开云体育

I've been looking through posts here, and other internet references, to try to understand the meaning of the term "measurement reference plane." My general understanding is that this refers to the place in a network (and once you attach your VNA, it, and the cables you use to attach it, effectively become part of the network), where one conducts the appropriate calibration procedures, which are intended to eliminate (or at least, significantly reduce) errors that are inevitable in the test equipment. Many of the reference materials provided by VNA manufacturers (manuals and such, which may be found on the internet) refer to the measurement being affected by the use of extensions or fixtures that may be added after calibration is completed, and how these may be compensated for without recalibration.

Okay, so far so good. (Please feel free to comment on the validity of the above, or to correct my interpretation.)

What I have been unable to find is any discussion of how much error might be introduced by use of cables, extensions, and/or fixtures that may be employed up to the measurement reference plane. To put it another way, if the purpose of conducting a calibration is to eliminate/reduce errors introduced by the measurement equipment (meaning the VNA itself, and the cables/fixtures used to attach it to the device under test), just how complicated can the "measurement equipment" be, before residual errors, after calibration, become significant?

Suppose I have a network consisting of, say, 3 connected elements, A, B, and C, where A is connected to B, and B is connected to C, like A==B==C. Suppose, further, that, for the purpose of measuring S11, I will be able to connect the s11 port of my VNA directly to element A. I begin by performing a calibration directly at the s11 port - this becomes my measurement reference plane. I then connect s11 directly to element A, and measure the impedance of the network that consists of A==B==C.

Now, I want to measure the impedance of the network without the presence of element A. Presuming I can connect the s11 port directly to element B, all well and good - I simply disconnect element A, connect the s11 port to element B, and bob's your uncle.

But, suppose, for some reason, I cannot connect directly to element B. Could I connect s11 to element A, disconnect elements B==C, perform a calibration at the far side of element A (thus establishing a reference plane there) reconnect elements B==C, and make the measurement? Element A has become, in some respect, a "test fixture" for measurement of a network consisting of B==C. Taking this a step further, could A==B become a "test fixture" for measurement of element C. And just how complicated a "test fixture" could I employ before I degrade measurement accuracy beyond any useful point? (Of course, the notion of what the "useful point" consists of will vary with the person doing the testing.)

how complicated a "test fixture" could I employ before I degrade measurement
accuracy beyond any useful point

VNA works by measuring signals, then calculating based on differences from known references
namely open, short and 50 Ohms. So long as there are measurable differences among those references,
then measurements can be of interest.

To anticipate how interesting, watch the Smith plot while performing calibration with whatever "test fixture".
So long as point clusters for open, short and 50 Ohm are substantially separated,
then there is hope.

The easiest one to explain is using a simple random length of RG316 (PTFE)
cable terminated in sma (instrument end) at one end and bare wires the other
(DUT end).

If we want to normalize the cable effect of of the system we do the following:

Do our Open cal with the cable wire ends literally open.
Then solder them shorted and cal Short.
Using a small SMT 50 ohm resistor between the ends of the wires we do cal-50.

If we are doing an S21 measurement then the other cable is prepped similarly
and we do isolation (open) and then connect the ends and perform though.

At this point the plane (interface to the DUT) of measurement is the ends
of those wires.

That would be suitable for a test set up that is literally connected to the end
of the wires like measuring the feed point of a 432mhz dipole.

Jigs are set up the same way save for the measurement point in the
jig gets treated with open/short/load in what ever form is handy.
It helps if the jig is built to help maintain a R50 interface to the DUT.

In all cases at lower HF the wire leads are less critical than at UHF.
Ideally you would like to keep the leads under lamda/200 in length.
Example at 100mhz that 3M, or about .015m=about .5". At 1000mhz
That would be best to stay under .05".

If you are dead ended for some reason and the plane is before the point
of measurement then with good data collection it can be calculated.
That is tedious but there are tools for that. Its a rare case you cannot
get a calibration to the point of the plane (interface to) the unknown
device or termination.

I've worked this way using PNAs to 6ghz and a few times to 60ghz
and often the resulting answer is more than sufficient. Especially for
antennas and their matching networks.

One last thing, in a lot of case carrying the measurement to more
than 3 maybe 4 places is sufficient. After all most parts are barely 5%.

Allison
-----------------
I do not accept private email due to forum scraping groups.io

-----------------
I do not accept private email due to forum scraping groups.io

Establishing a "measurement plane" makes the impedance or admittance displayed on the VNA valid at that specific point. For example, you want to know what the impedance is at the input of some device. You select a transmission line that will reach the device's input and you perform the open,short, load cal with a cal kit of the appropriate connector type for the device under test. Now the impedance measured is the impedance at the device under test's input. Maybe the device under test can be connected directly to the device under test so no cable is needed so the OSL cal is done right at the VNA's test port.
Now if you are simply measuring SWR or return loss the exact location of "measurement plane" is not that critical if its location is not too far down some lossy or poor cable of a different characteristic impedance, etc. (the SWR and the RL remains basically constant along a short transmission line)....Impedance is another thing unless you plot things on a Smith Chart and rotate to the "measurement plane".
Respectfully, I see a lot of discussion about calibration in this discussion group and I think the subject is being overly complicated. The only time I had to get wrapped around the axle is when I had to performance test a VNA in a cal lab. (when I worked at HP/Aglient from '77 to '01, we verified ALL of the published specs and stood behind them so we had to be very careful using the methods and equipment specified by the manual).
With this NanoVNA I think the results have been excellent. I have done a lot of tinkering with impedance matching using it and the measured impedances and the L and C's calculated to perform the matching have worked very nicely.