Designing a Varactor FM modulator w/ carrier freq = 1MHz

[KennyG]

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/fr...lation-circuit

[pebe]

I have tried the link (you should have kept to one thread only) and it is still no better. There is no mention of a varactor and it comes up with 'Page not Found 404'.

Click on the link for yourself.

[KennyG]

[quote="KennyG"]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/frequ ... on-circuit

[pebe]

Got it OK.

There is quite a wide range of suitable combinations of values, but these preferred values will probably be OK.

A resonant circuit with an inductance of 390µH will need 65pF in prallel to tune it to 1MHz.
The MMVL3102T1 varicap will give the following capacities with these voltages applied:
0.5V - 36pF
10V - 8pF
3V (nominal bias) - 20pF

Allow series cap < diode, so series cap C = 5pF
Sum of the two Caps = 4pF. With a tuning cap = 56pF will give a total tuning capacity of 60pF (nominal). That gives freq = 1.04MHz

If that is not close enough, then use a cored inductor, or add a small trimmer cap for adjustment.

If the modualtion is to be audio then the resistor feeding the voltage to the varactor is not fussy and anything between 47K and 1M will do.

I hope that helps.

[KennyG]

Got it OK.

There is quite a wide range of suitable combinations of values, but these preferred values will probably be OK.

A resonant circuit with an inductance of 390µH will need 65pF in prallel to tune it to 1MHz.
The MMVL3102T1 varicap will give the following capacities with these voltages applied:
0.5V - 36pF
10V - 8pF
3V (nominal bias) - 20pF

Allow series cap < diode, so series cap C = 5pF
Sum of the two Caps = 4pF. With a tuning cap = 56pF will give a total tuning capacity of 60pF (nominal). That gives freq = 1.04MHz

If that is not close enough, then use a cored inductor, or add a small trimmer cap for adjustment.

If the modualtion is to be audio then the resistor feeding the voltage to the varactor is not fussy and anything between 47K and 1M will do.

I hope that helps.

Wow it helped a lot. Now everything is clear
thank you pebe

-kenny- ^_^

[KennyG]

pebe, after googling MMVL3102T1 it shows that this is not a varactor diode.
What should I consider when looking for a varactor diode that would satisfy on what is being required? and what voltage value should place at Vo?

[pebe]

Kenny,
Here is the data sheet for that diode.

http://www.datasheetcatalog.org/datashe ... he6iwy.pdf

[KennyG]

okay. Thanks