Voice Transmitter Via FM Band 2 VHF by Hartley Oscillator

Figure 1 

The circuit was designed to operate a frequency modulation voice transmitter over the FM band 2 frequency range of VHF.

• 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 radio
• Frequency 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 signal
• Very High frequency (VHF) – the radio frequency range from 30 MHz to 300 MHz
• Band 2 VHF – an FM radio broadcasting band that operates within the frequency range of 88 to 108 MHz
• Hartley Oscillator – an electronic oscillator that derives its feedback from a tapped coil in parallel with a capacitor or the tank circuit

A Hartley type oscillator was used for the circuit because the frequency may be varied using a variable capacitor, either a tapped coil or two fixed inductors are needed, and the output amplitude remains constant over the frequency range. In a VHF configuration, the base emitter capacitance of the transistor acts as a short circuit as the capacitor in the tank circuit typically connect at the base of the transistor.

Wrapped around a quarter inch rod is a coil of four turns made of 18 standard wire gauge (swg) wire. By wrapping the turns in threads, a uniform separation was made between the coil windings. About one and a half turns from the supply end is the aerial tap or the antenna. Using an electret condenser microphone will provide good audio sensitivity and it can capture any audio in a room.

Application

FM is commonly used at VHF radio frequencies for high fidelity broadcasts of music and speech. It is also used at intermediate frequencies by all analog VCR systems, including VHS, to record both the luminance and the chrominance portions of the video signal. It can also be used at audio frequencies to synthesize sound. The Hartley oscillator was extensively used on all broadcast bands including the FM 88-108MHz band. An example is given of the Scott 310E RF oscillator for its FM section.

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