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After speaking with Rana and realizing that it would be better to use smaller inductances in the flying-component circuit (and after a lot of tinkering with the original), I rebuilt the circuit, removing all of the resistors (to simplify it) and making the necessary inductance and capacitance changes. A picture of the circuit is attached, as is a circuit diagram.
A plot of the measured and simulated transfer functions is also attached; the general shape matches between the two, and the resonant frequencies are roughly correct (they should be 11, 29.5, and 55 MHz). The gain at the 55 MHz peak is lower than the other two peaks (I'd like them all to be roughly the same). I currently have no idea what the impedance is doing, but I'm certain it is not 50 Ohms at the resonant peaks, because there are no resistors in the circuit to correct the impedance. Next, I'll have to add the resistors and see what happens.
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Stephanie,
This is a quite nice measurement. Much better than the previous one.
1) For further steps, I think now you need to connect the real EOM at the end in order to incorporate
the capacitance and the loss (=resistance) of the EOM. Then you have to measure the input impedance
of the circuit. You can measure the impedance of the device at Wilson house.
(I can come with you in order to consult with Rich, if you like)
Before that you may be able to do a preparatory measurement which can be less precise than the Wilson one,
but still useful. You can measure the transfer function of the voltage across the input of this circuit,
and can convert it to the impedance. The calibration will be needed by connecting a 50Ohm resister
on the network analyzer.
2) I wonder why the model transfer function (TF) has slow phase changes at the resonance.
Is there any implicit resistances took into account in the model?
If the circuit model is formed only by reactive devices (Cs and Ls), the whole circuit has no place to dissipate (= no loss).
This means that the impedance goes infinity and zero, at the resonance and the anti-resonance, respectively.
This leads a sharp flip of the phase at these resonances and anti-resonances.
The real circuit has small losses everywhere. So, the slow phase change is reasonable. |