# Max and min frequencies of a capacitor

1. Jun 5, 2005

### laminatedevildoll

A variable capacitor with a range from 10 to 365 p is used with a coil to form a variable frequency LC circuit t0 tune the input to a radio.

a. What ratio of max and min frequencies may be obtained with such capcitor.

I used that $$\omega=\frac 1{\sqrt(LC)}$$ to get the ratio of max to min frequencies.

b. If this circuit is to obtain frequencies from 0.54 MHz to 1.60 mHz, the rato computed is too large. By adding a capacitor in parallel, this range may be adjused. What should the capacitance of this added be?

I said that $$C_{eq} = C_{old}+C_{new}$$

I have to compute what $$C_{new}$$. From part 1, I know my $$C_{old}$$. But, how do I find $$C_{eq}$$ first? I am thinking that I have to compute that using the frequencies, but how do I get L?

Last edited by a moderator: Jun 5, 2005
2. Jun 5, 2005

### inha

You're not doing anything to the coil, are you? Why would L change from what it was before adding the second capacitor?

3. Jun 5, 2005

### laminatedevildoll

I thought that the L cancelled out when I was doing the ratio before. So, would I use L from part A? Do I solve for L?

4. Jun 5, 2005

### inha

Use the earlier inductance and the frequencies the circuit is supposed to be able to deal with. Solve for C_eq and solve for C_new.

5. Jun 5, 2005

### Gokul43201

Staff Emeritus
You are given $\omega_1, ~\omega _2$. You know $C_{old}$. The unknowns are $L,~C_{eq},~C_{new}$.

You have three equations in 3 unknowns...

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