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MC13027P Datasheet(PDF) 8 Page - Motorola, Inc |
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MC13027P Datasheet(HTML) 8 Page - Motorola, Inc |
8 / 28 page MC13027 MC13122 8 MOTOROLA ANALOG IC DEVICE DATA Figure 5. MC13122 Internal Block Diagram 28 27 26 25 24 23 22 123456 7 21 20 19 18 89 10 11 17 16 15 12 13 14 I L–R Q VCC VGA ±0.9 VGA ±0.9 LR Level Fast AGC 1.0 V 3.0 V AGC Matrix C–QUAM Comparator Blend Disable 450 <90 ° 450 <0 ° Signal Quality Detector ÷32 ÷137/144 ÷4 Count Control Clamp 25.6 Hz 24.4 Hz Fast Lock VCO Loop Driver Pilot Level Det ÷8 Pilot I Det Pilot Q Det cos θ 330 Loop Filt Blend Gnd Pilot Ind Osc Out Osc In Pilot Det I Pilot I Pilot Q Audio Blank L–R L+R IF Amp VCC Q Det L–R Det I Det IF In SS L Out L Filt In L Filt Ctr L Mat Out R Mat Out R Filt Ctr R Filt In R Out AGC 3.0 V Reg Det In E Det MC13122 FUNCTIONAL DESCRIPTION The MC13122 is designed to accept a 450 kHz C–QUAM input signal from approximately 1.0 mV to 1.0 V and produce L and R audio output signals. It has additional features: stop signal, variable bandwidth IF and audio response, stereo indicator driver and track and hold noise blanking. The IF amplifier on Pin 5 has its own AGC system. It operates by varying the input resistance on Pin 5. With weak signals below approximate 5.0 mV, the input resistance is very high and the amplifier is at maximum gain. For this AGC to be effective, it is necessary to feed the IF input signal from a relatively high impedance. The input resistance variation also reduces the Q of the coil (T1 in the application) so the receiver bandwidth is narrow for weak signals and wide for strong signals. The value of the input resistor (R5) is selected for the desired loading of the IF coil. The impedance of the IF coil on Pin 2 determines the IF gain. Pin 2 is also the input to the C–QUAM decoder. The IF signal drives the envelope (E), in–phase (I), quadrature (Q) and (L–R) detectors. The E detector is a quasi–synchronous true envelope detector. The others are true synchronous detectors. The E detector output provides the L+R portion of the C–QUAM signal directly to the matrix. The AGC signal of the IF amplifier drives the signal strength output at Pin 6. An external resistor on Pin 6 (sets the gain of the AGC). The Pin 6 voltage is used to control the Q of the audio notch filter, causing the audio bandwidth and depth of the 10 kHz notch to change with signal strength. It is also used as one of the inputs to the signal quality detector which generates the stop–sense and blend signal on Pins 6 and 23 respectively and tells the signal quality detector that the RF input is below the AGC threshold. VCO The 3.6 MHz ceramic resonator on Pins 19 and 20 is part of a phase locked loop which locks to the 450 kHz IF signal. The 3.6 MHz is divided by 8 to produce in–phase and quadrature signals for the I, Q and L–R detectors. It is also divided by 32, and 137/144 to provide signals for the pilot I and Q detectors. The pilot detector is a unique circuit which does not need filtering to detect the 25 Hz pilot. Blend Circuit The purpose of the blend circuit is to provide an AM stereo radio with the capability of very fast lock times, protection against stereo falsing when there is no pilot present and control of the L–R signal so as to provide as much stereo information as possible, while still sounding good in the presence of noise or interference. The circuit also provides an optional stop–sense usable by a radio with seek and/or scan. The stop–sense signal provides a “stop” signal only when the radio is locked on station, signal strength is above minimum level, and the level of interference is less than a predetermined amount. The last feature prevents stopping on frequencies where there is is a multiplicity of strong co–channel stations. It is common for AM radios without this capability to stop on many frequencies with unlistenable stations, especially at night. The blend circuit controls the PLL fast lock, pilot detector, IF amplifier AGC rate, decoder L–R gain, cos θ compensation and stop–sense as a function of the voltage on a signal external blend capacitor. Timing is determined by the rate of change of voltage on the blend cap. Timing is changed by varying charge and discharge current and pulled down by a current source, switch, and optionally an external switch. The current sources and switches are controlled by various measures of signal quality, signal strength, and presence or absence of pilot tone. |
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