Technical Features; Circuit Description - Sony CRF-150 Service Manual

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1-2. TECHNICAL FEATURES
1:3. CIRCUIT DESCRIPTION
Fm Tuner
FET mixer Usually an fm front end consists
The rf amplifier sometimes
worsens the crossmodulation
handling ability of the receiver
when ordinary bipolar transistors
are used, It is, however, diffi
cult to eliminate the rf amplifier
because its removal causes strong
spurious radiation, poor sensi-
tivity, and а poor noise figure.
To solve this problem, the Model
Fig. 1-1 Usual fm front end
Fig. 12 CRF-150 fm front end
Local oscillator
0102
Afc diode
D101
Fm Ы amplifier
0103
CRF-150 uses a low-noise junc-
tion FET for the mixer and a
triple-tuned passive input circuit
as shown in Fig. 1-2. The Model
CRF-150 is capable of clear fm
reception even in strong signal-
strength areas due to the
extremely superior interference-
rejection characteristics of the
passive input circuit,
The oscillator generates a fre-
quency 10.7 MHz higher than
the incoming signal frequency
and injects the generated voltage.
at the source of FET mixer
0101.
This diode is connected across
the resonant circuit of the oscil-
lator and works as a variable-
capacitance diode. А dc feed
back voltage from the discri
nator controls the bias applied
to the diode to keep the local
oscillator. frequency correct.
Transistor Q103 amplifies the
10.7 MHz i-f signal produced by
mixer 0101 and coupled to it
through Hf transformer IFT 101.
A block diagram of the sw front
end is shown in Fig. 1 - 3 . Such
an arrangement effectively sup-
presses image signals, since the
high value of the first i4 causes
the desired and image signals to
differ greatly in frequency. At
the same time, the relatively low
value of the second if makes it
possible to obtain high amplifi-
cation as well as sharp discrimi-
mation against signals differing
only slightly in frequency from
the desired signal.
The result is that this double-
superheterodyne front end pro-
vides a combination of greater
image suppression and higher
adjacent channel-selectivity than
can be realized in a simple super-
heterodyne receiver.