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of PLL synthesizing oscillator(1)
Voltage controlled oscillator ( VCO )
The oscillation frequency of the voltage controlled oscillator is controlled by the output of the PLL synthesizer and does the stable oscillation of 119.3 MHz.
As the oscillation circuit, it is using Colpitts LC oscillation circuit.Two diodes are using for expanding the adjustment range of the frequency.
C3 and C4 become the capacitors of the Colpitts circuit for the voltage division.
The oscillation frequency is changed by the capacitance of C5, D1 and D2.
D1 and D2 are the variable capacitance diode (the varicap diode). The capacitance of the diode changes when gaining opposite direction voltage. The capacitance becomes small when making opposite direction voltage high.
The signal for the control which is output from the charge pump of MB87014A is a pulse signal. A pulse signal is changed into the voltage change of the direct current through the filter by R4, R5, R6 and C6.
Colpitts oscillation circuit
This oscillation circuit makes oscillate in dividing collector voltage with the capacitors and returning to the emitter.
The oscillation frequency can be calculated by the following formula.
In case of the high frequency circuit, the capacitance and the inductance (the coil ingredient) of transistor itself and the wiring influence a resonant frequency.
Therefore, above-mentioned calculation result is used as roughly value.
To prevent from an influence from the external circuit over the oscillation, an output buffer is provided.
The FET transistor is using for the output buffer.
The FET is an electric Field Effect-type Transistor. The output current (the drain electric current) can be controlled at the voltage which is gained by the input (the gate).
Usual transistor controls an output current in the electric current which flows through the base.
In case of the FET, because to pass an input electric current isn't necessary, it is possible to compose with the simple circuit.
PLL frequency synthesizer(MB87014A)