What is the LM567 IC?

The LM567 is a versatile integrated circuit designed by National Semiconductor in the late 1960s. This IC can perform several functions, including tone decoding, tone generation, and phase-locked loop operations.

As the name suggests, the LM567 contains two 567 tone decoder circuits that share common external components. The 567 tone decoder was one of the earliest PLL circuits that became widely used in communications and control systems.

Some key features of the LM567 IC:

• Dual tone decoders with shared components
• Programmable center frequency from 0.01 Hz to 500 kHz
• Tone detect and quadrature clock outputs
• On-chip voltage controlled oscillator (VCO)
• Phase comparator, active filter, and amplifier circuits
• Wide supply voltage range of 3V to 12V

With its built-in VCO and phase comparator, the LM567 can function as a phase-locked loop or PLL. The phase comparator generates an error voltage that is filtered and fed back to control the VCO frequency. This negative feedback locks the VCO to an incoming reference signal.

Tone Decoding Applications

One of the main uses of the LM567 is as a tone decoder. This allows the detection of audio tones in the specified center frequency range.

Some example applications:

• DTMF touch-tone decoding
• Remote control tone decoding
• Call progress tone detection in telecom
• FSK decoding for data comms
• Tone detection in security systems

The tone decoder has an open collector output that goes low when an input tone is detected. Hysteresis around the center frequency prevents unwanted triggering from noise.

DTMF Touch Tone Decoding

Dual-tone multi-frequency or DTMF encoding is used on telephone keypads to represent the digits 0-9, * and #. DTMF uses two simultaneous tones – one from a high frequency group and one from a low frequency group.

By configuring two LM567 ICs with center frequencies matched to the DTMF tones, we can detect which key is pressed on a telephone keypad. One LM567 monitors the low frequency tones, while the other monitors the high frequency tones. When a valid tone pair is received, the logic output indicates the corresponding digit or symbol.

Remote Control Tone Decoding

The LM567 can also decode modulated tone signals used in remote controls. For example, garage door openers and electronic door locks often use amplitude shift keying (ASK) of a 40 kHz carrier tone. A series of pulse width modulated codes transmitted with the carrier represent the control functions.

By setting the LM567 center frequency to 40 kHz, the tone decoder can extract the digitally encoded signal. A microcontroller then interprets the PWM codes to perform actions like activating the garage door motor or unlocking the door.

Tone Generation Applications

In addition to tone decoding, the LM567 contains an on-chip voltage-controlled oscillator that can generate various audio tones and frequencies. This makes the IC useful in applications like analog tone dialing, warning tone generators, and frequency shift keying.

Analog Tone Dialing

Before touch tone DTMF dialing, telephones used an analog tone system to encode the dialed digits. The tone frequencies represented digits 0-9 according to the following table:

By toggling the LM567’s VCO between the appropriate frequencies, a telephone dialer circuit can synthesize the analog tones for dialed numbers. When the telephone network detects these tones, it will connect the call.

Warning and Alert Tone Generation

The LM567’s tone generation abilities are also useful for creating warning, alert, or notification tones. For instance, a smoke alarm could use the IC to generate a loud sweeping siren sound. Other applications include:

• Sirens for security systems, emergency vehicles
• Medical equipment alert tones
• Timer and clock alarms
• Computer error beeps
• DTMF tone dialing on systems without a DTMF encoder IC

For these applications, the LM567’s center frequency and timing components are configured to create the desired tone patterns. The tone output can directly drive a small speaker or audio amplifier.

Frequency Shift Keying

Frequency shift keying or FSK is a modulation technique used in data communications. Binary data is transmitted by shifting the frequency between two values – a “mark” frequency for a 1 bit and a “space” frequency for a 0 bit.

The LM567 can easily implement FSK modulation with its integrated VCO. A control circuit toggles the VCO between two center frequencies set by the timing resistor/capacitor. This generates the FSK modulated carrier which can then be sent through a transmitter.

An LM567 decoder IC on the receiver can detect the mark and space frequencies and reshape the serial data. FSK was commonly used in early low-speed modems and radio data links.

Phase-Locked Loop Applications

With the addition of an external low-pass loop filter, the LM567 forms a phase-locked loop or PLL. This allows frequency control, frequency multiplication, FSK decoding, and other useful functions.

The tone decoder circuit becomes a phase comparator. It generates an error voltage proportional to the frequency difference between the VCO and reference input. This error signal is filtered and applied to the VCO control input to complete the feedback loop.

Frequency Control and Multiplication

Applying a stable low-frequency reference clock to the PLL allows the VCO to lock to a precise higher frequency. This acts as a frequency multiplier. Since the VCO frequency is controlled by the timing components, we get:

VCO Frequency = Reference Frequency x (R1+R2)/R2

The loop filter smooths out noise and instability in the VCO output. This creates a clean and stable frequency multiplier.

For example, feeding a 10 kHz clock into the PLL could generate a precise 100 kHz VCO carrier locked to the reference. This technique was historically used to generate local oscillator signals for radio receivers.

FSK Demodulation

A PLL can also demodulate frequency shift keyed signals – recovering the binary serial data. The phase comparator detects if the incoming signal is centered on the “mark” or “space” VCO frequency. This generates the decoded data at the PLL error amplifier output.

An LM567 PLL decoder offers a simple way to demodulate FSK signals up to several hundred kHz. For higher data rates, a dedicated IC modem chip would be more appropriate.

Example LM567 Circuits

Here are some example circuits showing how the LM567 can be configured as a tone decoder, tone generator, or phase-locked loop.

LM567 Tone Decoder

This circuit will detect input tones around 1 kHz. The timing components R1 and C1 set the free-running VCO center frequency. Audio signals within the passband trigger the tone detect output.

LM567 Tone Generator

This configuration turns the LM567 into an audio tone oscillator. The 1 kΩ potentiometer adjusts the frequency set by the 220 nF timing capacitor. The output could drive a small 8 ohm speaker.

LM567 Phase-Locked Loop

Here the LM567 forms a phase-locked loop with the addition of an RC low-pass filter. The 10 kHz reference locks the VCO at 100 kHz, acting as a frequency multiplier.

Conclusion

First released over 50 years ago, the LM567 integrated circuit continues to prove useful in a wide array of tone decoding and phase-locked loop applications. With both decoder and oscillator functions combined with an easy to use PLL, this versatile chip offers a low-cost solution for analog tone and frequency control.

Advanced communication systems now utilize more sophisticated ICs with higher speeds, sensitivity and signal-to-noise ratios. However, for relatively simple tone decode/encode applications below 500 kHz, the LM567 remains hard to beat. Ease of use and low external component count make this a favorite IC among hobbyists and hardware tinkerers.

Next time your design calls for detecting remote control codes, dialing touch tones, generating alarm beeps, or synthesizing audio tones, take a look at the LM567. With a few resistors and capacitors, it can perform all these tasks and more.

Frequently Asked Questions

What are some typical applications for the LM567?

Some typical applications are:

• DTMF tone decoding for telephone dialing
• Decoding remote control signals
• FSK decoding and modulation
• Warning and alert tone generation
• Analog tone dialing for telephones
• Frequency synthesis/multiplication in radio tuners

How does the phase-locked loop work on the LM567?

The phase comparator (tone decoder) generates an error voltage based on the frequency difference between the incoming signal and the VCO. This error voltage is filtered and fed back to the control input of the VCO to complete the PLL feedback loop. The result locks the VCO to the phase and frequency of the reference signal.

What frequency range can the LM567 work over?

The center frequency of the LM567 can be programmed from around 0.01 Hz up to 500 kHz using the timing resistor R1 and capacitor C1. However, performance degrades considerably above 100-200 kHz. For higher frequencies, a different PLL IC would be more appropriate.

Can I use a microcontroller instead of the LM567?

Yes, many microcontrollers have tone detection, tone generation, and even PLL/VCO modules built in. However, for simple applications, the LM567 avoids the need for programming and provides analog components like filters external to the chip. It can also operate at lower supply voltages than most microcontrollers.

How stable is the frequency on the LM567?

The oscillator frequency stability depends on the external timing components used. But in general, LM567 frequency accuracy is around 1-2% without taking special precautions. For applications needing highly precise frequency synthesis, a crystal reference PLL should be used instead of the LM567’s LC oscillator.