How to calculate/measure S-parameter using Oscilloscope?

[Jeetkumar]

Hi All,

I had a Network Analyzer in a lab, but it is not working, so do you guys know any way, how to calculate or measure S21, S11 ans S 22 parameters using oscilloscope?

Thanks in advance.

 

[enjunear]

Remember that S-parameters are the ratio of one voltage to another. So, you can obtain the magnitude of S21 very easily (Vout/Vin). You could also get the phase information if you monitor both input and output simultaneously and phase match your inputs (delay compensation).

Getting S11 and S22 are much more difficult, since you don't have a way of separating the forward and backward traveling waves. You might be able to cob something together if you used a directional coupler, then monitored the signals coming off of the transmitted and reflected ports. Phasing would be tricky, but you could get |S11| and |S22| pretty readily that way.

In all cases, you'll need to set up a sinusoidal signal generator to dwell at one freq... take measurements with the o-scope, then shift to a new freq, take o-scope measurements, and repeat until you've swept your entire range of interest.

 

[biff44]

If you also had some good directional couplers, and your oscilloscope was a very high frequency kind, then sure....why not.

To measure S11, you would need two directional couplers, maybe 10 or 20 dB coupling value, and you would take one of the coupled outputs to scope channel 1 (and call it the incident power), and take the other coupled output and hook it to scope channel 2 (and call it the reflected power).

If you only care about magnitude measurements...you are done. Send a CW tone towards your device, and compute the ratio of 20 log (reflected voltage/incident voltage)= |S11|

If you are trying to make a vector measurement, you will have to calibrate out the cable phase lengths by using some known loads in place of the device under test.

 

[Jeetkumar]

Thanks a lot guys, i will try to figure it out by your methods.