Use of RF Inductors beyond their Self Resonance Frequency (SRF)

[kh847009]

If I have an inductor whose self resonance frequency is less than the operating frequency of the circuit where I employ, Will the circuit operate well or It will get affected?

In addition, Can ADS and Cadence get this performance effect due to the SRF of the RF Inductor?

Thnx

 

[mtwieg]

It depends on what the inductor is used for. If it's for a DC choke, then it might be okay as long as the SRF is not too low. If it's used in a filter, then it will probably cause terrible results.

If you have an accurate model of the inductor that includes the SRF (like a S11 file or parasitic capacitance), then any simulator should show the effects.

 

[biff44]

well, it is not going to look like an inductance? If you do not need an inductance (i.e. can tolerate almost any reactance it happens to have), then you CAN use an inductor above its resonant frequency.

 

[E Kafeman]

Your circuit will be affected of course. It is no problem if you know the inductor impedance for that frequency and if that value fits your needs.

I design a lot of RF filters 2.4 GHz. Mostly type impedance matching between a transmitter and an antenna. Inductors involved can have an SFR at 7-10 GHz.
Goal is to have best impedance matching at 2.4 GHz but a part of the matching net propose is also to reduce harmonics, even above 10 GHz.
The VNA measurement software I use for filter optimizing do both these things more or less automatically, taking account inductor behavior above SFR, if needed.
It proposes best circuit topology for a given number of components, and it happens sometimes that the circuit includes inductors that have SFR below main frequency.
The software must have impedance models at frequencies long above SFR. Murata SMD inductors do have good models, but some brands provides S-parameters not even covering SFR.
SFR is not any dramatically frequency limit, it is just the frequency where the reactance changes sign. Inductance Q degradation have started at much lower frequency then SFR.

An simulation software calculate what it is feed with. If S parameter data not is covering actual calculating frequencies, can it in worst case happen that no error message is raised. If impedance above a certain frequency suddenly becomes a straight line can that be a warning sign.
If data model is in SPICE format is it often not clear, up to which frequency the model is reliable. Some brands have SPICE inductor models that not even is covering SFR. This can result in a relative big error at 2*SFR and it is still possible to simulate up to +100 GHz based on a such SPICE model.

If your operating frequency is at high frequencies can PCB transmission lines and inductor environment be just as important factors as the actual inductor value.