aaron_do
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Hi all,
I have designed a LNA and down-conversion mixer, and I added a GSG probing point at the output node of the LNA for on-wafer measurement. The LNA output impedance is approximately 2 kohm and it roughly matches the mixer input impedance of 2 kohm. I'm only quoting the real part of the impedances. So anyway, in simulation, I turned off the mixer (which made the input impedance infinite), and ran 2-port s-parameter simulations on the LNA, with port 1 having a reference impedance of 50 ohm and port 2 having a reference impedance of 2 kohm. My thinking was that I would be able to run actual measurements on the LNA using 50-ohm input and output ports, and then convert them to s-parameters with 50-ohm and 2kohm port impedances.
When I ran the measurements using a vector network analyzer, the magnitude of the s-parameters were apprpoximately as expected (S11 and S21 in dB), but when I convert them to s-parameters with 50-ohm and 2-kohm port impedances, the magnitude of S21 is way off (S11 is correct but S21 is totally wrong). My only explanation is that there is considerable inaccuracy in the delay measurement of the VNA, or perhaps the calibration is wrong. One problem is that I was only able to calibrate up to the end of the cables connecting to the probes, rather than the probe tips themselves, but i don't know if it would affect the measurements that much.
So my question is, should I expect such inaccuracy with the delay measurement of the VNA, or perhaps does anybody have another explanation for the measurement discrepancy? I have yet to measure the rest of the front-end. Also, how much uncertainty is added by only calibrating up to the end of the cables rather than the probe tips? Or is the problem due to the large mismatch between the actual 50-ohm output port, and the desired 2 kohm output port impedance?
thanks, Aaron
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