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FM transmitter (mod4)

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, Oct 26, 2006.

  1. mneary

    mneary New Member

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    Most of your questions are answered (several times each, in most cases), in the previous 499 messages in this thread.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Because America was the first country to have FM radio stations then they decided to use a lot of pre-emphasis to reduce hiss. In those days microphones and recording equipment produced few high frequencies. Then later when Europe began using FM stations the microphones and recording equipment had more high frequencies and they decided to use less pre-emphasis so that there is less over-modulation. America kept their "mistake" and Europe used their different pre-emphasis.

    The audio signal from Q1 to the base of Q2 amplitude-modulates the base voltage of Q2. When the amplitude of the base voltage of Q2 is modulated then the changing voltage causes its capacitance to change. The changing capacitance changes the frequency of the oscillator which is FM. FM radios ignore the small amount of AM.

    A 9V battery voltage drops to only 6V or less during its life.
    Q1 works properly only when its supply is 5.0V.
    If the supply to the oscillator changes then its frequency changes.
    Cheap simple FM transmitter circuits without a voltage regulator have their frequency changing all over the place.

    Q2 is a Colpitts oscillator circuit. Q3 is an RF amplifier and buffer so that the oscillator frequency does not change when something moves toward or away from the antenna. Other circuits are Hartley or many other types.

    R1 powers the Jfet inside an electret mic. If R1 is removed then a dynamic mic (coil and magnet) can be used.

    I used books when I went to school and university 48 years ago. In the seventies I bought The TTL and The Cmos Cookbooks by Don Lancaster.The Art Of Electronics is a good book today.
     
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  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why are there 500 replies to this thread about this simple FM transmitter circuit?
    It is mono, not stereo. My design is more than 5 years old.
     
  4. dave

    Dave New Member

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  5. gauravkalra

    gauravkalra New Member

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    thanks very much audioguru for all your assistance
    actually i have made a temperature sensor using microcontoller
    but now i want to make the sensor go wireless like sensor on transmitter circuit and dispay on the reciever side
    ur transmitter has a good range . so i think it is possible to do it.
    so sensor in place of microphone
    value of r1 will be adjusted according to the voltage given to the sensor
    c1 change according to the output current
    no need for c4
    and the rest of the circuit will be same
    am i right

    thanks
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My FM transmitter has an AC input from a microphone.
    But a temperature sensor has a DC output, not AC.

    An FM radio has an AC output, not a DC output.
    So your idea will not work unless the temperature sensor has an audio output frequency that changes when the temperature changes.

    An FM radio has de-emphasis that reduces high audio frequencies. C4 in my circuit does the opposite so that all audio frequencies have the same output level.
     
  7. gauravkalra

    gauravkalra New Member

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    i dont think so audioguru
    maybe thermocouples give ac output .
    but i want to use ic temperature sensor lm 35 series
    so can i use an inverter which converts dc into ac of audio frequency
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You can use an LM35 that has a DC outout that drives a VCO if you want. Then decode it at the FM receiver.
     
  9. gauravkalra

    gauravkalra New Member

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    thanks audioguru
    i just searched about about many types of VCOs
    so i have to design vco which gives 88 to 108 MHz output frequency
    but designing is very difficult for beginner like me
    i also read about integrated circuit vco but they give pulsed output with 2 MHz maximum frequency 2 MHz which is very less than 88 to 108 MHz

    so i cannot use vco directly by replacing colpitts oscillator
    so can i use pulsed output as modulating signal at the base of q2
    but i know fm is the modulation of oscillator signals by the amplitude of modulating signal but not the frquency
    but pulsed output has constant amplitude at each cyle so i think frequency modulatio is not possible in the case of ic vco

    so help me by giving me some tips /tutorials about how to design a vco with frequency 88.5 to 108.5 MHz

    thanks
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Rohm make some very good FM stereo transmitter ICs. They use a quartz crystal controlled frequency synthesizer with a phase-locked-loop and use some dip-switches to select a frequency. Most MP3 player to car radio transmitters use one.

    Sorry, I don't know a VCO circuit that works near 100MHz and i don't know any tutorials.
     
  11. mbarazeen

    mbarazeen Member

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    you want to use the audio input of the FM modulator for the purpose, why do you need a VCO on FM range? your receiver will be tuned to only one frequency but the audio input can be from a low frequency VCO.
    or you can think about coding the value to multiplexed tones as a digital bits.
     
  12. gauravkalra

    gauravkalra New Member

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    actually i want to make a wireless temperature sensor whose input on transmitter side and display on reciever side
    but sensor actually gives 1 mV to 1000 mV dc output so i have to use a vco in order to convert dc input into output frequency

    many vco ic take that dc input and convert them into output frequency around 2 Mhz but square wave output
    so problem is that square wave output could not be used for fm
    so i want to directlry convert dc input intom fm frequency and send it to antenna
     
  13. gauravkalra

    gauravkalra New Member

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    hello audioguru
    can a varactor diode be added in your circuit for dc voltage

    can you give me othe oscillator designs like hartley or othes which use varactor diode
     
    Last edited: Jun 23, 2010
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You can add a varactor diode across the tuning inductor if you want.
    There are hundreds of FM transmitter circuits on the internet. Some use a varactor diode for tuning and for FM modulation.
     
  15. mbarazeen

    mbarazeen Member

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    why don't you covert the out put to binary, depends on your accuracy you can make it to 8 or 16 bit data. then use two low frequency audio to represent the data, like scanning the data and switch between both audio sources as per the value of the byte and feed it to the modulator.( it will give you a serial data of two different tones like you play a piano to the microphone input of the transmitter using two keys only). scan it at a rate of 1 cycle/ sec or faster depends on the required scanning rate of your system. (there may be some limitations)

    at the receiver end you have to decode the audio into a parallel data (reverse) and feed it to your display.
     
  16. gauravkalra

    gauravkalra New Member

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    hello audioguru

    i have made some modification in ur circuit with varactor diode and want to share it with you
    but not able to attach the schematic file inthe post

    so any solution
     
    Last edited: Jun 24, 2010
  17. gauravkalra

    gauravkalra New Member

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    so finally i attach the image in my album
    link is
    http://www.electro-tech-online.com/attachments/

    so i made changes
    plz correct me if i am wrong anywhere
    component numbers are differ in this image than urs image.
    1 varactor is added in parallel with inductor
    2 reverse voltage is applied across varactor and inductor 10 uH
    3 10 uH is added so as to block oscillator signals to short into the ground
    4 i think 10uH is sufficient to produce large impedence for oscillator signals?
    5 as figure shows there is open between L1 and D1 but it not open but ther is a capacitor in between .this capacitor is added to prevent shorting of Q1 collector .am i right in doing all these?
    6 let suppose this capacitor is c16 so c16,c1 and c varacotr are inseries and their is equivalent is parallel with L1 and C2
    7 so L1,c2,c15,c16 determine the frequency of carrier oscillations and c varactor determines frequency deviation
    8 Ceq=c1 series c16 series c varactor
    9 c total=Ceq+c2
    oscillation f=1/(2 pi sq root(c total*L1))
    10 first transistor of ur circuit is not needed at this point so i start the circuit with r1 and c4?
    11 i dont know whether c4 is required or not ?
    12 rest of the circuit is same
    13 in figure ,left topmost wire no included in figure is 5V regulated
    14 wire applied across varactor is amplified output from sensor

    plz correct me if i am wrong anywher
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your schematic is huge and has pastel lines on a dark grey cloth background so I can barely see it and I cannot correct it.

    1) Remove L2.
    2) Connect the anode of the varactor diode directly to the collector of Q1, not to its emitter.
    3) Reduce the value of trimmer capacitor C2 by the capacitance of the varactor diode.
    4) C4 causes the transistor to be a common-base oscillator and must be used.

    Your circuit has the varactor diode biased oddly. It would be difficult to change its capacitance with voltage.

    Varactor tuned FM transmitters have the varactor diode connected to ground so that a positive DC voltage can tune the radio frequency like this one:
     

    Attached Files:

  19. gauravkalra

    gauravkalra New Member

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    yes thanks
    i saw transmitters with varactor diode
    but still i want to experiment it on ur circuit
    here again modification
    will it work
    Picture 1 of 3 from modified fm transmittor

    sorry for blurred and enlarged image
    i dont know how to convert schematic file into image file
    so i am using my digital camera to take picture from schematic
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your schematic is much too big and the lines on iyt are almost the same color as the dark background so i cannot look at it.
    Use a schematic drawing program.

    I make schematics and copy parts of other schematics with Microsoft Paint program. Then I save it as a PNG file and attach the file to my reply here.
     
  21. gauravkalra

    gauravkalra New Member

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    now my schematic is okay

    ground connected to varactor diode is the ground of opamp connected to sensor
    it is not the ground of actual circuit
    i have done this so that collector of q2 doesnot short to the ground

    Picture 1 of 4 from modified fm transmittor
     

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