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RF CV measurement with VNA

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Lionel
Posts: 4
Joined: January 25th, 2016, 7:46 am
Country: Ecuador

RF CV measurement with VNA

Post by Lionel » January 25th, 2016, 8:11 am

Dear,

We are trying to setup a measurements of ultra low scaled MOS capacitance using the RF technic (i.e. mesuring the s-parameter). To do so we use a VNA for the high-frecuency RF signal and the SMU's of the K4200-SCS for the input bias of the VNA (indeed an Agilent or Keysight ENA). For the bias input we need to sweep the tension from 0 to 2V (or so, with 20 or 50mV step.)

We are using an external computer to control the K4200 and the Agilent (we are programming in Python). The main issue is that we are not understanding how the VNA and the K4200 could interact. More especifically, for each RF signal we are biasing the DC component with the K4200. However how to "ask" to the SMU to "wait" that the VNA does the s-parameter measurements before to go to the next step voltage?

Thank you in advance,
Lionel

brian.d.smith
Keithley Applications
Keithley Applications
Posts: 402
Joined: June 10th, 2013, 6:00 am
Country: United States

Re: RF CV measurement with VNA

Post by brian.d.smith » January 25th, 2016, 4:27 pm

I do not think that there is enough information to render an opinion. Can you please post your Python code for the 4200 and add some detail of what behavior is happening at what point of the python program.

Kenneth P
Keithley Applications
Keithley Applications
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Joined: October 25th, 2010, 1:31 pm
Country: United States
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Re: RF CV measurement with VNA

Post by Kenneth P » January 26th, 2016, 8:47 am

I agree with Brian, there isn't sufficient information to determine how you are attempting to resolve the handshake between the 4200 and VNA. A controller assumes that you are using the 4200 in KXCI mode. The next assumption is that you are trying to perform a voltage sweep using the SMU. It may be easier to use the SMU as a voltage source and program the voltage steps individually within a test loop in the Python script.
You can optimize the test loop by commanding the 4200 SMU to force voltage, issue a command to the VNA to measure and follow up with a *OPC?. Once the VNA has completed the measurement, repeat those steps in the loop. I don't believe the KXCI interface and 4200 accept external trigger inputs other than BUS type. Introducing a GPIB command loop, will add no more than 20 msec per voltage step however I would expect a few msec. per step typical.

Lionel
Posts: 4
Joined: January 25th, 2016, 7:46 am
Country: Ecuador

Re: RF CV measurement with VNA

Post by Lionel » January 27th, 2016, 8:03 am

Thanks for your anwer both,

Indeed we were thinking a strategy where, applying a sweep to the SMU, the VNA has to measure the RF signal with the required DC but our question/issu was: how does the VNA have enough time to measure the RF signal between two step or how to control this time-parameter.
Indeed the suggestion of Kenneth is a simple way to do this measurement and after discussion we decided to try this option, namely to loop the procedure "DC+AC for each DC bias" (using SMU as a force constant bias).We are now reworking the code and as soon as we have a new version of the Python code (and if we have troubles) we will send it to the code through this post (it can take a couple of weeks.)

Thanks again,
Lionel

Lionel
Posts: 4
Joined: January 25th, 2016, 7:46 am
Country: Ecuador

Re: RF CV measurement with VNA

Post by Lionel » September 14th, 2016, 11:05 am

We did a lot of progress and the routine is working. However the results show very large distortion. The setup looks ok since we use HF cable for the VNA with RF probes (CGC) and we work on-wafer on RF structures designed for this kind of measurements. The model of extraction sticks to the structure we are using and have been proved in different place.

Therefore our question is about our routine if it is doing what it should or not. We do not discard completely the possibility of the setup but we would like to get an opinion of people we have much more experience the Keithley.

I attach you the part of the routine where the both tools (VNA and K4200) are communicating in the execution of the DC+AC measurements.

thanks in advance to tell me what do you think about it.

Lionel
Attachments
Código Generación de comandos para el SMU en python.rtf
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