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MC13027 Datasheet PDF : 28 Pages
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MC13027 MC13122
Detectors
In AM stereo operation, the Q detector delivers pilot signal
via an external low–pass filter to the pilot detector input (Pin
18). The E and I detectors drive the C–QUAM comparator.
The L–R signal and the output of the envelope detector are
combined in the matrix to produce the L and R signals. The
C–QUAM system modifies the in–phase and quadrature
components of the transmitted signal by the cosine of the
phase angle of the resultant carrier, for proper stereo
decoding. An uncompensated L–R would be distorted,
primarily by second harmonics. Where there is noise or
interference in the L–R, it has been subjectively determined
that reducing the cosθ compensation at the expense of
increased distortion sounds better than full decoding. The
blend line operates over a small voltage range to eliminate
cosine compensation.
Signal Quality Detector – Blend Voltage Control
The signal quality detector output is dependent on signal
strength, over–modulation, and whether or not the blend pin
has been pulled low prior to searching. Over–modulation
usually occurs when a radio is tuned one channel away from
a desired strong signal, so this prevents stopping one
channel away from a strong signal.
In a radio tuned to a strong, interference free C–QUAM
station, the blend voltage will be approximately 3.6 V. In the
presence of noise or interference, when the modulation
envelope is at a minimum, it is possible for the I detector to
produce a negative, or below zero carrier signal. The Signal
Quality Detector produces an output each time the negative
I exceeds 4%. The output of the detector sets a latch. The
output of the latch turns on current source which pulls down
the voltage of the blend cap at a predetermined rate. The
latch is then reset by a low frequency signal from the pilot
detector logic. This produces about a 200 mV change each
time 4% negative I is detected. Tables 1 and 2 describe the
blend behavior under various conditions.
When the blend voltage reaches 2.2 V a blend control
circuit starts to reduce the amplitude of the L–R signal fed to
the decoder matrix. By 1.5 V the L–R has been reduced by
about 40 dB. At lower voltages it is entirely off and the
decoder output is monaural. This reduction of L–R signal, or
blend as it is commonly called when done in FM stereo
radios, reduces undesirable interference effects as a function
of the amount of interference present.
Stop–Sense
Stop–sense is enabled when the blend voltage is
externally pulled below 0.45 V. An input from the AGC
indicating minimum signal, or detection of 10% negative I will
cause the stop–sense pin to be pulled low. With signals
greater than the AGC corner and less than 10% interference
the stop–sense will be a minimum of 1.0 V below the 3.0 V
line. Very rapid scanning is possible because the radio can
scan to the next frequency as soon as the stop–sense goes
low. The maximum wait time, set by the radio, is only reached
on good stations.
The decoder will not lock on an adjacent channel because
it is out of the lock range of the PLL. The beat note produced
in the I detector by the out of lock condition will trigger the
10% negative I detector.
Sequence For Seek Scan
• Change Station – Pull–Down Blend
• Wait Approximately 50 ms for Synthesizer and Decoder
PLL to Lock
• Observe Pin 6 Voltage
• If it is Above 2.0 V and Stays Above 2.0 V for
Approximately 800 ms, Stay on the Station
• No IF Count Now Needed
• No AGC Level Detector Needed
Table 1. Normal Sequence When Changing Stations
External Pull–Down of
Blend Capacitor to Under
0.47 V
– Increased Current Supplied to
Loop Driver for Fast Lock
– Fast AGC Activated
– Extra Current Pull–Up Activated
on Blend Capacitor
– Pilot Detector Disabled
– Loop Locks
– Stop–Sense Activated
Blend Released
– Blend Capacitor Pulled Up to
0.7 V – Stops
– Fast Lock Current Removed
– Fast AGC Turned Off
– Pilot Detector Enabled
Pilot Detected
– Stereo Indicator Pin Pulled Low
– Blend Voltage Pulled Positive
Rapidly
Blend Voltage Reaches
1.4 V
– Audio Starts Into Stereo
– 10% Negative I Detector
Enabled
Blend Voltage Reaches
2.2 V
– Stereo Separator Reaches 20
to 25 dB
– Rapid Current Pull–Up Turned
Off
– 4% Negative I Detector Enabled
Blend Voltage Reaches
3.0 V
– cosθ Enabled – Full C–QUAM
Decoding
– Blend Voltage Continues to Rise
to 3.6 V and Stops
Table 2. Operation In Adverse Conditions
4% Negative I Detected
– Blend Pulls Down
Approximately 200 mV for Each
Event – Acts Like One–Shot
– Stops at 2.2 V – cosθ Has Been
Defeated, Almost Full Stereo
Remains
10% Negative I Detected
– Blend Pulls Down 200 mV for
Each Event
– Stops at 1.4 V – Stereo Has
Blended to Mono
– Resets Fast Pull–Up if Blend
Has Not Been Above 2.2 V
50% Negative I Detected
(Out of Lock)
– Blend Pulls Down Fast During
Event
– Stops at 0.47 V
– Resets Fast Pull–Up
– Pilot Indicator Turned Off
Minimum Signal Level
Detected
– Resets Fast Pull–Up
– Pulls Down to 0.7 V
MOTOROLA ANALOG IC DEVICE DATA
9
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