VNA calibration-measurement

[Julew]

Hello!

To measure the S11 of a LNA, manufacturer says that the generator output power of VNA has to be -30dBm so I can´t configure the attenuator of VNA so i bought an external one (20dB). Today i try to measure S11 but i can´t calibrate the equipment with the attenuator (if i do this, when i connect load/oc/sc it goes crazy!) (i attach load pictures) and if i attach it after calibration S11 of the attenuator is taken into account and the measure is not valid. How can make this measurement?

 

[FvM]

I already mentioned in reply to a previous post of your's, that a S11 measurement with an external attenuator doesn't work. Now you verified this statement in your test.

To allow a reduced level S11 measurement, a VNA must be equipped with an internal attenuator between source and reflected signal measurement. (or an option to connect an external attenuator to the internal signal path). When connecting the attenuator between VNA and DUT, you'll effectively measure the attenuator's rather than the DUT's S11.

 

[BigBoss]

Why ??
Because a 20dB attenuator after VNA will provide 40dB isolation and that's why Output Port of your VNA will see approx. 50 Ohm that is almost independent from your load or calibration standard.
In additional to this, -30dBm output power will increase your uncertainty..

 

[Julew]

Ok but, why i get this crazy smith chart? It has to be pointing to 50 Ohm ....

 

[FvM]

Ok but, why i get this crazy smith chart?

Most likely it's just noise. Possibly, the VNA calibration fails, because the software isn't prepared for effectively zero difference between open, short, load. If you use a more moderate attenuation, e.g. 6 dB, you should get at least plausible results.

 

[volker_muehlhaus]

It has to be pointing to 50 Ohm ....

... if you insert the attenuator after calibration.