9.3 Feature Description

9.3.1 Center Frequency

The center frequency of the LM567 tone decoder is equal to the free running frequency of the voltage controlled

oscillator. In order to set this frequency, external components should be placed externally. The component

values are given by:

fo ≃ 1 / (1.1 × R1 × C1)

(1)

where

• R1 = Timing Resistor

• C1 = Timing Capacitor

9.3.2 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.

9.3.3 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.

9.3.4 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.

9.3.5 Die Characteristics

Figure 9-1. Die Layout (C - Step)

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LM567, LM567C

SNOSBQ4F – MAY 1999 – REVISED JANUARY 2022

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Product Folder Links: LM567 LM567C