VNA Calibration with U.FL

Hi,
I have to measure S11 mag for some PCB antennas, in GSM bands and also in 2,5GHz. The connector I use is U.FL from Hirose. For the test port I use an N-SMA adapter, then a SMA-U.FL adapter, then a coax with U.FL plugs at both ends. How can I calibrate the reference plane at the end of the U.FL receptacle which is connected directly to the antennas on the PCB? I do not require high precision results, any more or less precise calibration method would be fine. The VNA is an Advantest R3765BH.

/pisoiu

I assume you may calibrate at SMA planes with a 3.5mm Cal Kit.

In my opinion, there is no one exact solution.
You may do an approximation.

If your VNA has Time domain option, you may mask the adapter.

If no time domain is available di the following:
Once you have calibrated, but before perform measuring, insert a series of SMA-U.FL-SMA, then find the electrical lenght, then extend the calibrated plane by 1/2 of the lenght of the 2 adapter. This is a poorely accurate method...but better than nothing.

Unfortunatelly, the time domain option is not available, and I also do not have yet a 3,5mm cal kit.
Can be of any good a SOL calibration with three U.FL receptacles mounted on a PCB with short, open and load with two paralleled 100 ohm resistors in 0402 case?

/pisoiu

Some time ago a colleague of mine asked me the same question. I've discussed and analyzed that scenario.

1) Usually the accuracy of the Cal Kit set the overall measurement accuracy, good Cal Kit=good measuremets, homemade CalKit=higher uncertainty

2) The most important causes of uncertainty for all spar are the discontinuities.
For Reflection measurements (s11, s22) also Reactive parasitics of the load, short , open plays an important role.

3) Of course, You may build your own Cal Kit but You'll take in mind that the accuracy will drop a lot.
More specificatelly, s21, s12 traces will be affected by a ripple.
While for s11 and s22 the accuracy will be degaded by at least two effect: a ripple (visible effect) and a reduction of the measurable range (non visible effect), in other worlds , if you calibrate with a 3.5mm CalKit, you may read s11 form 0 to -40 dB , but if you calibrate with your own Cal Kit, perhaps, you may read valid s11 form 0 to -20dB . The VNA always will show a dinamic of 60 dB but you cannot believe it.

4) how to practically build the LOAD and SHORT
Build the 3 standards. Do not recycle the same line by soldering (and de-soldering) the resistors. Use as low as possible solder alloy. You should perform exellent solderings.

Mount the two 100 Ω resistors upside down. I mean the carbon layer should be mounted to be considered as a small lenght of a 50 Ohm line.

Ground: Start from the consideration that the gnd is never good enough, if you use PCB, lots of via hole and stay closest to coax connector.

Thanks for clarifying these aspects. This is what I've build so far. There are no vias involved. I use "through" for normalisation, I am interested in S21 amplitude only. The R3765BH have only T/R test set, so I cannot make full two port calibration. I will mount two new resistors on the load, flipped down as you said.
Of course the cal kit will set the overall accuracy, but now I am in the situation of making any measurement better than nothing.

/pisoiu

At past time , I had a short adapter to transform SMA to the port in the picture.


So , I just do the CAL by the adapter to SMA calKIT.


After CAL, the reference plane is about at the port on PCB.


Now I can see SMITH chart , there are only two circles from 800MHz to 2GHz.

That is what just I want.


Then, tune the ANT to get 890-960, 1710-1880 VSWR better than 4 or 5.

That is all.