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| Audio ICs AM radio / FM IF stereo system IC BA1442A The BA1442A is an AM radio and FM IF stereo system IC developed for radio cassette players. The FM circuit is comprised of a differential IF amplifier, a double-balance type quadrature detector, and a PLL-type stereo MPX circuit. The AM circuit is comprised of a local oscillator, a double-balanced mixer, an IF amplifier, a detector, and an AGC circuit. FApplications Radio cassette players FFeatures 1) The "S" curve characteristic of the FM detector can be used for upper heterodyne ("N" curve) operation. 2) Audio high cut and low cut can be used to improve the AM fidelity. 3) A reference voltage supply is provided to ensure good frequency stability in the shortwave bands. 4) Good FM stability. 5) Built-in damping resistor (3.3k) for the quadrature coil. 6) The FM MPX uses a Laser Locked Loop, making adjustment of the VCO and addition of external components unnecessary. 7) Built-in forced monaural function for the MPX (VCO stops, LED off). 8) VCO of the MPX is switched off during AM operation. 9) The stereo indicator pin has a constant drive current, and a current limiting resistor is not required. 10) Few external components required. FAbsolute maximum ratings (Ta = 25_C) FRecommended operating conditions (Ta = 25_C) 337 Audio ICs FBlock diagram BA1442A FPin descriptions 338 Audio ICs BA1442A 339 Audio ICs BA1442A 340 Audio ICs BA1442A FElectrical characteristics (unless otherwise noted, Ta = 25_C, VCC = 5V, FM IF MPX: fin = 10.7MHz, 400Hz modulation, 22.5kHz dev (30%), 19kHz 7.5kHz dev (10%) AM: fin = 1000kHz, fm = 400Hz 30%) FMeasurement circuit 341 Audio ICs FTechnical reference materials (unless otherwise noted, Ta = 25_C and VCC = 5V) BA1442A FCircuit operation (1) The FM IF amplifier is a three-stage differential amplifier. The gain of this stage is approximately 60dB. The bias for the first-stage amplifier is taken from the bypass filter (pin 2). This improves the FM stability of the firststage amplifier by reducing noise. The input impedance is set by a resistor on the chip (330), and can be connected to a ceramic filter. (2) The FM detector circuit is comprised of a quadrature detector. The IF amplified signal is input to the detector via a limiter amplifier with a gain of 13dB. (3) The AM mixer is a double-balance type, and has been designed for low local oscillator leakage. The mixer bias current is added from Vreg via antenna coil. If you lower the mixer conversion gain for mid- to high-strength electric fields, AGC will operate, and for high-strength electric fields, the input shunt AGC gives good strong-input characteristics. (4) The AM local oscillator circuit consists of a differential positive feedback loop. To improve the frequency stability, particularly in the SW band, the circuit is operated with bias referenced to Vreg. To improve the startup characteristics, a 100 resistor is inserted in series with the tank circuit. (5) The AM IF amplifier consists of two-stage differential amplifier. Both amplifiers use capacitors to block DC, so bypass capacitors are not necessary. The first stage amplifier has AGC. The gain of this stage is 47dB, and the input impedance is 3k. (6) AM detector circuit A built-in envelope detector that uses a minute current serves as the AM detector. External components are not required. 342 Audio ICs (7) AM AGC circuit AGC is applied to the input shunt, mixer , and IF circuits. The AGC reduces the mixer and IF circuit current, and according the DC level of the detector output, reduces the gain to control the output at a fixed level. In addition, an input shunt is provided to prevent distortion caused by strong input signals. The AGC pin is also used as the MPX PLL filter pin. For this reason, when there is no input, the AGC voltage becomes Vreg (pin 19), and when there is a strong input signal, it is about Vreg - 0.2V. (8) AM / FM detector circuits There is one pin for both detector outputs (pin 6), and the AM / FM switching is done internally. The output impedance is 5k, and the DC output is 1.3V. BA1442A (9) The FM MPX circuit uses a PLL to recover the stereo. For FM, the input from pin 7 is switched by the decoder and output on pin 11 (L channel) and pin 12 (R channel). In the case of AM, the input is from pin 8, and this is output through to pins 11 and 12. The audio fidelity is improved by the low-pass filter comprised of the capacitor connected between pins 6 and 8 and the input impedance of pin 8 (20k). The input / output gain for both AM and FM is about 6dB. The VCO has been laser trimmed on the chip to set its time constant. External components and re-adjustment are not required. FApplication example 343 Audio ICs FFrequency data BA1442A FComponent data 344 Audio ICs BA1442A 345 Audio ICs FSelecting of attached components and operation notes Pin 1: FM ceramic filter (CF1) The input impedance of pin 1 is 330, so it should be connected to a ceramic filter that has an output impedance of 330. Connect the earth point of the ceramic filter to the input side GND. In addition, the connection distance between the ceramic filter and pin 1 should be as short as possible. Pin 2: FM IF bypass This pin is used to ensure stable FM, and it is normally connected to pin 19 (Vreg). If it is not possible to achieve stable FM, ground pin 2 via a 0.01F capacitor. The earth point in this case will be on the GND side of the FM ceramic filter. Pin 3: Power supply bypass capacitors (C7 and C8) C7 should be a capacitor with good high-frequency characteristics. Connect it between the base of the legs of pin 3 (VCC) and pin 5 (GND). The purpose of C8 is ripple rejectiun, and it can be connected a little further away. Pin 4: FM quadrature detector phase-shift coil (T3) Pin 3 and 4 are connected internally by a 3.3k damping resistor. The earth point for the phase-shift coil should be VCC, so ensure that it is at the same voltage as pin 3 (VCC). Pin 6: FM detector low-pass filter capacitor (C11) This capacitor smooths the 2 10.7MHz (21.4MHz) signal generated by FM quadrature detection, and prevents leakage of the 10.7MHz signal. The output impedance of pin 6 (5k) and C11 form a low-pass filter. Connect the earth point to a GND other than the input side GND. If you do not do this, the input carrier will return from the output, and may result in poor stability. If the capacitor is too large, poor separation will result. Pin 7: FM detector output coupling capacitor (C9) FM detector output is input to the MPX throngh C9. The input impedance of pin 7 (100k) and C9 form a highpass filter. If the value of C9 is too low, poor bass region separation will result. Pin 8: AM detector output coupling capacitor (C10) AM detector output is input to the MPX through C10. The input impedance of pin 8 (20k) and C10 form a high-pass filter and improves the AM fidelity. Characteristic curves for different values of C10 are given in the graph in Fig. 3. BA1442A Pin 9: AM high-cut capacitor (C12) The input impedance of pin 9 (8k) and C12 form a lowpass filter. To prevent beat from occurring between carrier leak from the detector output pin and the input signal, connect a capacitor of about 1000pF, even if not using high-cut. Connect the earth point to a GND other than the input side GND. Characteristic curves for different values of C12 are given in the graph in Fig. 4. 346 Audio ICs Pin 9: Forced monaural (SW1) If the voltage on pin 9 goes to 2.2V or higher in FM mode, forced monaural operation starts (VCO stops, the stereo indicator goes off, and audio output goes monaural). Do not apply a voltage to pin 9 that is higher than the voltage on pin 3 (VCC). Also, when in AM mode, the AM detector output is output on pin 9, so use a switch to switch pin 9 open circuit in AM mode, as shown in the application circuit example. If you change the voltage on pin 9 during AM operation, the AGC characteristics will be disturbed. Pin 10: Stereo indicator The pin 10 current is fixed at 10mA, so an external current-limiting resistor is not required. If you wish to reduce the brightness of the LED, connect a resistor in parallel to reduce the current. If you do not wish to use stereo indication, connect pin 10 directly to the power supply, however, the current consumption is that same as that when stereo indication is used. The withstanding voltage of pin 10 is 9V, so use caution when connecting it to power sources other than the IC power supply. BA1442A Pin 14: PLL filter (C15, C16, and R4) PLL loop filter for the MPX. Varying the values of these components changes the capture range of the PLL, but can also result in PLL beat, or the PLL going out of lock due to temperature changes, so do not change these component values. Pin 15: Pilot filter and AGC filter (C17) Pin 15 is used by both the MPX pilot filter and the AM AGC filter. Lower values for C17 will result in increased THD for bass-region AM audio, and larger values will increase the time required to switch between monaural and stereo for FM. Characteristic curves for various values of C17 are given in Figs. 7 to 9. Pins 11 and 12 De-emphasis capacitors (C13 and C14) The output impedance of pins 11 and 12 (5k), and C13 and C14 set the de-emphasis time constant (50sec. for 0.01F, and 75sec. for 0.015F). Do not connect C13 and C14 if composite (voice multiplex etc.) signal processing will be performed at the following stage. Pin 13: AM / FM switching noise reduction filter Pin 13 is the AM power supply pin. When power is applied, the circuit is in AM mode. By setting a CR time constant, it is possible to reduce the audible switching noise that occurs when switching between AM and FM. Choose the resistor so that the voltage drop across the resistor is 0.5V max. with respect to the pin 3 (VCC). Do not apply a voltage higher than the voltage on pin 3 (VCC) to pin 13. 347 Audio ICs BA1442A Pin 18: AM antenna (L3) The input impedance of pin 18 is 10k. The AM antenna earth point is Vreg; make the voltage at pin 18 the same as the pin 19 (Vreg) potential. If there is a voltage drop, local oscillator leakage to the mixer output will increase. Pin 19: Reference power supply bypass capacitors (C18 and C19) Connect the low-frequency bypass capacitor (C18) and the high-frequency bypass capacitor (C19). Pin 20: AM oscillator coil (L4) To improve the startup oscillation characteristics, a 100 resistor (built-in) is connected in series with the coil. If the shortwave oscillation startup characteristics are poor, connect an additional resistor (47 to 100) in series with the coil. Make the earth point for the oscillator coil pin 19 (Vreg). The relationship between the S / N ratio and sensitivity at 20dB S / N against pin 20 oscillation voltage is shown in Fig. 11. Pin 16: AM ceramic filter (CF2) The input impedance of pin 16 is 3k, so it should be connected to a ceramic filter that has an output impedance of 3k. The connection distance between the ceramic filter and pin 16 should be as short as possible. Pin 17: AM IFT (T2) The output impedance of pin 17 is 100k. The IFT earth point is VCC; make the voltage at pin 17 the same as the pin 3 (VCC) potential. The connection distance between the IFT and pin 17 should be as short as possible. 348 Audio ICs Pin 6: AFC circuit It is possible to use the pin 6 detector output DC voltage and apply AFC. Fig. 12 shows an application example where the BA4424N is used as the FM front end. Even if you will not use AFC, when recovering the subcarrier phase-modulated composite signal (voice-multiplexed TV) in the next stage, match the oscillator and "S" curve orientations in the same way as described above. If they are reversed, the left and right channels of the recovered audio will be reversed. BA1442A 349 Audio ICs FQuiescent pin voltages (VCC = 5V) BA1442A FElectrical characteristic curves MW characteristics 350 Audio ICs BA1442A FM characteristics 351 Audio ICs BA1442A FApplication board 352 Audio ICs BA1442A FExternal dimensions (Units: mm) 353 |
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