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TDA7541 数据表(PDF) 31 Page - STMicroelectronics |
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TDA7541 数据表(HTML) 31 Page - STMicroelectronics |
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31 / 76 page  TDA7541 Functional description 31/76 4.1.5 Weak signal mute The filtered field strength signal (FSWO) is the reference for soft slope mute control in weak signal condition to eliminate audible effects. The start point and mute depth are programmable (WMTH, WMD) in a wide range. These settings together with FSWO bits are influencing the weak signal mute behaviour. The time constant is defined by external capacitance at PIN 42. Additional adjacent channel mute function is supported. A high pass filter with -3dB threshold frequency of 100 kHz, amplifier and peak rectifier generates an adjacent noise signal from Demodulator output. This value is compared with adjustable threshold (ACMTH). For present strong adjacent channel the MPX signal is additional attenuated (ACMD) and has the same time constant as weak signal mute. 4.1.6 Weather band input If the weather band input is used the chip must be set in FM mode (FMON). In addition to that the ISS filter must be switched ON (ISSENA), mixer2 quality must set to IF2Q=”00” and the ISS filter must be set in manual control mode. The bit WBON is activating the WB input and is switching the ISS filter in WB mode (24 kHz band width). 4.2 AM section The up/down conversion in AM signal path is combined with gain control circuit sensing three input signals, narrow band information (DAGC) referred to PIN 56, up conversion signal (IFAGC) at PIN 61 and wide band information (RFAGC) at PIN 3.This gain control gives two output signals. The first one is a current for pin diode attenuator and the second one is a voltage for preamplifier. Time constant of RF- and IF-AGC is defined by internal 100k resistor and external capacitor at PIN 54. The intervention points for AGC (DAGC, IFAGC and RFAGC) are programmable by software. The oscillator frequency for upconversion mixer1 is generated by dividing the VCO frequency after VCO divider (VCOD) and AM predivider (AMD). Two 10,7MHz ceramic filters before mixer2 input increase 900KHz attenuation. Table 12. ISS control modes Input signals Mode1 Mode2 ac ac+ sm dev dev+ inton int80 Function inton int80 Function 0 0 0 0 0 0 0 wide 0 0 wide 0 0 0 1 0 0 0 wide 0 0 wide 0 0 0 1 1 0 0 wide 0 0 wide 0 0 1 0 0 1 1 narrow 1 1 narrow 0 0 1 1 0 0 0 wide 1 0 mid 0 0 1 1 1 0 0 wide 0 0 wide 10 00 0 1 1 narrow 1 0 Mid 1 1 0 0 0 1 1 narrow 1 1 Narrow 10 01 0 1 0 mid 10 Mid 11 01 1 1 0 mid 11 Narrow |
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