Harmonic generation by passive components (C, L)

[mowgli]

Hello all,
this morning I was having a coffee break with a collegue and we were talking about problems faced during our own working experiences, when he told me something that at the moment made me curious: the topic was too high spurious emission of a HF transmitter and he told me that the RF engineer of the customer company said that, in his opinion, the problem was generated by capacitors used in the matching network in the RF path between power amplifier output and antenna input. In particular the engineer said that some capacitors are built in a way that at certain frequencies exhibit parasitic components which behave like diodes, introducing strong non linearity due to clipping (or what else) which is the source of the spurious harmonics observed at the output of the matching network (the output of the PA itself was pretty good: only in conjunction with the matching network spurious are presents). I am not experienced on RF design (I am a SW developer with only a university background on RF subjects) but this talk raised my curiosity: does anybody have some suggestions about this?

Thank you
Mowgli

 

[madengr]

Some ceramic capacitor dielectrics are a function of applied voltage. I had one (I don't remember the formulation) that had 70% capacitance change over the applied voltage. This was used in a switching power-supply resonator and caused problems.

Some dielectrics are polarized. Some have a piezo effect. However normally you would not use any of the above in high-power RF circuits, but all capacitors have these small non-linearities so that's probably where it's coming from. Another source is the electrode metalization; the interface of metal and ceramic, and also the plating layers. Inter-metallics have non-linearities such as the plating on RF connectors.

 

[FvM]

Don't want to say it's impossible, but to my opinion, it's rather unlikely to have non-linear effects with usual low capacitance RF dielectrica. But it's easy to cause nonlinear behaviour of a PA itself by small modifications of the matching network. Particular increasing the RF level at the output node can have this effect.