How to design a Varactor FM Modulator?

Thread Starter

electrogirl

Joined May 2, 2010
47
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:

CDRIVE

Joined Jul 1, 2008
2,219
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.
 

Thread Starter

electrogirl

Joined May 2, 2010
47
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.

Read this PDF for more information on varicap diodes:
Variable Capacitance Diodes

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

Bertus
Sorry but I didn't get it...I'm just a newbie please do understand ^^,
 

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bertus

Joined Apr 5, 2008
22,270
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
 

CDRIVE

Joined Jul 1, 2008
2,219
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:

Audioguru

Joined Dec 20, 2007
11,248
Never mind talking about the politics of why your teacher doesn't teach anything in your country. Do gardening instead of electronics.
 
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