CK200 - 3V FM TRANSMITTER

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ASSEMBLY INSTRUCTION
Components may be added to the PCB in any order. Note
that the electret microphone should be inserted with the
pin connected to the metal case connected to the negative
rail (that is, to the ground or zero voltage side of the
circuit). The coil should be about 3mm in diameter and 5
turns. The wire is tinned copper wire, 0.61 mm in diameter.

CIRCUIT DESCRIPTION
The circuit is basically a radio frequency (RF) oscillator
that operates around 100 MHz. Audio picked up and
amplified by the electret microphone is fed into the audio
amplifier stage built around the first transistor. Output
from the collector is fed into the base of the second
transistor where it modulates the resonant frequency of
the tank circuit (the 5 turn coil and the trimcap) by varying
the junction capacitance of the transistor. Junction
capacitance is a function of the potential difference
applied to the base of the transistor. The tank circuit is
connected in a Colpitts oscillator


The electret microphone: an electret is a permanently
charged dielectric. It is made by heating a ceramic material,
placing it in a magnetic field then allowing it to cool while
still in the magnetic field. It is the electrostatic equivalent
of a permanent magnet. In the electret microphone a slice
of this material is used as part of the dielectric of a
capacitor in which the diaphram of the microphone forms
one plate. Sound pressure moves one of its plates. The
movement of the plate changes the capacitance. The
electret capacitor is connected to an FET amplifier. These
microphones are small, have excellent sensitivity, a wide
frequency response and a very low cost.

First amplification stage: this is a standard self-biasing
common emitter amplifier. The 22nF capacitor isolates the
microphone from the base voltage of the transistor and
only allows alternating current (AC) signals to pass.

The tank (LC) circuit: every Tx needs an oscillator to
generate the radio Frequency (RF) carrier waves. The tank
(LC) circuit, the BC547 and the feedback 5pF capacitor are
the oscillator in the Cadre. An input signal is not needed
to sustain the oscillation. The feedback signal makes the
base-emitter current of the transistor vary at the resonant
frequency. This causes the emitter-collector
current to vary at the same frequency. This signal fed to
the aerial and radiated as radio waves. The 27pF coupling
capacitor on the aerial is to minimise the effect of the aerial
capacitance on the LC circuit.