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VHF Center Frequency FM Transmitter

2016-04-05 14:24  
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The circuit was designed to function on the middle of the FM band, 98 MHz, since the FM frequency ranges from 88 MHz to 108 MHz.

Transmitter – an electronic device that can produce or amplify a carrier wave signal, modulates it with a significant signal, and radiates the resulting signal from an antenna which are being utilized in television, telecommunications, and radioFrequency Modulation (FM) – transmits its signal or information over a carrier wave by changing its frequency but it can also be taken into account as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signalTrimmer Capacitor – a type of variable capacitor that is operated that is operated with a small screwdriver or similar tool, and is designed to be set when the circuit is made and then left without further adjustment

The circuit works as the transmitted signal is a Frequency Modulatedwhich means that the carrier’s amplitude stays constant and its frequency varies according to the amplitude variations of the audio signal. When the input signal’s amplitude increases the frequency of the carrier increases too, and vice versa. For this circuit to be tuned at the center frequency (98 MHz) of the FM band, it utilizes an inductor rated at 0.112 uH with a variable capacitor VC1 in the center value of 33 pF. The inductor has a standard wire gauge of 22, made of enameled copper wire that created its winding 5 times around a 3/16” diameter and 5mm long bolt. Otherwise, an adjustable molded coil can be used to achieve a fixed capacitor value of 33 pF from the variable capacitor. Without setting the variable capacitor to a fixed value, the tuning can provide the range between 85 to 125 MHz.

To produce a repetitive electronic signal, to op-amp oscillators were used in the circuit. The electret microphone used was an omnidirectional type while the antenna is a single vertical wire above a ground plane known as whip, stub or monopole, around 76 cms of 22 swg copper wire. The combination of the transistor and LC oscillator (a variable capacitor in series with an inductor) produces a clapp oscillator on Q1 which prevents the circuit to have a variable feedback voltage and cause it to less likely achieve oscillation over a portion of the desired frequency range.

FM is commonly used atVHFradio frequenciesfor high fidelity broadcasts of music and speech. It is also used at intermediate frequencies by all analogVCRsystems, includingVHS, to record both the luminance and the chrominance portions of the video signal. FM is the only feasible method of recording video to and retrieving video from magnetic tape without extreme distortion, as video signals have a very large range of frequency components. It can also be used at audio frequencies to synthesize sound, known as FM synthesis, which was popularized by early digital synthesizers and became a standard feature for several generations of personal computer sound cards. A narrow band form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.

The FM broadcast band may also be used for small scale industry like those FM transmittersthat can transmit a signal from an audio device to a standard FM radio receiver which ranges from small units built to carry audio to a car radio with no audio-in capability up to full-sized, near professional grade broadcasting systems that can be used to transmit audio throughout a property. FM broadcast band can also be used by some inexpensive wireless microphones, but professional-grade wireless microphones generally use bands in theUHFregion so they can run on dedicated equipment without broadcast interference. Low power transmitters are also sometimes used for neighborhood or campus radio stations, though campus radio stations are often run over carrier current.

Source:www.zen22142.zen.co.uk/Circuits/rf/txcct.htm


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