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TECHNICAL
DESCRIPTION
GENERAL
Figure 7 is a block diagram of the Model 2252
Receiver showing the main functional elements
and input and output signal routing. For clarity,
only the left audio channel is shown; the right
audio channel is identical. The MONO switch is
common to both channels. All audio controls are
ganged to their counterparts in the right channel.
FM FRONT END
FM antenna signals are applied through the an-
tenna coil to the dual gate MOSF ET RF ampli-
fier. The signals from the RF amplifier are fed
through the tuned RF tank circuit to the mixer
stage where the carrier frequency is converted to
the 10.7 MHz intermediate frequency. It is then
fed to the input of the I F amplifier.
Careful attention to the thermal and electrical
characteristics of the local oscillator has mini-
mized drift,
thus eliminating the necessity for
AFC.
FM IF AMPLIFIER
The FM IF amplifier system is composed of one
IC expressly developed for FM I F use, plus one
transistor and three dual-element ceramic filters.
The IC includes a complete I F amplifier, limiter
stages, FM discriminator,
and a 6-stage double
ended type differential amplifier providing excell-
ent AM rejection.
The FM discriminator
stage is composed of a
Double balanced type quadrature detector circuit.
The signal from the front end is fed to the IC
through the 3 ceramic filters and the one transis-
tor amplifier.
The characteristics of these filters are ideal in that
the 200 kHz passband is phase linear, with sharp
cutoff
slopes. Its exceptional phase linearity as-
sures the elimination of a major source of high
frequency distortion
and a loss of stereo separa-
tion. The sharp cutoff slopes provide outstanding
selectivity, permitting reception of closely space
stations.
FM STEREO DEMODULATOR
The stereo composite signal obtained from the
quadrature detector is fed to the MPX IC. The L
and R signal outputs from the MPX IC are applied
through the low-pass filter and the de-emphasis
circuit to the one (per channel) transistor ampli-
fier. The output (about 1100 mV) obtained from
this amplifier is fed to the Land
R output ter-
minals at the rear panel through the function
switch.
This stereo demodulating
IC employs a PLL
(Phase Locked Loop) technique, whereby the
phase of the 38 kHz switching carrier signal in the
decoder is constantly interlocked with the phase
of the 19 kHz input pilot signal. Phaseshift of the
switching carrier (38 kHz) is thereby eliminated
and deterioration
of separation and distortion
does not occur.
19 kHz and 38 kHz muting circuits are not em-
ployed so that stability is greatly improved. Mut-
ing distortion,
as a result of element aging or
adjacent temperature
changes, is consequently
eliminated.
The stereo demodulating 38 kHz carrier switching
signal is very clean with no even harmonic con-
tent. SCA rejection of approximately
80 dB is
obtained since resonant beats are not generated
between the secondary harmonic (76 kHz) of the
38 kHz carrier switching signal and the SCA signal
(67 ± 7 kHz).
MUTING CIRCUIT
If there are no signals present, any FM tuner will
generate some noise. The muting circuit elimi-
nates this noise. In addition, spurious noise that is
generated while tuning is also removed. The mut-
ing circuit in the Model 2252 is integrated" into
the I F stage. The circuit is also designed to mini-
mize popping noises which tend to be generated
when muting is switched in and out.
AM TUNER
The AM tuner section of the Model 2252 is com-
posed of an IC (incorporating an RF amplifier,
local oscillator, mixer, I F amplifier, and detector)
and a one transistor audio amplifier which ampli-
fies the detected audio signals. A two section
variable capacitor is used to tune the antenna and
mixer stages. The ceramic filters utilized in the
AM I F amplifier are designed for high selectivity
and wide bandwidth
for interference-free high
quality
AM reception.
Following
the AM IF
amplifier,
the AM detector recovers the audio
modulation and presents this signa! to the audio
amplifying
stage and the mode selector switch.
The AM tuner and I F amplifier are subjected to
the action of an effective automatic gain control
circuit which maintains a constant volume level
for all stations in the AM band.
SELECTOR SWITCH
The Model 2252 has the capability to operate
from a variety of program sources, such as AM or
8
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