# Tuning Frequency

#### frisky

Joined Jul 25, 2005
3
I am not sure if I am doing this question right:

A coil and 2 capacitors are connected in parallel. If the inductance of the coil is 100 uH and the fixed capacitance C1 is 80 pF, what should be C2 to tune in a broadcast frequency of 1000 kHz?

Here is what I was thinking:

C1 and C2 are in parallel, so their combined capacitance is C1 + C2.

A resonant circuit has maximum current when 2πfL = 1/(2πfC), or

C = 1/((2πf)^2L)

Since C = C1 + C2

C2 = 1/(2πf)^2L) - C1

Is this right?

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by frisky@Jul 25 2005, 10:24 AM
I am not sure if I am doing this question right:

A coil and 2 capacitors are connected in parallel. If the inductance of the coil is 100 uH and the fixed capacitance C1 is 80 pF, what should be C2 to tune in a broadcast frequency of 1000 kHz?

Here is what I was thinking:

C1 and C2 are in parallel, so their combined capacitance is C1 + C2.

A resonant circuit has maximum current when 2πfL = 1/(2πfC), or

C = 1/((2πf)^2L)

Since C = C1 + C2

C2 = 1/(2πf)^2L) - C1

Is this right?
[post=9253]Quoted post[/post]​
Hi Frisky,

Your equations all look good. You are obviously absorbing the material being presented.

I think that the phrase in your post that I have highlighted in RED may be more generally stated by saying that an RLC circuit whether it is a parallel or a series configuration is said to be in resonance when 2πfL = 1/(2πfC).

Good job,
hgmjr

#### frisky

Joined Jul 25, 2005
3
Originally posted by hgmjr@Jul 25 2005, 06:37 PM

173 pF

#### hgmjr

Joined Jan 28, 2005
9,027
Originally posted by frisky@Jul 26 2005, 11:28 PM
173 pF
[post=9287]Quoted post[/post]​
That's the value I calculated when rounded to the nearest pF.

Good work,
hgmjr