McIntosh MR80 Owner's Manual page 15

tions in load impedance. This prevents changes in
audio cable lengths outside the tuner from affecting
the detector performance.
FM STEREO MULTIPLEX
The heart of the multiplex section is a new third
generation phase locked loop (PLL) stereo decoder
integrated 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 adja-
cent 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
separation for the prevailing signal conditions. The
circuit operates only during stereo reception. It swit-
ches automatically to monophonic if the 19kHz pilot
tone is absent.
In the PLL the internal oscillator operates at
228kHz, locked onto the 19kHz pilot tone. The
228kHz feeds a 3 stage Johnson countervia a binary
divider to generate a series of square waves.
Suitably connected 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 adjacent stations since the third har-
monic of the sub-carrier (114kHz) is excluded. It also
eliminates interference from SCA broadcasts since
the third harmonic of the pilot tone (57kHz) is exclud-
ed. Unwanted 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 induc-
tors to minimize drift, integral lamp driving capabili-
ty to indicate the presence of the 19kHz pilot carrier,
excellent channel separation over the entire audio
frequency range, extremely low distortion, low out-
put impedance, and transient-free mono/stereo
switching.
AUDIO CIRCUITS
Following the MPX decoder is the three position
deemphasis switch. The three different positions
allow the MR 80 to be used in North America with
standard 75ms deemphasis and in Western Europe
and the Far East with 50ms deemphasis. The 25ms
position is for use with an external noise reduction
adapter.
An electronically switched f i l t e r circuit, im-
plemented with two J-FETs of a quad J-FET
package, is used to reduce out of phase noise when
in the stereo mode and tuned to a weak station. This
filter is actually a twin-T bandpass that blends the
high and low frequencies, leaving separation unaf-
fected at mid-frequencies. This results in a greatly
improved stereo image when the filter is required.
A combination of three inputs are used to control
the switch point of the filter. The first is the front
panel switch. The filter is only active when this
switch is in the AUTO or ON position. In the ON posi-
tion the filter is in the circuit for all stations mono or
stereo. In the AUTO position the filter will switch in
when the station is transmitting a stereo pilot signal
and the signal strength is below 100mV.
In the same quad J-FET package are the two FETs
that are used for the muting circuit. These FETs are
controlled by the logical ANDing of three signals.
The first signal is the deviation or center tune, this
detects when the tuner is correctly tuned to the
center of the FM channel. The second signal is the
signal to noise ratio. Only when there is sufficient
signal will the tuner be allowed to unmute. Signal to
noise ratio is measured by a special bandpass
amplifier centered around 138kHz. This frequency is
used to prevent false triggering due to adjacent
channel and overmodulation products. The third
signal is the RF signal level that is derived from the
five signal detector circuits. A front panel control
(MUTING LEVEL) adjusts the reference level to a
voltage comparator. The other input to the com-
parator is the signal strength voltage. When the RF
level exceeds the preset reference level, a signal is
sent to the muting logic to be ANDed with the other
two signals. This adjustment allows the unmute
level to be set to any value from 2mV to greater than
1000mV.
This wide range is necessary for the tuner to scan
properly in all signal conditions and provide the best
combination of ease of use with maximum intersta-
tion noise suppression.
19 AND 38kHz FILTER
An LC tuned notch filter is used to eliminate any
beatnote interference with the bias oscillator in a
users tape recorder. This filter is driven from a filter
buffer amplifier and terminated by the output opera-
tional amplifier. These amplifiers provide the
necessary input and output impedances for proper
phase response. The output amplifier is also used as
an audio bandpass filter to prevent unwanted noise
and transients from being passed on to the head-
phone amplifier.
HEADPHONE AMPLIFIER
The headphone amplifier is capable of driving a
pair of dynamic headphones with less than .02%
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