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fm modulator circuit schematic

FM Modulator Circuit

The FM modulator circuit (frequency modulation) is built with a Motorola MC1648P oscillator. Two varactors, Motorola MV-209 are used to frequency modulate the oscillator. The 5000 Ω potentiometer is used to bias the varactors for the best linearity. The output frequency of approximately 100 MHz can be adjusted by changing the value of the inductor.

The output frequency can vary as much 10 MHz on each side. The output level of the modulator is -5 dBm. In this fm modulator prototype the varactor bias was 7.5V for the best linearity but this could be different with other varactors.

FM modulator circuit diagram

fm modulator circuit schematic

18 Comments

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  • alexpmgr8

    Hi folks, for those of you who want to look at a good site, i suggest http://www.sm0vpo.com/ (ultimate site for learners and experienced alike.) Dont send him silly questions though. From a one tranny circ. to more elaborate transmitters such as V7b and v7
    Here you will all find the circuits and theories behind radio circuits which are simple to follow.
    As you can see above, there is a coil with two tuning diodes(mv209, 2 x bb105 or similar may work too) (find, the veronica 1 watt guys , similar tuning deal) the mc1648 chip is a stabiliser type chip regulated by the 5v zener, although you will find that this was quite a stable circuit without if the circuit ran off say a 9v regulator, the earliest model i can think of was a kids c.b circuit and then the 3w pantek came from that (good sound , rubbish harmonics but stable). This is prone to harmonics with out the chip and a filter though. This will not propogate more than one room i would say. I see no amplification either. This should put out the few miliwatts that could be amplified.

    • smseeja

      Alexpmgr8 i want to learn about Antenna very interested in Radios and Antennas, please refer me any excellent site about Antennas.

    • alexpmgr8

      That site name again, sm0vpo.com typo in my above comment. thats a zero in the middle. sorry bout that.

  • darya khan lund

    mujy polarity samaj mein nhn a rahi kia karoon????????

  • RONAK

    your efforts are appreciatable

  • alexpmgr8

    karan, integrated circuit nuber#1 is Motorola MC1648P chip.

  • karan

    what is ic#1????

  • burak

    is the IC#1 the circuit as known as 555 ?

  • Syam

    i need a fm audio loss alarm circuit,when a particular fm station is off at working an alarm will produce.kindly send anybody know the circuit diagram

  • BUKKY KTS

    Please, i need component list of this circuit.Thank

  • livingstone

    i love the website with it’s explanation of reseach. i’ll like to “frequency modulator circuit diagram with details” thanks so much.

  • dharmy

    please i need a complete circuit diagram for a one transistor radio kit

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