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Capacitors and resonant frequency

M

meyousikmann

Jan 1, 1970
0
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.
 
B

Baron

Jan 1, 1970
0
meyousikmann said:
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.

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

Charles Schuler

Jan 1, 1970
0
meyousikmann said:
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.

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 ;>)
 
T

Tom Biasi

Jan 1, 1970
0
meyousikmann said:
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.

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
 
J

John Popelish

Jan 1, 1970
0
meyousikmann said:
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.

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).
 
E

Eeyore

Jan 1, 1970
0
meyousikmann said:
Where does ppm fit into any of the calculations

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
 
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