Revox B261 Serviceanleitung Seite 37

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8261
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sEcTroN
3/2
3.2
RF INPUT
SECTION
BANDPASS
ANTENNA
1ü
RF-At\rPLl
CIRCUIT
FIER STAGE
llh DOUBLE
TUNED
2'd
RF
AMPLI-
CIRCUIT
FIER STAGE
BAUNCED
MIXER
PHASE
LINEAR
,FILTER
TO IF.AMPLI'
FIER
1.726.740
87...108.0MH2
INP
NNA
.8
FROM
IF-AMPLIFIER
1.726.1
40
+
AGC
FROM SYNTHESIZER PCB
1.1
26.1
7
o
+
3.2.1 RF
amplifier 1.726.730
From
the 75
ohm
IEC socket or the optional
antenna selector
switch, the antenna signal is conducted through the
FM
band-
pass to the RF i,nput. Through the antenna circuit,
the si-gnal
is taken to the first
RF
amplifier
stage Q1 (dual gate
MOSFET).
The gain of this stage is decreased for large input
signals. The control
voltage (AGC)
is generated on the rF
ampl-ifier ),.726.1 40.
The
amplifi-er stage is followed by a double-tuned circuit
band-pass fiLter. After the second RF
amplifier stage
Q2
and
the second tuned band-pass fj-1ter,
the signal is coupled to
the balanced mixing stage (Q4 and Q5). The tuning voltage for
the series-paralle1-connected
varactors of the bapd-pass
filter is generated in the synthesizer module.
3.2.2 Antenna switch
An electronic
antenna selector
switch for changing over
between two 75
ohm
fEC antenna sockets A/B is avail-able as
a
retrof itt able
opt
j.on
.
3.2.3 Synthesizer and local- oscill-ator 1.726.770
The local osciflator with a buffer
stage connected to the
output supplies its signal to the m.ixing stage on the
RF
section as well as to the synthesj-zer module
(IC1 ) . This
semiconductor devj,ce contains the complete PLL circuit for
generating the tuning voltage for the varactors of the
1oca1
oscillators
and the band-pass filters on the RF section.
The quartz-accurate local- oscillator
frequency is higher
by
the IF frequency of 10.7
MHz
than the input frequency.
The
frequency steps are r-2.5
klH,z.
A1l operating data and controf
information are transferred
by the microprocessor to the
snythesizer module through the serial
CBUS
interface.
ANTENNA SWITCH
Uro (98.2...118.7 MHz)
4.5...24v