When I design a radio, I have two components to fudge with, an inductor, and a capacitor. Is it always better to use a lower value inductor and a higher value capacitor or a higher value inductor and a lower value capacitor?

I am dealing with the VHF band.

 

You don't have a great deal of choice at VHF, you can't reduce the capacitance much or stray capacitance will take over. Likewise, you can't do too much with the coil either, it doesn't have many turns to start with.

Generally you would tend to use a variable capacitor, probably 5-25pF or so, with around 4-6 turns of wire - this should cover the FM broadcast band.

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I buy and use fixed inductors. I don't create my own.

What value of an inductor is equivalent to 4 - 6 turns of wire?

I need to know in uH, so that I can choose the best inductor.

 

There are a number of small FM transmitters in the Projects section, I seem to recall one of them says the coil is 0.17uH?.

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Since the tuned circuit will most probably be a parallel one, using a small value inductor with large capacitor values to resonate will increase the loaded Q of the circuit and also the selectivity. The drawback is that with increased Q, the insertion losses will go up. So you need to find a balance here.

 

Is bandwidth = 1/Q?

and how do I determine Q using L (inductor value) and C (capacitor value)?

 

Bandwidth is related to the Q-Factor of a resonance cct

Q = Centre Freq / Bandwidth

Or

Q = 1/R * SQRT( L/C)

to be prurely theoretical

 

Thanks for the equation. Now I shall rearrange it to suit my own frequency and bandwidth.

 

This is for the case where R is in series with the inductor. For parallel R, the equation is the reciprocal:

Q=R/√(L/C)

 

I looked at http://www.fcc.gov/mb/audio/bickel/oddno.html, and it seems that the bandwidth of an FM station is 200Khz wide.

When I use your formulas, it seems that the resistance must be less than 1 to achieve a 200Khz bandwidth. What does "R" actually mean in the equation? and why don't I use 2 and pi with the Q equation you specify?

When I calculate frequency, I use 1/(2*pi*sqrt(L*C))

 

what is insertion loss?

 

You seem a little confused about what you are doing?, for a start are you talking about transmitters or receivers?, and unless you are trying to build a broadcast quality transmitter the FCC spec doesn't seem very relevent?.

What EXACTLY are you trying to do?.

The bandwidth of the receiver is governed by the bandwidth of the IF amplifier, the front end tuning is ONLY to reduce image interference, not to set the bandwidth - and in a receiver with any pretensions of quality will consist of more than one tuned stage.

The transmitter bandwidth is dependent on the level of modulation, and bears no resemblance to the Q of any tuned circuit.

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