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6.
Detector and AM Stereo Decoder (Cont.) During AM operation, transistors Q5 and Q6 conduct to apply the regulated 10 volts to pin 6 of U3. The AM audio outputs pin 7 (left) and pin 8 (right) are applied to the filter/Amp IC (U7).
C.
FM CIRCUIT
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FM Front End - The completely integrated FM front end, U4, performs the following functions AGC, RF amp, mixer, oscillator and IF. Increased quality and reliability are achieved by integrating the discrete components. The signal from the antenna is hard wired to pin 13 and the output on pin 6 is the intermediate frequency. Tuning voltage is applied to pin 10 to determine the resonant frequencies of the varactor diodes, capacitor and coil combinations, for the RF, mixer and oscillator stages. Search sensitivity is controlled by pin 4 and the AGC input is applied to pin 3. Pin 1 supplies the IF output to the synthesizer. During FM operation, switched supply voltage will be applied to pin 5 because of the conduction of Q9 and Q8. Audio Detection and Station Detector - The IF output out of the front end is applied to pin 2 of u5. The IF amplifier output has two outputs, one to the level detector for AGC output pin 6 and the other to a buffer amplifier. The buffered IF output on pin 16 is connected to pin 15 by R37. Pins 14 and 15 are the inputs to the peak detector which has two outputs, one for the AF amplifier and the other to the frequency change detector. The detected audio is amplified and outputting on pin 10. Operation of the frequency change detector is determined by R39, C48 and C49. Station detect sensitivity is determined by R36 and C47. The station detect output pin 7 turns on Q7 when a station is detected. An internal regulator regulates the Vcc voltage level. Stereo Decoder and Blend - The separation of the left and right audio is accomplished in the FM stereo decoder U6. The multiplexed audio signal goes in on pin 3 and is outputted as left and right audio pins 4 and 6. The frequency of the PLL 19 kHz pilot detector is set at pin 16. The internal Vco frequency can be measured at pin 11 with a sufficiently high impedance counter through a 56K resistor or greater. The IC also contains a stereo
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