This is only a phase shift from different feed line length.and find similar results when I look on the S11 but not on the Smith Chart...
Yes, this will rotate the entire curve in Smith chart.Shall I modify the feed line length through the port definition using a TRL with a length of my SMA connector?
Having taken care of the ripples in calibration, at this point, I highly suspect the deviations are just the use of FR4. The relative permittivity of the material can typically vary widely from ~3.8 to ~4.8, and is also spacially inhomogenous and varies with frequency (note that the datasheet defines 4.7 at 1 MHz [!]). It is also fairly lossy, and the losses increase with frequency and are substantial at ~4 GHz.I have calibrated the VNA also and the waves are disappearing, nevertheless, we can still observe this shift.
And the archive of my work:
Possible, but the differences of the PCB-only testcases are small, and the large difference in the last case (with diode) is NOT caused by PCB tolerances.you may get less variation between simulated and measured results, due to PCB variations.
Varying the load impedance from the Schottky diodes will not get much shift in frequency of S-parameters.
Yes, but we don't need to guess here because measured vs. simulated results are already available: the frequency shift for the 3 GHz resonance is 1%, which means that PCB permittivity is approx 2% lower than simulated. This is only a small effect for this specific case.But varying a bit the Er of the substrate you will get a lot of shift in frequency.
What model(s) did you use for simulation of the diodes? Does your simulation use the correct bias point (current) for each of the two diodes, including the shorted diode?the shift appears only when the diode is short circuited
How can I check that the correct bias point is use during my LSSP simulation?
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