Ask a question,about measure a capacitor !Thank you very much!
try to find a fixture, or a little length microstrip, install the SMA on both ends. And insert the tested CAP in the microstrip. Use VNA test S21 etc., say S21<0.1dB @10GHz, that is microwave capacitor.
a good trick is to get a length of 50 ohm microstrip line, and place 5 capacitors in series with some spacing in between. Then just run an insertion loss measurement. If they stink, you will have considerable roll off at higher frequencies and some insertion loss spikes.
tony_lth
FYI,but my VNA's measuring MAx frenquency is 8G.so when I inpect S21<0.1dB @8GHz ,the capacitor is a high frenquency capacitor?
A lot depends on the capacitance value, the dielectric, the package the capacitor uses, the internal construction and also what frequency range you intend to use it over.
Also you need to consider the PCB characteristics as these will affect the performance of the capacitor especially if you want to use it over a broad range of frequencies.
One tip is to mount a SMD capacitor on its skinny side onto the PCB in order to remove one potential resonance mode. This can extend the useful bandwidth quite a bit.
If you want to measure the capacitor value you can do this on the VNA if you calibrate your test port for S11 and simply fit the capacitor in shunt across the test fixture. You can then view the response on a smith chart and it will display the capacitance at a test marker frequency, eg 1MHz or 10MHz etc.
it's best to look at the manufacturer's published data as a starting point. eg Q curves, ESR, s-parameters etc etc. However, you may still need to do some practical tests using your chosen PCB material in case you are worried about resonance or loss problems with the capacitor.
A long time ago I measured a capacitor with VNA, without any PCB.
I held the SMD capacitor with something like plastic tweezers directly on the VNA connector and performed a single port measurement. No need of Test fixture or specific calibration...
Please don't laugh, I know this is far from the proper way to do but... I did it quickly.
Yes, I've done the same a few times. However, I sometimes use an SMA end launcher with some extra ground soldered to it as it allows the cap to be pressed across the pins or soldered across.
This is fine if measuring the cap at a lowish test frequency.
JOHNSON/EMERSON|142-0701-851|JACK SMA, END LAUNCH, TAB | Farnell United Kingdom
GOHZU,
I really don't know how i can thank you enough! By your opinion,measure the resitance of capacitor must use s11 of the VNA?S12/s21 is ok?