MC13028A
FUNCTIONAL DESCRIPTION
Introduction
circuit where the Left channel and the Right channel outputs
can be extracted at Pins 15 and 16. (The outputs from the I
and Q detectors are also filtered similarly.) At this time, a
stereo indicator driver circuit, which can sink up to 10 mA, is
also enabled. The stereo output will occur if the input IF
signal is: larger than the stereo threshold level, not too noisy,
and if a proper pilot tone is present. If these three conditions
are not met, the blend circuit will begin to force monaural
operation at that time.
A blend circuit is included in this design because
conditions occur during field use that can cause input signal
strength fluctuation, strong unwanted co–channel or power
line interference, and/or multi–path or re–radiation. When
these aberrant conditions occur, rapid switching between
stereo and mono might occur, or the stereo quality might be
degraded enough to sound displeasing. Since these
conditions could be annoying to the normal listener, the
stereo information is blended towards a monaural output.
This circuit action creates a condition for listening where
these aberrant effects are better tolerated by the consumer.
Intentional mono operation is a feature sometimes
required in receiver designs. There are several ways in which
to accomplish this feat. First, a resistor from Pin 10 to ground
can be switched into the circuit. A value of 1.0 k is adequate
as is shown in the schematic in Figure 18. A second method
to force the decoder into mono is simply to shunt Pin 10 to
ground through an NPN transistor (collector to Pin 10, emitter
to ground), where the base lead is held electrically “high” to
initiate the action.
A third method to force a mono condition upon the
decoder is to shunt Pin 8 of the decoder to ground through
an NPN transistor as described above. Effectively, this
operation discharges the blend capacitor (10 µF), and the
blend function takes over internally forcing the decoder into
mono. This third method does not necessarily require extra
specific parts for the forced mono function as the first two
examples do. The reason for this is that most electronically
tuned receiver designs require an audio muting function
during turn on/turn off, tuning/scanning, or band switching
(FM to AM). When the muting function is designed into an
AM Stereo receiver, it also should include a blend capacitor
reset (discharge) function which is accomplished in this case
by the use of an NPN transistor shunting Pin 8 to ground,
(thus making the addition of a forced mono function almost
“free”). The purpose of the blend reset during muting is to
re–initialize the decoder back into the “fast lock” mode from
which stereo operation can be attained much quicker after
any of the interruptive activities mentioned earlier, (i.e. turn
on, tuning, etc.).
The MC13028A is designed as a low voltage, low cost
decoder for the C–QUAM AM Stereo technology and is
completely compatible with existing monaural AM
transmissions. The IC requires relatively few, inexpensive
external parts to produce a full featured C–QUAM AM Stereo
implementation. The layout is straightforward and should
produce excellent stereo performance. This device performs
the function of IF amplification, AGC, modulation detection,
pilot tone detection, signal quality inspection, and left and
right audio output matrix operation. The IC is targeted for use
in portable and home AM Stereo radio applications.
A simple overview follows which traces the path of the
input signal information to the MC13028A all the way to the
audio output pins of the decoder IC.
From the appropriate pin of an AM IC, the IF amplifier
circuit of the MC13028A receives its input at Pin 4 as a
450 kHz, typically modulated C–QUAM signal. The input
signal level for stereo operation can vary from 47 dBµV to
about 90 dBµV. A specific threshold level between these
limits can be designed into a receiver by the choice of the
resistor value for R1 connected to Pin 1. This IC design
incorporates feedback in the IF circuit section which provides
excellent dc balance in the IF amplifier. This balanced
condition also guarantees excellent monophonic
performance from the decoder. An IF feedback filter at Pin 3
is formed by a 0.47 µF low leakage capacitor. It is used to
filter out the unwanted audio which is present on the IF
amplifier feedback line at higher modulation levels under
weak input RF signal conditions. Elimination of the unwanted
signal helps to decrease the amount of distortion in the audio
output of the stereo decoder under these particular input
conditions. An AGC circuit controls the level of IF signal
which is subsequently fed to the detector circuits. An AGC
bypass capacitor is connected to Pin 2 and forms a single
pole low pass filter. The value of this part also sets the time
constant for the AGC circuit action.
The amplified C–QUAM IF signal is fed simultaneously to
the envelope detector circuit, and to a C–QUAM converter
circuit. The envelope detector provides the L+R (mono)
signal output which is fed to the stereo matrix. In the
converter circuit, the C–QUAM signal is restored to a Quam
signal. This is accomplished by dividing the C–QUAM IF
signal by the demodulated cos φ term. The cos φ term is
derived from the phase modulated IF signal in an active
feedback loop. Cosine φ is detected by comparing the
envelope detector and the in–phase detector outputs in the
high speed comparator/feedback loop. Cosine φ is extracted
from the I detector output and is actively transferred through
feedback to the output of the comparator. The output of the
comparator is in turn fed to the control input of the divider,
thus closing the feedback loop of the converter circuit. In this
process, the cos φ term is removed from the divider IF output,
thus allowing direct detection of the L–R by the quadrature
detector. The audio outputs from both the envelope and the
L–R detectors are first filtered to minimize the second
harmonic of the IF signal. Then they are fed into a matrix
The VCO in this IC is a phase shift oscillator type design
that operates with a ceramic resonator at eight times the IF
frequency, or 3.60 MHz. With IF input levels below the
stereo threshold level, the oscillator is not operational. This
feature helps to eliminate audio tweets under low level,
noisy input conditions.
8
MOTOROLA ANALOG IC DEVICE DATA
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