Need further info on network analysis terms...

[paul@burridge8333.fsbusiness.co.uk]

Hi gang,

Reading through the instructions for this VNA it mentions some
important accessories; power spitters, 6db attenuators, 50 ohm
terminations, opens, shorts and throughs. What exactly is a "through"?
And is it possible to make up accurate opens and shorts oneself that
are good enough for calibration purposes up to 1.3Ghz? Not talking
about any absolute standard here, like you'd get from HP themselves;
but just mid-tolerance stuff that'd be sufficient for reasonably
accurate amateur experiments?
Thanks,
p.

 

[Joerg]

Hello Paul,

Reading through the instructions for this VNA it mentions some
important accessories; power spitters, 6db attenuators, 50 ohm
terminations, opens, shorts and throughs. What exactly is a "through"?

Probably a matched connection from point A to point B. This is often
used to provide a certain line length for purposes of equalizing a time
delay. You'll see that on many directional coupler assemblies, like when
you open the front pod of a HP4191A.

And is it possible to make up accurate opens and shorts oneself that
are good enough for calibration purposes up to 1.3Ghz? Not talking
about any absolute standard here, like you'd get from HP themselves;
but just mid-tolerance stuff that'd be sufficient for reasonably
accurate amateur experiments?

For hobby use, probably. The lab analyzers come with a calibration kit.
The 50ohm resistor is a barrel with a connector. The open is just that,
a tube that looks like the innards of the connector have been scraped
out. The short is a connector with a precision gold plate in there that
guarantees a solid flat contact between center and rim. Believe it or
not, the 50ohm resistor of mine is now about 52.5ohms. Couldn't believe
it, considering the profit margins on that stuff.

For the terminator I suggest to look at chip resistors. What is most
important is to keep all the contact surfaces clean.

Regards, Joerg

 

[Joerg]

Hello Paul,

BTW: Which VNA do you have? (looking for something that might replace
the HP4191A...)

Regards, Joerg

 

[Wes Stewart]

Hi gang,

Reading through the instructions for this VNA it mentions some
important accessories; power spitters, 6db attenuators, 50 ohm
terminations, opens, shorts and throughs. What exactly is a "through"?

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.

And is it possible to make up accurate opens and shorts oneself that
are good enough for calibration purposes up to 1.3Ghz? Not talking
about any absolute standard here, like you'd get from HP themselves;
but just mid-tolerance stuff that'd be sufficient for reasonably
accurate amateur experiments?

Depends. For SMA stuff, an open and shorted connector wouldn't be too
bad. Type N is a little more problematic but probably doable for
moderately accurate measurements.

BTW, even HP (Agilent) doesn't offer "absolute" standards. :-;

 

[K7ITM]

You can make a very decent 50 ohm load using two 100 ohm 0805 SMT
resistors and an SMA PC-mount jack. Cut the center pin of the jack
down to a height about the same as the thickness of the resistors, and
solder the two resistors diametrically opposite, one side to the shell
and one to the sawn-off center pin. The ones I've made check out fine
up to a couple GHz...as I recall, better than 30dB return loss out that
far. I've found the 0805 resistors work slightly better than 0603s,
but as they say, YMMV, and unfortunately you probably have no good way
to check them.

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.

You can make a two-way levelling splitter if you are really careful.
For a two-way, it's an input port that connects to two 50 ohm
resistors, one to each of the two output ports. One output port is
normally used to drive the reference port of the VNA, and the effect
similar to the virtual ground of an op amp: the input port level is
know or controlled accurately, so it's a virtual zero-impedance point
there. Through the 50 ohm resistor to the test port makes for a
virtual 50 ohm source impedance. Again, keep lead lengths to zero, or
make any leads look like 50 ohms.

You won't get any VNA cal kits from HP these days...

Could be helpful to look for an S-parameter test set or a
reflection/transmission test set to go with your VNA. You can find
them on ebay sometimes. They can make life a lot easier.

Cheers,
Tom

 

[Ian]

You won't get any VNA cal kits from HP these days...

Could be helpful to look for an S-parameter test set or a
reflection/transmission test set to go with your VNA. You can find
them on ebay sometimes. They can make life a lot easier.

Cheers,
Tom

VNA cal kits maybe not from HP, but most certainly from Agilent.
I do like my N4431 electronic cal kit, no lead swapping and really
fast (once it has warmed up).

If you try ebay, be sure to check connector condition.

Regards
Ian

 

[paul@burridge8333.fsbusiness.co.uk]

Hi Joerg,
Sorry for the delay in replying; I've been away in Europe for a few
days.
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.
Best wishes, Paul.

 

[paul@burridge8333.fsbusiness.co.uk]

Okay, thanks all.
I've obtained proprietory 50 ohm loads and power splitters, so just
need to sort out the throughs, shorts and opens. The obvious question
now is: in respect of throughs and open and shorted terminations, is
the coax length of these relevant? I envisage making up N-type
connectors with say one inch long coax stubs for both open and short
unless there's some problem with this. Is the electrical length of the
coax relevant?
Thanks,
P.

 

[K7ITM]

Well, yes, length is in general important in a vector network analyzer.
Let's say you have port 1 of an S-parameter test set, or a
reflection-transmission test set, accurately calibrated at its
connector. What is S11 if you connect nothing there? What is S11 if
you connect an inch of coax there? If you tell me it's different in
the two cases (which you better!), then you should pay attention to the
length of the line to the open or short.

Another way to look at it: on a Smith chart, where will your 50 ohm
termination be? Where will a short be? Where will an open be? If the
"short" and "open" are at different distances down a line from their
connector, even if you calibrate so the "open" is where it should be on
the reflection coefficient display (Smith chart display), where then
_should_ the short be, if you don't have the line lengths exactly the
same?

And while I'm asking questions, what happens if you put your 50 ohm
load, made with really good 0.1% 0805 resistors, at the end of a
section of 50 ohm line which is not actually 50 ohms, but is instead 55
ohms or 45 ohms? The impedance tolerance on typical coax is pretty
poor. Beware. It's actually unlikely to be as good as a carefully
made load as I've described before here. (It's a real pain in the rear
trying to set up a system to test things like VNAs accurately and
guarantee your calibrations because of stuff like this.)

Cheers,
Tom

 

[Wes Stewart]

Okay, thanks all.
I've obtained proprietory 50 ohm loads and power splitters, so just
need to sort out the throughs, shorts and opens. The obvious question
now is: in respect of throughs and open and shorted terminations, is
the coax length of these relevant? I envisage making up N-type
connectors with say one inch long coax stubs for both open and short
unless there's some problem with this. Is the electrical length of the
coax relevant?

Yes.

No sense elaborating, you don't seem read the answers.

 

[Saandy , 4Z5KS]

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

 

[paul@burridge8333.fsbusiness.co.uk]

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. :-(

 

[Wes Stewart]

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.

 

[Joerg]

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

 

[K7ITM]

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.

Cheers,
Tom

 

[Wes Stewart]

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

 

[paul@burridge8333.fsbusiness.co.uk]

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.

 

[Joerg]

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

 

[paul@burridge8333.fsbusiness.co.uk]

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?