1. Home
  2. Hobby Electronics Basics
  3. Capacitors and resonant frequency

Capacitors and resonant frequency

I have a problem that asks about the movement of the second plate of a capacitor in order to change the resonant frequency by 10 ppm in an LC circuit. I am getting stuck in terminology.

Here is the information given:

Area of each capacitor plate (one attached to solid nonmoving post while other can move small distances perpendicular to the plates) .01 m^2

Initial plate separation

100 micometers

Inductance

120 nH

I have found the initial capacitance and frequency, but the second part of the question says "How far must the second plate move to change the resonant frequency by 10ppm". I am not entirely sure what this question is asking for. Where does ppm fit into any of the calculations. I don't want answers, but I would appreciate if somebody can translate the question into something I might understand.

Reply to
meyousikmann
Loading thread data ...

Think about the definition of a capacitor ! The value of capacitance for a fixed plate area will vary in inverse proportion to the separation !

--
Baron:
Reply to
Baron

delta f = 10/1e6 x f1

fx = f1 - delta f

solve for new value of C based on fx

solve for new plate spacing based on C

(If I understand the question ;>)

Reply to
Charles Schuler

I believe you are being asked to determine the capacitance needed for that frequency change and then determine the plate areas that must face each other at the given distance.

Tom

Reply to
Tom Biasi

First you need to derive the relationship between capacitance and frequency (the power of proportionality).

Then you derive the relationship between plate spacing and capacitance (another power of proportionality). Then combine these two things to get 10 ppm frequency change.

10ppm is just a fraction 10/1,000,000 times the original frequency. So the original frequency must become either 1 + 10/1,000,000 of its original value, or 1 - 10/1,000,000 of it (depending on whether the plates are moved apart or together).
Reply to
John Popelish

It's another way of representing a small number ( as in parts per million ). Compare with per_cent for example which is parts per hundred.

Graham

Reply to
Eeyore

Off the top of my head, I think it's:

100um (plate spacing) * (10 ^ -6) = 10^ -9 m = 1 nanometer
Reply to
mphillipps2

Wait, my last post was wrong.

f = 1/(2 * pi * sqrt(LC))

so I get 2 nanometers.

snipped-for-privacy@yahoo.com wrote:

Reply to
mphillipps2

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.