M8SL2-00-002-812 T855 Circuit Operation B2.11
© Copyright Tait Electronics Limited August 2004. All rights reserved.
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 and D2) to facilitate tuning. The VCO output is coupled into a cascode
amplifier stage (Q2 and Q3) which supplies +10dBm (nominal) output. Further amplifi-
cation 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 syn-
thesiser (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 355 to 395MHz, 395 to 435MHz or 435 to 485MHz
according to product type. The VCO is tuned to 45MHz below the desired receive fre-
quency (low side injection) to produce a 45MHz IF signal at the output of the mixer.
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