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4.2
FM
TUNER
Front-End
A frequency
linear 3-gang variable capacitor is
used with a single stage FET RF amplifier.
The FET
possesses high input impedance com-
pared with a transistor, and allows simple coupling
with the input tuning circuit, plus a significant ad-
vantage in terms of noise.
The
local
oscillator,
Q3
employs
a modified
Clapp
circuit,
thereby
reducing the amount
of
drift caused by changes in time, ambient tempera-
ture,
and power
supply voltage.
Also since the
oscillator signal is obtained from the tuning circuit,
there are far fewer higher harmonics,
resulting in
a much cleaner waveform with less spurious inter-
ference.
The
oscillator
signal is applied
to the base of
the mixer transistor Q2.via low capacity capacitor.
When the FUNCTION switch (S3) is in the AM
position,
+B is applied to the cathode of D1
via
R34. D1 will be cut-off and Q3 will turn off.
Q1 RF AMP.
Q2 MIXER
IFT
Fig. 4-2 FM front end
IF Amplifier and Detector
This
stage includes 2 dual-element ceramic
fil-
ters, a transistor and an IC (PA3001-A) with a high
SN ratio and very little distortion.
The transistor
inserted between the two ceramic filters is employ-
ed for impedance matching purposes as well as to
increase the gain.
PA3001-A includes the IF limiter amplifier, FM
detector
(quadrature
detector),
AF
amplifier,
muting circuit and the meter. drive circuit.
The muting circuit is turned on when pins 5 and
12 are connected
by means
of the FM MUTING
switch.
If the
dial pointer
is moved
approx.
+70kHz away from a station, and the input level is
very low (equivalent antenna input less than 4uV),
a 5V DC signal is generated at pin 12, and applied
to pin 5, thereby
activating the muting circuit
within the IC.
FM MPX Stereo Demodulator
The IC (PA1001-A) employed in the FM multi-
plex stereo demodulator stage also features a high
SN: ratio and reduced
distortion.
Due to the in-
corporation
of a pilot auto-canceller circuit, very
good frequency characteristics
are obtained.
Un-
like the more
conventional
pilot signal (19kHz)
canceller circuits, which fail to completely remove
the
pilot
signal
if it is not
at standard
level,
PA1001-A pilot auto-canceller circuit also includes
a pilot signal level detector
circuit.
Changes in
pilot signal level are consequently
responded
to
immediately, resulting in very effective suppression
of the pilot signal leak level.
Filter Circuit
Besides eliminating the sub-carrier signals (more
than 23kHz), this PNP transistor 18dB/oct. active
filter also serves as an amplifier for the low-pass
region, and as a crosstalk canceller.
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