Ron H said:
I think you need to take another hack at the equation for resonant frequency.
L*C=1/((2*pi*fc)^2)
Solve for LC, pick a value for one then solve for the other. You probably should keep XL=XC=(2*pi*fc*L) in the range 300<X<3000 ohms. I pulled this range out of my hat, so it's not a hard and fast rule. If you try to use a large L value in order to be able to use a variable capacitor, the self-capacitance of the inductor may be too large.
Ack! You're right, I was writing uH, but calculating with mH, didn't check my units. So my calculations were off by 3 orders of magnitude.
One thing I don't understand is this: Why would a large inductor fail to work? I am not used to using inductors at all, and so I am very unfamiliar with their characteristics.
Ok then... doing a bit more searching, I found some HiQ tunable coils but they are each near 1uH, instead of 1mH. How would these changes go:
Tunable coil: .450uH - .550uH
Capacitor: 16uF
This would tune from 53.651kHz - 59.313kHz, a bit narrower than I wanted. Is there a way to increase this range?
The other problem is the fact that I can only find 8uF capacitors in 10% meaning we push ourselves out of frequency if each is 8.8uF, and nearly so if each is 7.6uF, this seems like a bad result to me.
Digikey has some here:
**broken link removed** but I don't see the range of each in the listings. Ideally I'd love to have one that tunes over .3uH - 1.5uH.
The voltage follower is there because the impedance looking back into the tank circuit at resonance is the value of the series resistors (the tank circuit itself has very high impedance). You don't want to load it with the next stage. If the next stage has very high input impedance at 56 kHz, then you don't need the voltage follower.
Q is a measure of the bandwidth of the bandpass filter. The higher the Q, the lower the bandwidth.
Gotcha, both of those make sense to me, except, what are the units on Q?