Quote:
Originally Posted by G0HZU_JMR
I had a go at modelling both the P6202A probe and the poorman's probe and in both cases the input reflection coefficient is extremely close to 1 all the way up to about 300MHz.
At 150MHz it is in the region of 0.9999 for both probes and this is asking too much of a real world VNA to measure using a classic 1 port s11 measurement. Therefore, whatever the VNA displays, it won't be the correct result. Even my expensive lab VNA will fail here when trying to measure s11.
Both probes are quite similar in terms of the input circuit. The P6202A uses an 8M series tip resistor and a 2M resistor to define a resistive tap. There's also a tiny fraction of 1pF across the 8M tip resistor as part of a compensated capacitive divider. This compensation capacitance is going to be similar to the series cap at the tip of the poor man's probe. Up at RF, the 8M resistor will effectively be invisible.
The P6202A uses a JFET, and this normally has higher input capacitance. However, there is another JFET in the source connection that acts as a current source and the source is only loaded by a common collector BJT. Therefore, the Cgs capacitance of the input JFET will be virtually nulled out.
The P6202A does things this way because the input JFET has to act as a faithful and accurate voltage follower for use with an oscilloscope. The price for this will be lots of negative resistance at the input because the overall circuit at the source pin of the input JFET will mimic a shunt capacitance with a very high shunt resistance. There will also be some Cdg capacitanceat the input of the JFET. The RC snubber circuit attempts to cancel the negative resistance at the cost of slightly increased input capacitance.
I think a fair bit of the input capacitance of the P6202A probe will be in the elongated tip section. If this tip section wasn't there, I think it would have lower input capacitance.
Realistically, I think the best way to measure the Rp and Cp performance of these probes is to measure a very narrow (single resonator) 100MHz BPF with a VNA with an s21 measurement and then see how much the capacitance of the probe pulls the BPF response down in frequency when the probe is connected to the resonator of the BPF. Then see how much the level of the s21 measurement drops as well. This will give an idea for the probe Rp. If the level goes up then Rp is negative. I suppose you could tweak the probe design to get no change in S21 level and this would mean the input Rp of the probe was as high as possible (at the 100MHz test frequency) without actually turning negative.
|
Thanks. Fundamentally, there are intrinsic uncertainties with the methodology of the measurements when the DUT has very large impedance mismatch with the VNA. In my case, I inserted a short 50 ohm transmission line for the measurements of the Poorman's and P6202A probes. The measured S21 results for the open and 50 terminated line are different because the impedance of the probes are somewhere between 50 ohms and infinity. The actual gain (or insertion loss) is somewhere between the open and closed transmission line measurements as verified by the use of the oscilloscope measurement.
I have looked at my 100MHz bandpass filters. By design, they are meant to be maximally flat so the Q are moderate. So I have quickly tried to measure my 465MHz cavity filter with 3 helical resonators(Q=166-250 depending the insertion loss and bandwidth adjustment).
In the 2nd attachment, it was measurement with direct through measurement having the short transmission line included in the calibration and measurement. The S11 is quite poor which suggests that this filter may not be designed for 50-ohm system.
In the 3rd attachment, it is measurement with the Poorman's probe (0.7pF input capacitance) pogo pins pressing against the open transmission line. You can see the bizarre dip at the resonance frequency. I dont know what is going on here.
4th attachment, Poorman's probe with the transmission line terminated by 50 ohm load.
I think the stray capacitance of the P6202A is not due to the length of the tip but it is due to distributed capacitance between the probe tip and the outer ground ring (see attachment 5). My poorman's has much longer probe tip and ground pin but it still has far less input capacitance...my probe tip and ground pin are very far apart..and also I removed most of the ground traces nearby to cut the stray capacitance.
I have made some custom 50-ohm, short, open and through standards in order to measure the Rp, Rs, Cp, Cs of a 10K 1206 resistor, my results look similar to the link below at high frequencies, but they are very different at lower frequencies:
https://www.w0qe.com/Measuring_High_Z_with_VNA.html
PS, correction of my previous post; the P5202A works well with the alignment of 10.7MHz LC IF filters.