Specifications

DMOSFET RF amplifier, Double Balanced Mixer and 4
RF circuits that are tuned by matched varactor
diodes. The results are high spurious rejection and
great sensitivity. This circuitry and high tuning
voltage eliminate RF inter modulation distortion
caused by diode nonlinearities.
The FM IF section uses 2 transistors, 3 linear phase
piezoelectric filters, and 2 integrated circuits. They
combine to provide over 120dB of gain and a
selectivity greater than 70dB. Limiting, muting, signal
strength drive, and FM detection are all functions of
the LA 1235 integrated circuit.
A phase locked loop (PLL) stereo decoder integrated
circuit, the TCA 4500A, is the heart of the multiplex
section. It has a high signal to noise ratio, low
distortion, high channel separation, and high SAC
rejection. The PLL MPX IC eliminates inductors to
minimize drift, provides integral lamp driving
capability for stereo indication, and has transient free
mono/stereo switching.
Following the PLL MPX decoder an LC tuned notch
filter is used to prevent tape recorder bias
interference. The LC filter is driven from the MPX
output amplifier and is terminated by an operational
amplifier. This provides the necessary filter input and
output impedances for proper phase response.
AM TUNER
Push terminals are provided for the AM antenna.
Due to the unusual design of the AM RF input
section, almost any type of antenna can be used.
The first element in the input section is a new Low
Impedance Loopstick Antenna. It receives all the AM
stations and rejects noise and other interference. J
FET-Transistor cascode amplifier follows. this
amplifier, with its J FET inputs stage, provides
excellent sensitivity and spurious response rejection.
It also has a delayed AGC system that does not
degrade the overload performance margin.
Two varactor tuned circuits, a second RF amplifier
and a double balanced mixer are next. This type of
mixer has excellent image and IF rejection.
The mixer's 450kHz output feeds a matching
transformer for the wide band linear phase
piezoelectric 4 element lattice filter, then the IF
amplifier, another transformer, and finally the AM
detectors, filter and muting circuits.
AUDIO PROCESSOR
The MR7083 contains an audio processor that looks
at L + R and L • R information and ENHANCES the
sound.
If the tuner is in the MONO mode, AM or a weak
FM station, it introduces frequency and phase
differences between the two channels giving a stereo
effect. Since this is done at audio frequencies, it
does not influence the signal to noise ratio. When
receiving a strong FM station in stereo, it ENHANCES
apparent separation giving a wider stereo image.
ANTENNA INPUTS
FM
The MR7083 is provided with a 75 ohm coaxial
antenna input. It is a standard "F" connector that
mates with most common antenna and cable service
feedlines. This type of input combines low loss with
interference rejecting shielding.
A matched dipole antenna is provided for average
reception. However, optimum performance is
obtained with an outdoor highly directional beam
antenna and rotator.
Although a 75 ohm coaxial feedline offers the best
noise and multipath rejection, many antennas use 300
ohm twinlead. If this is the case, a matching
transformer (balum) can be used. They are
inexpensive and available at most radio and TV store
(i.e., Radio Shack, part number 15-1140 or 15-1253).
AM
The AM antenna input on the MR7083 is unusual in
that it will accept almost any type of antenna. In a
location of moderate signal strength and little
interference (few florescent lights, motors, TV sets,
etc.) the new Low Impedance Loopstick will give
good performance. In a rural area, an outdoor long-
wire might be desirable. However, the best overall
performance in a weak signal area can be obtained by
using a shielded loop antenna.
To construct a shielded loop antenna, one must take
25 to 50 feet of single conductor shielded wire,
microphone cable, coaxial cable, etc., and arrange it
MR7083 Product Preview/Page 5
MclNTOSH LABORATORY INC., 2 CHAMBERS STREET, BlNGHAMTON, NEW YORK 13903