with BNC connectors? You COULD use
a UHF/BNC adapter but this will change the input C by a couple of pf and
therefore the accuracy of the adjustments. This is not necessarily a serious
problem but it could throw the input C off enough to make it impossible to
make correct adjustments. This is why Tek made TWO series of these
"Standardizers/Normalizers" . . . one series for UHF inputs and one series for
BNC inputs. There are at least 9 different "Standardizers/Normalizers" in the
complete set. Also, The earliest ones used "011-xxxx-xx" part numbers and the
later ones used "067-xxxx-xx" part numbers. Here is a list of the ones I know
about. I will list "067" number first, previous "011" number second (if it
had one), input C in pf, and connector series:

067-0533-00, 011-0022-00, 20pf, UHF
067-0534-00, 011-0029-00, 24pf, UHF
067-0535-00, 011-0030-00, 47pf, UHF
067-0536-00, 011-0051-00, 12pf, BNC
067-0537-00, 011-0073-00, 15pf, BNC
067-0538-00, 011-0066-00, 20pf, BNC
067-0539-00, 011-0067-00, 24pf, BNC
067-0540-00, no "011" number, 33pf, BNC
067-0541-00, 011-0053-00 and 011-0068-00, 47pf, BNC

I have not researched why there are TWO 011 numbers given as equivalent to the
067-0541-00 but I guess I could if anyone really thinks its important . . .

So if you want to do this job the way Tek recommends, you need the whole set
to be sure you have the right one for the next Tek instrument you may see. I
think I have the whole set. Some of these normalizers such as the 33pf BNC
one are rarely used since there aren't many instruments that have 33pf input
C.

So, you can see why Dean's other suggestion about making your own 2X variable
RC Normalizer makes good sense. It could save you a lot of money and
frustration in trying to acquire the complete set for a relatively unimportant
reason. The reason I have them is because I have an obsession with trying to
complete a collection of Tek stuff and this is part of it. There is no other
good reason. My wife has certified that I am crazy in this regard. Who else
(other than a bonafide nut) would own more than 1150 old Tek products?

If we go WAY back in Tek history, Tek was adjusting input C and attenuator
compensations before there were any "Capacitance Standardizers". They did it
using probes and they even had an X2 probe to use on the higher ranges.

Stan
w7ni@...

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