Home |
Search |
Today's Posts |
#1
|
|||
|
|||
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. |
#2
|
|||
|
|||
Need further info on network analysis terms...
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 http://www.analogconsultants.com |
#3
|
|||
|
|||
Need further info on network analysis terms...
Hello Paul,
BTW: Which VNA do you have? (looking for something that might replace the HP4191A...) Regards, Joerg http://www.analogconsultants.com |
#4
|
|||
|
|||
Need further info on network analysis terms...
|
#5
|
|||
|
|||
Need further info on network analysis terms...
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 |
#6
|
|||
|
|||
Need further info on network analysis terms...
"K7ITM" wrote in message oups.com... snip various good suggestions. 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 |
#7
|
|||
|
|||
Need further info on network analysis terms...
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. |
#8
|
|||
|
|||
Need further info on network analysis terms...
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. |
#9
|
|||
|
|||
Need further info on network analysis terms...
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 |
#10
|
|||
|
|||
Need further info on network analysis terms...
|
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Utillity freq List; | Shortwave | |||
Here's Your Answer, Todd.... | Policy | |||
OLD motorola trunking information | Scanner | |||
Derivation of the Reflection Coefficient? | Antenna |