M850-00
T856/857 Circuit Operation
C2.7
Copyright TEL 31/09/98
2.4 VCO
(Refer to the VCO circuit diagram in Part E.)
The VCO transistor (Q1) operates in a common emitter configuration, with an LC tank
circuit coupled between its gate and drain to provide the feedback necessary for oscilla-
tion. The VCO control voltage from the loop filter (IC750) is applied to the varicaps
(D1-D2) to facilitate tuning within an 8MHz band of frequencies. A trimcap (C6) is used
for coarse tuning of the VCO. The output from the oscillator circuit drives a cascode
amplifier stage (Q2, Q3) which supplies +10dBm (typically) to a further stage of amplifi-
cation, Q5. This is the final amplifier on the VCO PCB, and delivers +20dBm (typically)
to the exciter drive amplifier.
A low level "sniff" is taken from the input to Q5 and used to drive the divider buffer for
the synthesiser (IC740).
The VCO operates at the actual output frequency of the exciter, i.e. there are no multi-
plier stages. It is modulated by superimposing the audio signal onto the control voltage
and by phase modulating the reference signal.
2.4.1 VCO Supply
The VCO is supplied from two switched +9V supplies under the control of the Tx-Reg.
supply.
The VCO and buffer amplifier are supplied from one +9V switched supply by Q540 via
the C multiplier (Q550, C530 on the T856 and C550 on the T857).
The output amplifier is supplied from the other +9V supply by Q520, Q530, and Q510.
A delay circuit holds the VCO on for a short time after the Tx-Reg. supply has been
switched off. This is to allow the RF power circuits (both exciter and PA) to ramp down
in the correct manner before the VCO is switched off.
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