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  1. #1
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    how to improve simple FM circuit ??

    hi all,

    i want ask you about ...

    1. how i can transmit my message in narrow bandwidth ??

    2. how i can reduce a noise " i want clear sound " ??

    for example



    this circuit is working ,, if you choose C3 and L1 Properly

    thanks

    •   Alt1st August 2010, 00:06

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  2. #2
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    how to improve simple FM circuit ??

    Is it an FM transmitter? It looks like an AM one to me (and not a very good one at that).

    Keith
    I started life with nothing and I've still got most of it left. (Seasick Steve)



    •   Alt1st August 2010, 07:42

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  3. #3
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    Re: how to improve simple FM circuit ??

    Hi,

    The circuit can be used as an FM transmitter but it's likely to have poor frequency stability and harmonics in its output. Also, the tuned circuit in the oscillator isn't isolated from the antenna, so when something moves near the antenna, the frequency may change.

    It's a good idea to power the oscillator from a regulated voltage source. A 7805 voltage regulator is suitable, provided that the input voltage to the regulator doesn't drop very much when the circuit is operated. The 7805 isn't quite the best choice for battery operation as it draws approx. 5mA of current. A resistor with a zener diode can also be used to keep the voltage stable.

    The Colpitts oscillator topology isn't very stable to begin with and it can have too much phase noise and other issues. I'd recommend using a Vackar oscillator instead, with a varactor diode to achieve a good modulation index. (The generation of FM in your circuit is based on the effects of the voltage-dependent junction capacitances of the oscillator transistor, effects that alter the frequency and that the Vackar oscillator can eliminate to a great extent, reducing frequency drift. The junction capacitances will also change with temperature.) The Vackar oscillator can also have less phase noise, however, the use of a varactor diode is bound to increase it somewhat.

    Either way, to eliminate the capacitive effects of the antenna, an RF amplifier should follow the oscillator. Its output can be tuned to reduce the level of harmonic oscillations going into the antenna.

    To reduce noise (other than phase noise, at least), low-noise transistors can be used in the circuit. The biasing networks of the transistors could also be optimized and low-noise resistors could be used. However, I think some part of the noise you mentioned can be phase noise, generated in the oscillator.

    To reduce frequency deviation and bandwidth, you can use a resistor in series with C2 (it's preferred to place the resistor between the capacitor and Q1's base). This resistor will increase the negative feedback in Q1, reducing gain and the signal level to the oscillator/modulator. Another way is to use an emitter degeneration resistor on Q1, connected from the emitter to ground. It'll have almost similar effects. Negative feedback can also reduce noise.

    Your circuit doesn't have a frequency pre-emphasis; if used with a normal FM radio receiver, the de-emphasis circuit in the receiver will attenuate high frequencies too much, resulting in poor frequency response. The baseband signal from the microphone or from Q1 should be filtered before it enters the modulator.

    It may be a good idea to filter out any RF coming down the microphone wires, especially if they're long as it may cause unwanted feedback from the antenna to the input circuitry.

    Check out Audioguru's FM transmitter design in this thread: http://www.edaboard.com/ftopic372160.html
    It uses a voltage regulator for the oscillator (which is Colpitts style) and it has a tuned RF amplifier. It also has frequency pre-emphasis.



    •   Alt1st August 2010, 21:30

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  4. #4
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    Re: how to improve simple FM circuit ??

    Thank you so much for these important observations.

    Now, I am working to improve input source, feedback , ...



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