Re: how to test s11 and s21 dynamic range?

Reinier Gerritsen

Op 10-2-2021 om 17:15 schreef Jim Lux:
On 2/10/21 7:36 AM, wrote:
I made some small changes of my Nanovna-H.
Is there a way to test s11 and s21 dynamic range?
S21 - start putting attenuators in and seeing where it bottoms out (or where the displayed attenuation change doesn't match the actual attenuation change)

S11 - calibrated mismatches - attenuator into an open or short. If you put a 20 dB pad on the Tx port, can you see the difference between short, open, and load?  (20dB pad is -40dB S11, 20 dB going out, 20dB coming back from the reflection).

A couple decent step attenutors (one in 10dB steps, the other in 1 dB steps) makes this easy.  But a handful of fixed attenuators can also do it, just more time consuming as you swap them around.

Watch out for leakage from cables etc as you get to higher isolations. 80 dB is hard. >120dB is really hard.

I would do it in a different way. Calibrate the analyzer. Terminate port 2 in 50 Ohm. The noise floor is the lowest level you can measure. To get the dynamic range you would need to know the maximum signal that can be applied. Therefore you would need an RF amplifier (input to port 1). Do not overload port 2, but to find the maximum usuable signal, you must find the level where it is no longer linear. You can find this level bij adding a know attenuator in series with the amplifier output. S21 should follow the attenuation.
S11 dynamic range: calibrate the VNA and do the 50 Ohm load as the last one. Leave it connected. Apply the calibration. Make sure you tell the analyzer (or the PC software) that the load is a perfect load. Now the displayed value of s11 is the dynamic range since it sees a perfect load, perfect in the sense that it is exactly the same as the reference.
For noise floor, take the peak values and add a few dB for safety margin. For S11 a dynamic range of 30 to 40 dB is enough for all practical applications.
All values are frequency dependent.

Note that a 20 dB attenuator does *not* mean a 40 dB return loss. Most likely it is less in practice if the input is not exactly 50 Ohm (or better, exactly the same as your reference)

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