Johnson DM3474, Руководство по обслуживанию

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CIRCUIT DESCRIPTION
4-5
Part No. 001-3474-002
4.2.2 VOLTAGE-CONTROLLED OSCILLATOR
Oscillator (Q850)
The VCO is formed by Q850, several capacitors and varactor diodes, and a ceramic resonator. It oscillates at
the transmit frequency in transmit mode and first injection frequency in the receive mode (approximately 450
MHz in transmit and 500 MHz in receive).
Biasing of Q850 is provided by R862, R867 and R868. An AC voltage divider formed by C859, C861 and
C862 initiates and maintains oscillation and also matches Q850 to the tank circuit. The ceramic resonator is
grounded at one end to provide shunt inductance to the tank circuit.
Frequency Control and Modulation
The VCO frequency is controlled in part by DC voltage across varactor diodes CR854, CR855, CR856 and
CR851. As voltage across a reverse-biased varactor diode increases, its capacitance decreases. Therefore, VCO
frequency increases as the control voltage increases. CR854/CR855 and CR856/CR851 are paralleled varactor to
divide the capacitance and improve linearity. The varactors are biased at -2V to adjust to the voltage output of
U801. The control line is isolated from tank circuit RF by choke L851 and L854 and decoupling capacitor C854.
The amount of frequency change produced by CR854/CR855/CR856/CR851 is controlled by series capacitor
C853.
The -2V applied to the VCO is derived from the TCXO frequency that is amplified by Q833, rectified by
CR831 and filtered by C844, C845, C846 and C847 on the RF board.
The VCO frequency is modulated using a similar method. The transmit audio/data signal is applied across
varactor diode CR852 which varies the VCO frequency at an audio rate. Series capacitors C856/C870 set the
amount of deviation produced along with CR853 and C858. R854 provides a DC ground on the anodes of CR852/
CR853, and isolation is provided by R852 and C855.
The DC voltage across CR853 provides compensation to keep modulation relatively flat over the entire
bandwidth of the VCO. This compensation is required because modulation tends to increase as the VCO frequency
gets higher (capacitance of CR854/CR855/CR856/CR851 gets lower). CR853 also balances the modulation
signals applied to the VCO and TCXO. An external voltage from J201, pin 14 can also adjust the modulation.
The DC voltage applied across CR853 comes from the modulation adjust control R810. R811 applies a DC
biasing voltage to CR852; C814 provides DC blocking; and C818 attenuates AC signals applied through R811. RF
isolation is provided by C858, R853, C817 and R812.