Need further info on network analysis terms...

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.

Reply to
paul
Loading thread data ...

Hello Paul,

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.

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

formatting link

Reply to
Joerg

Hello Paul,

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

Regards, Joerg

formatting link

Reply to
Joerg

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.

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. :-;

Reply to
Wes Stewart

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

Reply to
K7ITM

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

Reply to
Ian

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.

Reply to
paul

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.

Reply to
paul

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

Reply to
K7ITM

Yes.

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

Reply to
Wes Stewart

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

Reply to
Saandy , 4Z5KS

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. :-(

Reply to
paul

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.

Reply to
Wes Stewart

Hello Paul,

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.

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

formatting link

Reply to
Joerg

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

Reply to
K7ITM

And I your's.

Regards,

Wes

Reply to
Wes Stewart

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.

Reply to
paul

Hello Paul,

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

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.

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

formatting link

Reply to
Joerg

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?

Reply to
paul

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.