LM567, LM567C: IC Tone Decoder
The LM567 and LM567C are general purpose tone decoders designed to provide a saturated transistor switch to ground when an input signal is present within the passband. The circuit consists of an I and Q detector driven by a voltage controlled oscillator which determines the center frequency of the decoder. External components are used to independently set center frequency, bandwidth and output delay.
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Features
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Applications
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Output Filter
To eliminate undesired signals that could trigger the output stage, a post detection filter is featured in the LM567C. This filter consists of an internal resistor (4.7K-Ω) and an external capacitor. Although typically external capacitor value is not critical, it is recommended to be at least twice the value of the loop filter capacitor. If the output filter capacitor value is too large, the turn-on and turn off-time of the output will present a delay until the voltage across this capacitor reaches the threshold level.
Loop Filter
The phase locked loop (PLL) included in the LM567 has a pin for connecting the low pass loop filter capacitor. The selection of the capacitor for the filter depends on the desired bandwidth. The device bandwidth selection is different according to the input voltage level. Refer to the Operation With Vi < 200m – VRMS section and the Operation With Vi > 200m – VRMS section for more information about the loop filter capacitor selection.
Logic Output
The LM567 is designed to provide a transistor switch to ground output when the input signal frequency matches the center frequency pass band. The logic output is an open collector power transistor that requires an external load resistor that is used to regulate the output current level.
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