Technical Description
FM SECTION
The radio frequency (RF) assembly houses the com
plete FM-RF front end and part of the AM-RF circuitry.
An eight section variable capacitor is the heart of the
RF assembly. Five sections, 1, 2, 4, 6 and 8 are used
for FM and three sections, 3, 5 and 7 are for AM. Sec-
tions 1 and 2 of the capacitor are for the FM antenna
RF input circuits. Section 2 is always in use, however,
if additional RF selectivity is desired to protect
against strong signal overload, additional preselector
filtering is accomplished by adding section 1. This is
accomplished by operation of the preselector switch
on the rear panel. This switch directs a DC voltage
that controls PIN semiconductor diodes to perform
the RF switching functions electronically.
A MOS-FET RF amplifier follows section 2 to pro-
vide exceptional sensitivity while maintaining super-
ior cross-modulation rejection over a wide dynamic
range. Capacitor sections 4 and 6 follow the RF ampli-
fier to increase selectivity and to provide correct im-
pedance matching for the following fully balanced
MOS-FET mixer.
The local oscillator is tuned by capacitor section 8.
A MOS-FET buffer amplifier is employed between the
oscillator and the mixer. The high input impedance of
this amplifier insures that oscillator loading is
minimal, therefore, oscillator stability is greatly
enhanced. The oscillator is fine tuned by a varactor
diode operated by a correction voltage generated by
the Automatic Frequency Lock (AFL) circuit. AFL is a
Mclntosh patented circuit that turns on a lock voltage
when manual tuning reaches the center of the FM sta-
tion carrier. This AFL tuning aid insures the tuning is
accurate for minimum distortion and best performance.
The FM-IF system that follows the mixer consists of
five integrated circuit amplifiers and four piezoelectric
filters. They combine to give a total amplification of
140 dB. The IF response characteristic is very selec-
tive, being 170 kHz wide at -3 dB and only 500 kHz
wide at - 5 0 dB. The response is symmetrical about
the center frequency of 10.7 MHz. The IF filters are per-
manently sealed and cannot drift nor vibrate out of ad-
justment. The exceptionally high gain of the IF system
assures proper limiting at all input signal levels. A
Foster-Seely discriminator completes the IF system.
The detected output of this discriminator has a high
signal to noise ratio and extremely low distortion. This
output feeds to the FM stereo multiplex section.
FM STEREO MULTIPLEX
The heart of the multiplex section is a new third
generation phase locked loop (PLL) stereo decoder in-
tegrated circuit (IC). This PLL IC incorporates two
special systems, an automatic variable separation
control circuit to reduce background noise when
receiving weak stereo stations, and tri-level digital
waveform generation which eliminates interference
from SCA signals and from the sidebands of adjacent
channel FM signals.
The variable separation control is operated from
the IF amplifier's signal strength detector. A smooth
transition is provided from mono to stereo or from
stereo to mono at weak signal levels to provide the
optimum signal to noise ratio and best stereo separa-
tion for the prevailing signal conditions. The circuit
operates only during stereo reception. It switches
automatically to monophonic if the 19 kHz pilot tone
is absent.
In the PLL the internal oscillator operates at 228
kHz, locked onto the 19 kHz pilot tone. The 228 kHz
feeds a 3 stage Johnson counter via a binary divider
to generate a series of square waves. Suitably con-
nected NAND gates and exclusive OR gates produce
the tri-level drive waveform for the various
demodulators in the circuit. The usual square
waveforms have been replaced in the PLL and
decoder sections by tri-level waveforms. These tri-
level forms contain no harmonics which are multiples
of 2 or 3. This eliminates frequency translation and
detection of interference from the side-bands of adja-
cent stations since the third harmonic of the sub-
carrier (114 kHz) is excluded. It also eliminates in-
terference from SCA broadcasts since the third har-
monic of the pilot tone (57 kHz) is excluded. Un-
wanted spurious audible components and phase jitter
in the PLL are inherently eliminated by this technique.
Additional advantages of the phase locked loop
stereo demodulation are the elimination of inductors
to minimize drift, integral lamp driving capability to
indicate the presence of the 19 kHz pilot carrier, ex-
cellent channel separation over the entire audio fre-
quency range, extremely low distortion, low output
impedance, and transient-free mono/stereo switching.
After multiplex detection, 19 kHz pilot and 38 kHz
carrier suppression circuits are used to prevent tape
recorder interference.
The FM muting circuit is unusual. It operates both
by detecting ultrasonic noise and by sensing correct
tuning of the detector circuit. To 'un-mute' it is
necessary for the signal to have an adequate signal-
to-noise ratio and to be tuned to the center of the FM
carrier. The MUTING circuit can be activated or de-
feated by the front pane! muting switch. The muting
of the audio signal is done with FET analog switches.
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