# FM Transmitter(92 MHz)

Discussion in 'The Projects Forum' started by abdullah8391, Apr 7, 2016.

1. ### abdullah8391 Thread Starter Member

Sep 6, 2015
53
1
Hi everyone, i have chosen to make an fm transmitter as my semester project. but after several days of research, I couldn't get many answers.
I am making transmitter from colpit using transistor. A circuit design that gives almost 94 MHz output is attached,(though in calculations, it should give 116 MHz).
I know:
1. signal received would be maximum of 150 mV, and I need to amplify its current and voltage.
but how?(i.e through common emitter or common collector?).

2. I need to generate 90 MHz frequency, and modulate the input sound signal with generated 90 MHz signal.
but how will it modulate and how should I design it? because the attached pic doesn't satisfies theoretically.

So i shall be really great full if you could provide me with links how to design it. Or at least tell my if you know anything about it.
For my last project (obstacle avoiding robot), I got great help from here, so i am expecting for good response again.

• ###### circuit.png
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2. ### GopherT AAC Fanatic!

Nov 23, 2012
6,303
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What have you found when you searched for circuits that can do frequency modulation demodulation?

3. ### abdullah8391 Thread Starter Member

Sep 6, 2015
53
1
I am not working on demodulation, i am working on Frequency modulation, and need help regarding that topic.

4. ### GopherT AAC Fanatic!

Nov 23, 2012
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Ok, what have you found in your searches of "frequency modulation circuits"?

5. ### abdullah8391 Thread Starter Member

Sep 6, 2015
53
1
1. take signal through mice or through headphone (from mobile).
2. amplify the input signal using common emitter amplifier.
3. design a colpit by fr=1/(2*pi*root(LC)).
4. attach the output of amplified signal to the base of transistor of colpit circuit.
5. attach antenna at the output of colpit circuit.

now my questions:
1. up to what voltage and current should i amplify in point 2.
2. should I use common emitter for amplification because it gives phase shift of 180, which will definitely cause wrong output sound.
3. what should be output voltage/amplitude of generated wave.
4. I have designed a colpit (you can say through hit and trial method and figure is attached). theoretically it should give 109 MHz, but on simulator it is giving 97 MHz. why???

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6. ### GopherT AAC Fanatic!

Nov 23, 2012
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4. The answer is usually because components like capacitors have a tolerance of only +/-20% unless you have paid for components with tighter tolerances.
-or-
The tank circuit of the oscillator is somehow influenced by the input signal or the output load.

For other questions, have you searched for an IC that can do the modulation for you instead of using all discrete transistors?

7. ### abdullah8391 Thread Starter Member

Sep 6, 2015
53
1
we are not allowed to use that. :-(

what about points 1,2 and 3?

Last edited by a moderator: Apr 9, 2016
8. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,967
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Since you have a simulator you should be able to answer those questions yourself. It's easy to try various schemes and compare results.
How are you going to get frequency modulation; not amplitude modulation?

9. ### abdullah8391 Thread Starter Member

Sep 6, 2015
53
1
I have tried many thing and thanks to that my colpit is working.
now i don't know how much should i amplify input signal before sending it to colpit fo mudulation, and what should be the amplitude of waves generated by colpit.