Another new design in the MR 78 front end is the
integrated circuit balanced mixer. This mixer is prac-
tically impossible to overload. Oscillator pulling,
crossmodulation, and other types of distortion so com-
mon in ordinary transistor or FET mixers is minimized.
A balanced bifilar transformer couples the 10.7 MHz
output to the IF amplifier.
Completing the MR 78 front end is the oscillator.
It uses a high-Q ceramic form tank coil. The oscillator
is free form spurious radiations, and operates at high
efficiency. AFC is not required.
RIMO FILTER IF AMPLIFIER
The MR 78 uses linear-phase IF filters. The Rimo
filter, designed from a FORTRAN computer program,
is the most general class of minimum-phase constant-
delay filter. The filter design has great mathematical
complexity. 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 an engineer, working twen-
ty-four hours a day, seven days a week, and working
error-free three-hundred years to perform the same
mathematical calculations.
The Rimo filter has flat time delay in its pass band
region. Any error in time delay causes FM distortion.
All other IF filters have delay distortion, as much as
100%. The MR 78 filter has less than 1.0% delay dis-
tortion from antenna input to discriminator output.
Constant delay in the IF filter bandpass (which is
equivalent to linear phase) is essential for low distor-
tion FM reception. The MR 78 tuner uses linear phase
(Rimo) filters at NORMAL and NARROW positions of
the SELECTIVITY switch settings.
For almost all reception conditions the tuner pro-
vides stereo signals that have the lowest possible
distortion.
Constant delay design techniques are used in the
4-pole 4-zero crystal filter used in the SUPER NARROW
position. However due to the extreme selectivity of-
fered, delay distortion is very slightly increased over
the other selectivity positions.
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