Roger,
You are correct that it does not change the actual impedance presented to the filter.
However, it is possible to mathematically compute S parameters in, say, a 500 ohm system knowing the S parameters in a 50 ohm system, at least for a passive DUT.
A full set, S11, S12, S21, and S22 is ideally need, but for most passive devices(isolators excepted) S12=S21, and for a symmetrical device like many filters, S22 is approximately equal to S11.
The latest DisLord and earlier ojisankoubou NanoVNA-V2 firmware does this calculation. It does work.
--John Gord
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On Sun, Sep 18, 2022 at 12:32 PM, Roger Need wrote:
On Sat, Sep 17, 2022 at 11:27 AM, John Gord wrote:
The latest DisLord firmware for the NanoVNA does allow you to see what the
filter response would be with a load impedance different than 50 ohms.
I respectfully disagree with you. The Port Z feature in the latest DiSlord
software only does the mathematical calculations necessary to convert the
measurements to a different system impedance (like 600 ohms instead of 50)
It does NOT change the actual load impedance on the filter itself. The
filter characteristics will change if you do not use the actual source and
load impedance that the filter was designed for. For example take an IF
filter that was designed for a 1000 ohm in/out impedance. If you put 50 ohms
on the output the rolloff characteristics will be much worse than at 1000 ohms
and no mathematical calculations in the NanoVNA will compensate for that.
Roger
