Need further info on network analysis terms...
 

one by one.
through is actually a piece of cable used to connect the two ports of
the VNA, prion to making what we call 2 ports measuremets: gain or
transmission response. it just lets you set you baseline and calibrate
out the incosistencies of the measuring setup.
for a short use the connector of your choice, with a well made short on
the back side. cut a small piece of copper or brass drill a hole for
the pin and solder everything as close as possible to the connector's
shell
for an open, use again a connector of your choice with the pin cut
flush with the back of the shell and left open. I used these made out
from SMA and TnC connectors up to 3 GHz and they were quite allright
for all practical purposes. I had the HP units to compare and they
wewren't far off.
Saandy 4Z5KS

Need further info on network analysis terms...
 

Well I can understand how it might appear that way, but in truth I
certainly *do* read *all* the answers. Understanding them is another
thing, however. :-(

Need further info on network analysis terms...
 

On 8 May 2006 09:01:51 -0700,
wrote:

Well I can understand how it might appear that way, but in truth I
certainly *do* read *all* the answers. Understanding them is another
thing, however. :-(

Okay. It's just that in an earlier post you asked, "What exactly is a
"through"?"

I said, among other things:

"A through is a direct connection of the two test ports (cables). If
all devices under test (DUTs) were "insertable", i.e. had one male and
one female connector (or GR or 7mm sexless connectors) then the test
port cables could be joined directly, thus forming a through ("thru")

Unfortunately, a lot of DUTs have the same sex connector on both
sides, thus a calibration thru must be a double, same sex adapter
(barrel). Using such an adapter during calibration adds some
unavoidable length (and loss and mismatch) to the test path length.
One method of removing (some of) the effects of the barrel is to
replace it after calibration with a matched length male-female adapter
that remains in the test path."

Now maybe I didn't make it clear enough, but if nothing else, I
inferred that extra length, even in an adapter, is a bad thing.

You then ask, "...The obvious question now is: in respect of throughs
and open and shorted terminations, is the coax length of these
relevant?..."

Sorry to have been short, but it appeared that you didn't read, or
believe anyway, my answer.

Unless you know otherwise, as you would with fully characterized
reference standards, the open-short-load-thru's are assumed to be
"perfect". A perfect short or open terminating a piece of coax,
regardless of length, is no longer "perfect". A non-zero length thru
isn't perfect either.

Need further info on network analysis terms...
 

Hello Paul,


Sorry for the delay in replying; I've been away in Europe for a few
days.


Why do you guys call the continent "Europe"? Aren't you also Europeans?
I know, I know, you kept the pound sterling and that was probably a
smart decision.


It's the same HP 8754A analyser that appears in another thread of mine
hereabouts (4-1300Mhz). Probably rather too old for commercial
development use nowadays, although I do know an RF developer in
Brighton who still rates his one very highly and uses it daily.


That is a fine piece of equipment and not so large as the usual ones
are. IIRC it weighs under 40lbs. The difference between the 8754 and a
3577 can mean the difference between back pain and no back pain.

Regards, Joerg

http://www.analogconsultants.com

Need further info on network analysis terms...
 

On 8 May 2006 21:27:03 -0700, "K7ITM" wrote:

And--I had posted something similar about length being important--it
was the posting to which you replied asking about if length was
important. You really would get more respect if you looked a bit more
carefully at the info already posted. It would be a lot better if you
would ask specific questions about what was already posted. The
specific paragraph from my earlier posting is quite clear about the
effects of length being important:

"You can make a decent open that's nominally the same electrical length
as the load by sawing the center pin off flush with the dielectric, and
a short by shorting the pin to the shell. Do it with a tiny disc of
copper foil if possible--but in a pinch you can use multiple lengths of
wire radially out from the center pin. All these things should have as
close to ZERO lead length as possible. At 1GHz, 1 millimeter is a
little over 1 degree of phase shift, with air dielectric, and more with
Teflon or other dielectric. "

I can understand Wes' frustrations.

And I your's.

Regards,

Wes

Need further info on network analysis terms...
 

Okay, thanks for that clarification (likewise to others who've chipped
in, too).
I'm building up a picture, but it's not so easy in this text-only
medium.
Okay, so I think it's overwhelmingly clear that adding or subtracting
any electrical length from this "baseline" - (or "reference plain" as I
think it's also called) introduces measurement error which must be
allowed for/accounted for in some way to maintain accuracy.

Let's say I make up a short and an open using N-type plugs as described
elsewhere on this thread, using identical, high quality plugs. Can I
then use these with my proprietory Suhner 50 Ohm precision N-type load
by calibrating out any differences between the home-made jobs and the
proprietory one?
Further, I now know that a "through" is simply that U-shaped patch lead
that's often seen sticking out of the front of VNAs in catalogue
pictures. I also know that you can't make one of these with zero coax
length for obvious reasons. Given the fact then that it has an
unavoidable length; does it have to be any *specific* length in
relation to the open, short and 50 ohm terminations?
Thanks.

Need further info on network analysis terms...
 

Hello Paul,


Okay, so I think it's overwhelmingly clear that adding or subtracting
any electrical length from this "baseline" - (or "reference plain" as I
think it's also called) introduces measurement error which must be
allowed for/accounted for in some way to maintain accuracy.


Depends on how much you are talking about. Man often only needs this
much precision ;-)


Let's say I make up a short and an open using N-type plugs as described
elsewhere on this thread, using identical, high quality plugs. Can I
then use these with my proprietory Suhner 50 Ohm precision N-type load
by calibrating out any differences between the home-made jobs and the
proprietory one?


Let me describe what the three cal devices look like that came with our
HP4191A (after all, this is the stuff that Hewlett-Packard has deemed
acceptable):

The "Short" is a flat nut with a closed end. It puts a flat plate
directly across the face of the DUT connector. No length there.

The "Open" is a longer threaded tube with a hollow inside, about an inch
or so long and closed off at the far end.

The "50 Ohm" is something I wasn't able to open w/o special tools but
will some day. It looks like the resistor is about 1/2" to 3/4" up there
from the DUT connector surface, firmly encapsulated.

All gold-plated, of course.


Further, I now know that a "through" is simply that U-shaped patch lead
that's often seen sticking out of the front of VNAs in catalogue
pictures. I also know that you can't make one of these with zero coax
length for obvious reasons. Given the fact then that it has an
unavoidable length; does it have to be any *specific* length in
relation to the open, short and 50 ohm terminations?

The HP4191A has that, too. It needs to match the length that makes the
DUT and ref path equal. The one on the HP4191A is about 1-1/2" for a
zero length on the DUT. In order for the analyzer to be useful in
settings where the DUT has to be a bit remote, like in a vacuum chamber,
you can remove a little face plate, remove the "through" and install
your own longer "through". It's made from rigid coax.

Regards, Joerg

http://www.analogconsultants.com

Need further info on network analysis terms...
 

Okay, thanks for that description. I'm pleased to hear your standards
are all different lengths to such a degree that it's apparent by visual
inspection. Hope for mine yet, then!
I guess I could check the accuracy of the short and open I fabricate by
reference to the known good 50 ohm standard? For example, say I begin
with the 50 ohm load and calibrate so I get a single spot dead in the
center of the screen.
If I then substitute the short and the open in turn I would expect to
see that spot shift to the far left then the far right edge of the
Smith chart overlay. If it doesn't do so and deviates to any extent
from these ideals across the whole frequency range, then I've screwed
up their construction. Does that make sense?