Download: Audio ICs AM radio / FM IF stereo system IC BA1442A

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 com- prised 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. Applications Radio cassette players Features 1) The “S” curve characteristic of the FM detector can 6) The FM MPX uses a Laser Locked Loop, making be used for upper heterodyne (“N” curve) operation. adjustment of the VCO and addition of external 2) Audio high cut and low cut can be used to improve the components unnecessary. AM fidelity. 7) Built-in forced monaural function for the MPX (VCO 3) A reference voltage supply is provided to ensure stops, LED off). good frequency stability in the shortwave bands. 8) VCO of the MPX is switched off during AM opera- 4) Good FM stability. tion. 5) Built-in damping resistor (3.3kΩ) for the quadrature 9) The stereo indicator pin has a constant drive cur- coil. rent, and a current limiting resistor is not required. 10) Few external components required. Absolute maximum ratings (Ta = 25C) Recommended operating conditions (Ta = 25C), Block diagram Pin descriptions, Electrical characteristics (unless otherwise noted, Ta = 25C, 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%) Measurement circuit, Technical reference materials (unless otherwise noted, Ta = 25C and VCC = 5V) Circuit operation (1) The FM IF amplifier is a three-stage differential am- (4) The AM local oscillator circuit consists of a differen- plifier. The gain of this stage is approximately 60dB. The tial positive feedback loop. To improve the frequency sta- bias for the first-stage amplifier is taken from the bypass bility, particularly in the SW band, the circuit is operated filter (pin 2). This improves the FM stability of the first- with bias referenced to Vreg. To improve the startup char- stage amplifier by reducing noise. The input impedance acteristics, a 100Ω resistor is inserted in series with the is set by a resistor on the chip (330Ω), and can be con- tank circuit. nected to a ceramic filter. (5) The AM IF amplifier consists of two-stage differen- (2) The FM detector circuit is comprised of a quadra- tial amplifier. Both amplifiers use capacitors to block DC, ture detector. The IF amplified signal is input to the detec- so bypass capacitors are not necessary. The first stage tor via a limiter amplifier with a gain of 13dB. amplifier has AGC. The gain of this stage is 47dB, and (3) The AM mixer is a double-balance type, and has the input impedance is 3kΩ. been designed for low local oscillator leakage. The mixer (6) AM detector circuit bias current is added from Vreg via antenna coil. If you A built-in envelope detector that uses a minute current lower the mixer conversion gain for mid- to high-strength serves as the AM detector. External components are not electric fields, AGC will operate, and for high-strength required. electric fields, the input shunt AGC gives good strong-in- put characteristics., (7) AM AGC circuit (9) The FM MPX circuit uses a PLL to recover the ste- AGC is applied to the input shunt, mixer , and IF circuits. reo. For FM, the input from pin 7 is switched by the de- The AGC reduces the mixer and IF circuit current, and coder and output on pin 11 (L channel) and pin 12 (R according the DC level of the detector output, reduces channel). In the case of AM, the input is from pin 8, and the gain to control the output at a fixed level. In addition, this is output through to pins 11 and 12. The audio fidelity an input shunt is provided to prevent distortion caused by is improved by the low-pass filter comprised of the capac- strong input signals. The AGC pin is also used as the itor connected between pins 6 and 8 and the input imped- MPX PLL filter pin. For this reason, when there is no in- ance of pin 8 (20kΩ). The input /output gain for both AM put, the AGC voltage becomes Vreg (pin 19), and when and FM is about 6dB. The VCO has been laser trimmed there is a strong input signal, it is about Vreg - 0.2V. on the chip to set its time constant. External components (8) AM/FM detector circuits and re-adjustment are not required. There is one pin for both detector outputs (pin 6), and the AM/FM switching is done internally. The output imped- ance is 5kΩ, and the DC output is 1.3V. Application example, Frequency data Component data, Selecting of attached components and operation notes Pin 1: FM ceramic filter (CF1) The input impedance of pin 1 is 330Ω, so it should be con- nected to a ceramic filter that has an output impedance of 330Ω. 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 0.01µF capacitor. The earth point in this case will be on the GND side of the FM ce- ramic filter. Pin 3: Power supply bypass capacitors (C7 and C8) C7 should be a capacitor with good high-frequency char- acteristics. Connect it between the base of the legs of pin Pin 9: AM high-cut capacitor (C12) 3 (VCC) and pin 5 (GND). The purpose of C8 is ripple rejec- The input impedance of pin 9 (8kΩ) and C12 form a low- tiun, and it can be connected a little further away. pass filter. To prevent beat from occurring between carri- Pin 4: FM quadrature detector phase-shift coil (T3) er leak from the detector output pin and the input signal, Pin 3 and 4 are connected internally by a 3.3kΩ damping connect a capacitor of about 1000pF, even if not using resistor. The earth point for the phase-shift coil should be high-cut. Connect the earth point to a GND other than the VCC, so ensure that it is at the same voltage as pin 3 (VCC). input side GND. Characteristic curves for different values Pin 6: FM detector low-pass filter capacitor (C11) of C12 are given in the graph in Fig. 4. This capacitor smooths the 2 10.7MHz (21.4MHz) sig- nal generated by FM quadrature detection, and prevents leakage of the 10.7MHz signal. The output impedance of pin 6 (5kΩ) 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 (100kΩ) and C9 form a high- pass 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 (20kΩ) 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., 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 cir- cuit example. If you change the voltage on pin 9 during AM operation, the AGC characteristics will be disturbed. Pin 14: PLL filter (C15, C16, and R4) Pin 10: Stereo indicator PLL loop filter for the MPX. Varying the values of these The pin 10 current is fixed at 10mA, so an external cur- components changes the capture range of the PLL, but rent-limiting resistor is not required. If you wish to reduce can also result in PLL beat, or the PLL going out of lock the brightness of the LED, connect a resistor in parallel due to temperature changes, so do not change these to reduce the current. If you do not wish to use stereo in- component values. dication, connect pin 10 directly to the power supply, Pin 15: Pilot filter and AGC filter (C17) however, the current consumption is that same as that Pin 15 is used by both the MPX pilot filter and the AM when stereo indication is used. The withstanding voltage AGC filter. Lower values for C17 will result in increased of pin 10 is 9V, so use caution when connecting it to pow- THD for bass-region AM audio, and larger values will in- er sources other than the IC power supply. crease the time required to switch between monaural and stereo for FM. Characteristic curves for various val- ues 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 (5kΩ), and C13 and C14 set the de-emphasis time constant (50µsec. for 0.01µF, and 75µsec. for 0.015µF). Do not connect C13 and C14 if composite (voice multiplex etc.) signal proces- sing 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 ap- plied, 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., Pin 18: AM antenna (L3) The input impedance of pin 18 is 10kΩ. 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 100Ω resistor (built-in) is connected in series with the coil. If the shortwave oscillation startup characteristics are poor, connect an additional resistor (47Ω to 100Ω) 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 3kΩ, so it should be con- nected to a ceramic filter that has an output impedance of 3kΩ. The connection distance between the ceramic fil- ter and pin 16 should be as short as possible. Pin 17: AM IFT (T2) The output impedance of pin 17 is 100kΩ. 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., 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 sub- carrier phase-modulated composite signal (voice-multi- plexed 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 re- covered audio will be reversed., Quiescent pin voltages (VCC = 5V) Electrical characteristic curves MW characteristics, FM characteristics, Application board, External dimensions (Units: mm)]

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