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MC13122 数据表(PDF) 9 Page - Motorola, Inc |
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MC13122 数据表(HTML) 9 Page - Motorola, Inc |
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9 / 28 page  MC13027 MC13122 9 MOTOROLA ANALOG IC DEVICE DATA 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 cos θ 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 – cos θ 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 – cos θ 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 |
类似零件编号 - MC13122 |
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类似说明 - MC13122 |
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