Tait T860 Series II, Service Manual

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M860-00-Preliminary
T865 Circuit Operation B2.11
Copyright TEL 30/06/98
2.11 VCO
(Refer to the VCO circuit diagram in Part E.)
The VCO transistor (Q1) operates in a common source 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 a 4MHz band of frequencies. A trimcap (CT) is used
for coarse tuning of the VCO. The output from the oscillator circuit drives a cascode
amplifier stage (Q2, Q3) which su10dBm (typically) to a further stage of amplifi-
cation, Q5. This is the final amplifier on the VCO PCB, and de19dBm (typically)
to the receiver mixer input pad.
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 frequency required by the first mixer, i.e. there are no
multiplier stages.
The VCO frequency spans from either 198-225MHz or 218-250MHz according to prod-
uct type (refer to Section 1.4). The VCO is tuned to 21.4MHz above the desired receive
frequency (high side injection) to produce a 21.4MHz IF signal at the output of the
mixer.
2.12 Received Signal Strength Indicator (RSSI)
(Refer to the IF section circuit diagram (sheet 3) in Section 6.)
The RSSI provides a DC voltage proportional to the signal level at the receiver input and
is an on-chip function of IC350. Buffering is provided by IC390 and the voltage is avail-
able at D-range 1 (PL100 pin 3).
The RSSI also provides the capability for high level signal strength muting, which may
be selected on PL250 (refer to Section 3.5). The mute threshold may be set between
-115dBm and -70dBm at RV235.