# How to design a Varactor FM Modulator?

Discussion in 'The Projects Forum' started by electrogirl, Mar 9, 2011.

1. ### electrogirl Thread Starter Member

May 2, 2010
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0
Hi, In our basic electronics class we were asked to design a Varactor diode FM Modulator. Each of us were assigned with a different carrier frequnecy in our FM modulator project. The carrier frequency assigned to me was fc = 1 MHz. I know tha principles of frequency modulation and on how a varactor diode can be used to varry the oscillating frequency.

What is not clear to me at this point is on what component values should I used in order to fulfill what was assigned to me. That is the resistor values, the capacitor values, the inductor values and on what type of varactor diode am I going to use.

Any help is very much appreciated.
Go to the link to see the circuit of a Varactor FM Modulator.
http://www.tutorvista.com/physics/frequency-modulation-circuit

Last edited: Mar 30, 2011
2. ### electrogirl Thread Starter Member

May 2, 2010
47
0
up...up....upp

3. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
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The data sheet for any particular Varicap will show you a curve of voltage input vs. resultant capacitance. Roughly speaking you have to factor the min and max capacitance into your resonant freq equations.

4. ### electrogirl Thread Starter Member

May 2, 2010
47
0
up...up update

5. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
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If you keep doing this the Mods are going to spank you..

Apr 5, 2008
17,143
3,009
Hello,

There is no need to do the bumping.
It can take more than 24 hours for an answer, as this is a worldwide forum.

Variable Capacitance Diodes

This is a link from the diode types page of the EDUCYPEDIA:
Diode types

Bertus

7. ### electrogirl Thread Starter Member

May 2, 2010
47
0
Sorry but I didn't get it...I'm just a newbie please do understand ^^,

• ###### fm modulator.gif
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Apr 5, 2008
17,143
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Hello,

The posted circuit will not work as L1 will short the DC voltage needed for the varicap.

Bertus

9. ### electrogirl Thread Starter Member

May 2, 2010
47
0
this circuit uses a crystal oscillator to generate the carrier frequency

• ###### fm mod2.png
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Apr 5, 2008
17,143
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Hello,

That circuit will work, as the crystal will act as a series LC circuit.
The two resistors connected to the varicap will provide the needed DC voltage to set the base capacitance.

Bertus

11. ### electrogirl Thread Starter Member

May 2, 2010
47
0
How do I figure it out what component values to use?

12. ### Audioguru AAC Fanatic!

Dec 20, 2007
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Why do you want to modulate with FM in the middle of the AM broadcast band??

13. ### electrogirl Thread Starter Member

May 2, 2010
47
0
huh?what do you mean?

14. ### Audioguru AAC Fanatic!

Dec 20, 2007
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Your circuit is at 1MHz. The AM broadcast band in North America (I don't know where you are) is from 0.54MHz to 1.6MHz.

15. ### electrogirl Thread Starter Member

May 2, 2010
47
0
But I'm not from north america( ^^,)
Can you help me figured out those component values?

Dec 20, 2007
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17. ### electrogirl Thread Starter Member

May 2, 2010
47
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He wont tell T_T

18. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,223
101
Your first circuit (LC) should work if you add a DC blocking cap between the cathode of D1 and the junction of L1 and C1. This cap will be very large in comparison to your tank caps. A 1000pF should work OK.

Last edited: Mar 25, 2011

Dec 20, 2007
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