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  • Contents

I Introduction

LM567 is a tone decoding phase-locked loop integrated circuit. It is compact in design, simple in circuit, and has a wide range of applications. However, if using improperly, it will bring troubles in debugging.

This blog introduces the working principle and tips about how to use LM567 tone decoder for readers' reference.

Figure 1. LM567 Tone DecoderFigure 1. LM567 Tone Decoder

Catalog

I Introduction

II LM567 Internal Structure & Pin Function

III LM567 Working Principle & Application

IV Tips of Using Tone Decoder LM567

Ordering & Quantity

II LM567 Internal Structure & Pin Function

The name of LM567 is "phase-locked loop tone decoder", its package form uses 8-pin dual in-line, its internal structure and pin functions are illustrated in Figure 2. Its internal circuit structure is composed of quadrature phase, detector, phase-locked loop, amplifier, etc.

The operating voltage range of LM567 is 4.75 - 9V, the operating frequency can reach 500 KHz, and the static operating current is only 8 mA. The pin ③ is the signal input terminal, which requires the input signal to be greater than 2 5 mV. The pin ⑧ is the logic output terminal. It can be seen from the figure that it is an open collector transistor output, which allows a maximum sink current of 100 mA. The external resistance and capacitance of pins ⑤ and ⑥ determine the center frequency f0≈1/1.1RC of IC internal voltage controlled oscillator. Pins ① and ② are usually connected to the ground separately to form an output filter network and a loop low-pass filter network. The capacitor connected to pin ② determines the capture bandwidth. The larger the value of the capacitor, the narrower the loop bandwidth is.

Figure 2. Top View of LM567

Figure 2. Top View of LM567

III LM567 Working Principle & Application

When the tone decoder LM567 works, its phase-locked loop internal current-controlled oscillator generates an oscillation signal of a certain frequency and phase. This signal is sent to the quadrature phase detector together with the signal input at pin ③ for comparison. When the frequency of the signal falls within a given passband, the phase-locked loop locks this signal, and at the same time the internal transistor of the LM567 is controlled to be powered up, and the output terminal of the LM567 outputs low level.

The ⑤ pin of LM567 outputs the rectangular signal of the internal oscillator, and the ⑥ pin outputs the sawtooth pulse. The frequency of both is the same as the center frequency of the internal oscillator. The ② pin is the output of the phase-locked loop phase detector. The voltage on is the signal after F/V conversion. If the tone signal is input to the ② pin, then the ⑤ pin outputs the FM square wave signal modulated by the ② pin input signal.

From the basic function of LM567, LM567 can be used as an oscillator, modulator or demodulator. Therefore, it can be used as a basic device in the circuit. The application of LM567 has the function of decoding a specific frequency in the input signal, and it is widely used in communication, remote control, measurement, frequency monitoring, etc.

Figure 3. LM567 Tone DecoderFigure 3. LM567 Tone Decoder

IV Tips of Using Tone Decoder LM567

Although LM567 has a very wide range of applications, if it is not handled properly during design and application, it still fails to achieve the expected results, and even brings trouble to the debugging or affects the reliability of the product. Therefore, it should be considered from the following aspects when using it :

    1. Set Operating Frequency and Bandwidth of LM567 Accurately and Appropriately

We know that the internal oscillation frequency f0 of LM567 can be pre-set within the range of 0.1 KHz to 500 KHz, and its corresponding bandwidth can also be determined as required within the field range of 7% f0 to 14 % f0.

Therefore, after the detected signal is determined, the internal oscillation frequency f0 of LM567 should be set to coincide with the center frequency of the measured signal, and the timing components R and C connected with the 5 and 6 feet of LM567 should be used with high precision. Among them, the setting of the center frequency can be determined by the resistance value of the tuner R. When adjusting, it is necessary to prevent R short circuit or open circuit, otherwise the output level of ⑧ pin will be low level whether there is input signal or not. The external capacitance of the ② pin of LM567 determines the capture bandwidth. The smaller the capacity, the wider the capture bandwidth. However, the capacitance cannot be reduced blindly to increase the bandwidth, so as not to reduce the anti-interference ability or even trigger falsely, which affects the reliability of the product.

    2. Make the Center Frequency of the Passband Coincide with the Center Frequency of the Oscillation As Much As Possible

It should be noted that the center frequency of the passband does not always coincide with the center frequency of the oscillator, and sometimes it will deviate severely. This will inevitably cause a decrease in reliability and sensitivity. Therefore, measures should be taken to make the two centers coincide as much as possible. The circuit shown in Figure 4 can minimize the frequency offset of the two centers.

Figure 3. Circuit of LM567Figure 4. Circuit of LM567

    3. Working Voltage of LM567 Should Be Stable

The stability of the operating voltage of the LM567 has a fixed response to the stability of the center frequency of the tone decoder.

    4. Avoid Misoperation When the Output Terminal is Powered On

LM567 outputs a low level at the moment when the power is turned on. Therefore, for some remote control circuits, it is necessary to add a CR integration delay circuit to the output end to avoid erroneous operation when the power is turned on. This is especially important in the on-off control circuit.

 

 

After reading the blog, have you better understand LM567? 

Finally, if you have any questions about LM567, please do not hesitate to leave a message in the comment section below!

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