Advertisement
Technical Description
TUNING MECHANISM AND DIAL DRIVE
In the MX 113, the unique design and careful manufac-
ture of the mechanical assembly gives smooth flywheel
tuning.
By controlling the relationship of mass and mechanical
resistance, and by dividing the workloads in the dial drive
system, it becomes nearly impossible to detect any backlash.
Yet, the entire dial drive is a model of mechanical stability.
For added ease and increased tuning accuracy, a section
of the dial pointer is illuminated.
FM SECTION
The Radio Frequency (RF) section houses the complete
FM-RF front-end and part of the AM RF circuit.
A seven-section variable capacitor is the heart of the RF
section. Four sections of the variable capacitor are in the
FM front end and the remaining three are in the AM section.
By interleaving the sections (AM-FM-AM-FM-etc.) spurious
responses are significantly reduced. The four FM sections
of the variable tuning capacitor provides a high degree of
RF selectivity and excellent spurious rejection. Using the
latest "state of the art" field effect transistors with a well-
designed variable tuning capacitor has provided the
MX 113 with an excellent RF front-end.
A dual insulated gate metal oxide silicon field effect
transistor (MOS-FET) is used as first and second RF ampli-
fier. Each gate of the transistor is internally protected by
back-to-back diodes against incoming transients that may
occur due to severe external conditions. Use of MOS-FET's
greatly reduces the cross-modulation products over a
wider dynamic range. A wider dynamic range permits the
input circuits to accept extremely strong signals without
overload. Since both RF amplifiers have insulated gate
configurations, external neutralization is not required.
This design results in a very stable RF amplifier circuit.
Low temperature coefficient components for the FM local
oscillator prevent frequency drift. The frequency stability
inherent in the local oscillator makes automatic frequency
control (AFC) unnecessary. The rate of drift of the local
oscillator is less than ten parts per million per degree
centrigrade.
The mixer design uses a JFET for high sensitivity and
freedom from overload. The mixer delivers the composite
FM signal at the 10.7 MHz intermediate frequency. The
path of the IF signal is controlled by the front panel
SELECTivity pushbutton.
At the OUT position, the SELECTivity switch directs the
signal through an IF preamplifier stage, that uses a J-FET
and a double-tuned IF transformer. The signal then goes to
the FM-IF and discriminator module for further amplifica-
tion. Activating the SELECTivity pushbutton routes the
signal to two double-tuned transformers, a ceramic filter
network, and a single-gate MOS-FET. The sides of the IF
curve are compressed by this circuit narrowing the IF band-
pass. In this mode of operation weak stations adjacent to
strong stations can be tuned with surprising clarity.
All of the RF circuits, including the selectivity circuit
and the AM sections of the variable capacitor are encased
in a metal module. Each FM-RF section is isolated in a
separate compartment by metal shielding. Careful design
and manufacturing increase the protection against radiation
and interference. The MX 113 exceeds the FCC require-
ments for suppression of local oscillator radiation.
Antenna connections for either 300 ohm twin lead trans-
mission line or 75 ohm coaxial cable are provided on the
back panel of the MX 113. The normal input impedance of
the RF amplifier is 75 ohms. Impedance match to 300 ohms
is provided by a Mclntosh designed, negligible loss, balun
transformer. Connections for both 300 ohm twin lead and
75 ohm coaxial cable are made with push type terminals.
FM-IF AND DISCRIMINATOR SECTION
The MX 113 uses linear-phase IF filters. Each filter was
designed from a FORTRAN computer program for minimum-
phase and constant delay. The mathematical complexity of
the filter design procedure is almost beyond belief. Using
numerical integration in the S-Plane, an "IBM" 1130 high
speed computer spent eighteen minutes on the mathematics
for the design of the IF filter. It would have taken a human
engineer, working twenty-four hours a day, seven days a
week, and working error-free three-hundred years to per-
form the same mathematical calculations!
The IF filter has equal time delay in its pass band region.
Any error in time delay causes FM distortion. All other IF
filters have delay distortion, some as much as 100% of
the 10.7 MHz transit delay. The MX 113 has less than 1.0%
delay distortion from antenna input to discriminator output!
This makes possible the overall low distortion performance
limit for the FM tuner and multiplex section of the MX 113.
Amplification of the intermediate frequency signal is
provided by two high gain integrated circuits, each contain-
ing 16 transistors, 3 zener diodes, 5 diodes, and 23 re-
sistors, all on a signal monolithic silicon chip. The excep-
tionally high gain of the integrated circuit assures "hard
limiting" at very low levels of input signals.
Paralleling the main signal path through the FM-IF and
detector module, a secondary amplifying and de-modulat-
ing process is used to activate the tuning meter and to pro-
vide testpoint TP1 and TP2 with a signal to be used with
the Mclntosh Maximum Performance Indicator.
FM STEREO MULTIPLEX SECTION
Mclntosh has developed a special detecting circuit used
in the multiplex section. A particular advantage of this
circuit is the elimination of the critical adjustments neces-
sary with commonly used matrixing circuits. The circuit
detects the L - R sidebands, then automatically matrixes
15
Advertisement
