Balun self resonanse frequency

Hello everyone!
I've the following problem: I use balun for the single ended-to-differential transformation, but the self resonanse frequency of the primary winding is almost at my working frequency- 7.75GHz(working - 7.7GHz). Can I use such transformer? Someone says, that normally recommended to use inductor or transformer no higher than 3/4 of its self-resonanse frequency.. - why is that (is the constant inductance value plays here the main role)? Can you suggest me some literature were its explained?

Thank you!

The Balun self resonance freq. is too low for your freq. In inductor/transformer design, you want to push the self res. freq as high as possible so that you have margin for simulation inaccuracies... also you don't want your inductor becoming a capacitor !!

Windings or inductors do not work at 7GHz. I think tapered transition
might be a choice. Merchand balun may be too small to work properly
depending on your substrate.
S.H.

psmon said:

... also you don't want your inductor becoming a capacitor !!

Click to expand...

True, but anyway together with parallel capacitor I force resonanse on my working frequency, so just on the line of the inductor/capacitor behaviour. Inductance variation +/- 0.2nH is acceptable. Problem is, who ever I asked, my colleagues, says that they wouldn't recommed to use it on self resonant frequency - but nobody! can't explain why(mathematically or this some other basis)??

Added after 3 minutes:

shwoo said:

Windings or inductors do not work at 7GHz. I think tapered transition
might be a choice. Merchand balun may be too small to work properly
depending on your substrate.
S.H.

Click to expand...

I'm making MMIC receiver - completly integrated. On chip windings and inductors wokrs, just the values no higher than 10..15nH.

When you're close to self resonance, that means that parasitic capacitance, either substrate or winding-to-winding, is causing a resonance. If the Q is high, then resonance can be very narrow band! You therefore can't tune both matching and single-ended to differential convertion over a wide-band.

Lets consider the sources of capacitance. Substrate and winding capacitance is distributed over the entire length of the inductor. The closer you are to resonance, the more important it is that you model the distributed nature of that capacitance.

For substrate capacitance, you need to model the distributed substrate below the inductor accuratlly in order to get a correct response. Your primary source of variance will be the inductor to substrate height variation, or the oxide variation. Simulate your inductor for the extremes of the process, and extract the S-parameter repsonse using tools suchs as momentum.

Next is winding-to-winding capacitance. This capacitance generally doesn't vary as much because the spacing rules are generally better controlled then say the oxide thickness (as a percent of tottal). Use momentum again and consider the wrost-case width's of the metal (dl variations).

Lastly is your load impedance variation. The closer to resonance you are, the more sensitive the matching and single-to-differential convertion is.

If you take all these into account and simulate it and it still meets your requirements, then you are o.k. Typically though, the oxide variation is approx +/-10%, the substrate modelling is too difficult to guarantee and accurate result and the load variation is too high to operate close to resonance.

Greg