This electronic circuit is a fm transmitter circuit that suitable for beginners for stereo encoder testing. Provided the input stage is designed to accommodate line input levels i.e. approx. 2Vrms into 50kOhm. The transmitter output power is approx. 80mW, which is sufficient to cover an area of about one hundred meters.The RF portion consists of a totally screened oscillator which is operated by a stabilised voltage and a loosely coupled buffer stage. With this two-stage lay-out a good frequency stability and low harmonic interference radiation is achieved. The frequency of the oscillator can be adjusted in the commercial range (88-108MHz) by a trimmer capacitor Cl.
The FM-modulation of the transmitter is achieved by a varicap BA138. The frequency ratio can be adjusted by the capacitor C6.
To enlarge the transmission range, the output circuit which is tuned with capacitor C14, can be terminated into a full wavelength antenna.
RF filters (RFC1-3) minimize power supply hum in the transmitted signal.
Specifications:
Operating voltage: 9-12V
Operating current: 8mA
Frequency (adjustable): 88-108MHz
Inductors:
L1: 3 turns, 5mm coil diameter (air-core), 1mm copper wire with a tap-off at a wire length of 12 mm, measured from ground terminal.
L2: 2 turns, 5mm coil diameter (air core), 1.5mm copper wire.
RFC1-3: 5 turns, 6-hole ferroxcube, thin Copper wire.
Antenna construction:
The antenna consists of a full wavelength dipole configuration. The construction details are indicated in figure below.
See more : FM Stereo Encoder - FM Transmitters - Stereo Transmitter
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