Inductor Self-Resonance Frequency: What's high enough?

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DigiTan

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Wikipedia says the self-resonance frequency (SRF) of an inductor is the point where it ceases to function as an inductor. Right now I have a 180MHz circuit, and a choice of inductors with SCFs ranging from 240MHz to 1.8GHz.

I'm guessing the 240MHz components are too close. In general, what SRF range do you shoot for if you already know your operating frequency? Two or three times higher?
 
What are you building?

If it's an oscillator then just use a smaller tuning capacitor to compensate for it.

You can figure out the parasitic capacitance by rearanging the formula for capacitance.
 
It's going in a tank circuit in an FM Transmitter. The vendor's impedance vs. frequency graphs didn't cover my inductance range, so I can't really base an accurate estimate off what I saw there. Until, I do I was hoping there might be some rule of thumb for avoiding low self-resonance frequencies. Like should I operate under half the SRF or something like that?
 
I would guess the SRF is a very high Q two-pole peak. As such, significant phase shift will be present +/- one octave of SRF. I would say at least a factor of 2 if phase is of any importance.
 
I do not know of any rule of thumb (how many rules can a thumb hold anyways), but I would say. just operate under the SRF with margin.
 
Just make your own coil with thick wire and air as its core. Nearly every FM transmitter project has one made like that.
 
Well, only thing is I'm buying those surface-mount chip inductors in the size 2012 packages. I suppose I could sub a hand-wound one in the prototype. The others are using SMT inductors.
 
DigiTan said:
It's going in a tank circuit in an FM Transmitter. The vendor's impedance vs. frequency graphs didn't cover my inductance range, so I can't really base an accurate estimate off what I saw there.
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If it is a new design, why would it matter what the self resonant frequency is, as long as it is higher than your operating frequency. In a tank circuit, it's like getting part of your capacitance for free. I'd just check to make sure that the self capacitance is stable with temperature, and that its value is stays consistent from part to part and batch to batch.
 
DigiTan said:
Well, only thing is I'm buying those surface-mount chip inductors in the size 2012 packages. I suppose I could sub a hand-wound one in the prototype. The others are using SMT inductors.
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Doesn't sound a good idea to use commercial SM inductors, use a simple handwound air cored coil.

Inductance isn't the only thing you need to be concerned about.
 

Exactly, I suppose the only problem is that the Q might be low if you're relying too much on the parasitic capacitance.

Calulate the resonant requency you desire, using F0 = 1/root(LC), then transpose it for C, make F0 the self resonant frequency and subtract it from the capacitance you desire.

For example, suppose you want have a 100nH coil with a SRF of 250MHz and you want a resonant frequency of 107MHz. Ignore the SRF for now and calculate the value of capacitor requred for 100MHz, 22pF, now work out the parasitic capacitance, 4pF so use an 18pF capacitor.
 
I'll probably just go with the 1.8GHz SRF inductor. I can still get an SMT model with a 5% tolerance. I can borrow a C meter, so parasitic capacitance isn't nearly as important right now as finding an accurate inductor. And incidentally, the 1.8GHz inductor has a higher Q than all the ones with the lower SRFs.
 
And when this approach falls flat on its face you'll probably go back and wind an air core coil.
 
And when this approach falls flat on its face you'll probably go back and wind an air core coil.
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How many air wound coils do you see in a PCS cell phone?

@1.8 Gig the PCB traces become inductors. much care must be taken.
 
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Mikebits said:
How many air wound coils do you see in a PCS cell phone?
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Exactly! I mean, forgive me for keeping up with the times--but I was told SMT was a proven technology. Cellular's got em. WiFi's got em. Bluetooth? You'd better believe it. "Wind an air core?" No thanks.
 
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DigiTan said:
Exactly! I mean, forgive me for keeping up with the times--but I was told SMT was a proven technology. Cellular's got em. WiFi's got em. Bluetooth? You'd better believe it. "Wind an air core?" No thanks.
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You need to consider the frequencies involved, at those frequencies you don't use coils at all, just etched PCB tracks - try reading about 'lecher lines' and such.
 
At only 180 MHz, I would use SMT inductors up to a hundred milliwatts or so. Just be sure to consider the current rating.
 
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