M850-00
T855 Circuit Operation
B2.11
Copyright TEL 31/09/98
2.11 VCO
(Refer to the VCO circuit diagram in Part E.)
The VCO transistor (Q1) operates in a common emitter, and uses a transmission line res-
onator (TL1). The transmission line is used in a two port configuration with varicaps
positioned at one end. The VCO control voltage from the loop filter (IC750) is applied to
the varicaps (D1 & D2) to facilitate tuning. The VCO output is coupled into a cascode
amplifier stage (Q2 & Q3) which supplies +10dBm (nominal) output. Further amplifica-
tion in Q5 brings the output drive level to +20dBm to drive the mixer.
A low level "sniff" is taken from the input to Q5 to drive the divider buffer to the synthe-
siser (IC740).
The VCO operates at the actual frequency required by the first mixer, i.e. there are no
multiplier stages.
The VCO frequency spans from either 355-395MHz, 395-435MHz or 435-485MHz
according to product type (refer to Section 1.4). The VCO is tuned to 45MHz below the
desired receive frequency (low side injection) to produce a 45MHz IF signal at the out-
put of the mixer.
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