On 23 December 2012 15:11, Peter Bunge <bunge@...> wrote:
Merry Christmas all!
Merry Christmas
My friends and I have been comparing what we see from the S Parameter Test Set on the two ports. These tests are with the VNA uncalibrated and with proper opens or shorts (even from APC7 cal kits).
Typically we get completely different shaped curves ranging from -35 to -55 dB.
If you have not calibrated the VNA, then you are just seeing the
uncorrect performance, and the fact that varies is no surprise. In
fact, I'm a bit surprised you managed to get 35 dB uncorrected. There
are a large number of systematic errors in a VNA. It does not matter,
as the whole idea of the error correction is to remove those.
I get very similar results using the same S Parameter Test Set on an HP8753B and an HP8753C so the curve shape is caused by the test set not the VNA.
Don't worry about it.
Typically we see one port 10 dB better and flatter than the other.
Any comments?
It is only important once calibrated. The calibration removes the
systematic errors.
p.s. the cal kit discussion was very informative and timely. I still don't understand the comment (that was later corrected) about the open usually missing a part. My friend's Type N cal kit does not have extra parts for the open.
It depends on what cal kit he has, but on for example and 85032B you
will often see something like this:
The small bit of gold played meter you see, is designed to increase
the diamteer of the last few mm of the pin, so it is 50 Ohms, and not
higher.
If you think about it, to get as near as possible to an open, you want
to have a 50 Ohm line that is cut off sharp at the end. Now consider
what an open N male looks like. The male pin is thicker at the bottom.
That bits makes 50 Ohm transmission line. Then the diameter is reduced
for about 5 mm, so it can go in the female. That will not make a 50
Ohm transmission line, but a higher impedance line, due to the formula
Zo=60 log(d_outer/d_inner)
So an open N plug is not as close as possible to an open. Does that
make sense? Hence the female part has an extender.
I would add it is possible to calibrate a VNA with an open N connector
without that. Accuracy suffers a bit, but that technique is used in
Agilent's portable VNA's. A better bet is an shielded N. But again you
need to know the paramters.
Also I have seen an open/short combo advertised and wonder if this comes apart to use the open??? I have not used one and find the concept of one end a short (OK) and the other an open (how?) confusing.
If you see an open/short advertised you MUST make sure you have the
paramters on it. It will have an offset in ps, and three capacitance
coefficents C0, C1, C2 and C3. You should then enter them in your VNA.
If you don't have that information, the kit is effectivly useless. The
short will have an offset too. The short might also have inductance
paramters L0, L1, L2, and L3, but you can't enter them in the 8753
series, so don't worry about them. They are not important for the the
frequency range of an 8753.
I hope that makes a bit more sence.
Dave
