Application of Phase-Locked Loop in Frequency Modulation and Demodulation Circuit-EEWORLD

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1 Introduction
A phase-locked loop (PLL) is a closed-loop automatic control system that can track the phase of an input signal. It has been widely used in various fields of radio technology. Integrated loop components are favored for their low cost, excellent performance, and ease of use. It has been used in many aspects such as frequency modulation and demodulation, frequency synthesis, TV color subwave extraction, FM stereo decoding, remote control systems, frequency encoding and decoding, etc. This article introduces the application of the integrated phase-locked loop CD4046 in frequency modulation and demodulation.

2 Introduction to the integrated phase-locked loop CD4046

2.1 CD4046 structure and performance characteristics

Its internal structure block diagram is shown in Figure 1,

Figure 1 CD4046 internal structure block diagram
It is a low-power CMOS type, multi-function digital ring. The main parameters are as follows: (1) operating voltage 3V-18V; (2) static operating current (terminal 15 open circuit) 10uA; (3) maximum operating frequency is 1.2MHZ; (4) Zener diode stable voltage 4.45V-6.15V. It contains two phase comparators PCІ and PCII. PCІ requires the input signal to be a square wave, while PCII does not have this requirement and has a voltage-controlled (frequency) oscillator VCO. There is a preamplifier at the input end of the two phase comparators, which can convert a weak signal of 100mV into a full-level square wave pulse. A2 is a low filter output buffer amplifier. CD4046 uses a 16-wire dual in-line package, and the functions of each pin are shown in Table 1:
Table 1 CD4046 pin function table

[page]2.2 Working principle of frequency modulation and demodulation circuit composed of CD4046

When an audio signal is input from pin 9, a frequency modulation signal modulated by the input signal can be output from pin 4. The circuit is shown in Figure 2. Since the VCO is required to have a certain frequency range (frequency deviation) during frequency modulation, R2 is not required to shrink the frequency band, that is, R2 is infinite (pin 12 is vacant), and only R1 and C1 are used to determine the center frequency f0 of the VCO. When designing parameters, it is only necessary to find C1 and R1 from f0 by looking up Figure 4 (the curve when the power supply voltage VDD is 9V, and the horizontal axis is the value of C1).
When a frequency modulation signal modulated by an audio signal (the center frequency is the same as the center frequency of the VCO of CD4046) is input from pin 14, the output end of the phase comparator will output an envelope signal with the same frequency change as the audio signal. After the carrier is filtered out by the low-pass filter, the audio signal after the frequency modulation signal demodulation is left. PCI is generally used. In this case, only R1 and C1 determine the center frequency f0 of the VCO, and R2 is not used to shrink the frequency range (which is infinite). Similarly, find R1 and C1 from Figure 4. When there is no FM signal input, the VCO works at f0. The demodulation circuit is shown in Figure 3.

Figure 2 Frequency modulation circuit composed of CD4046

Figure 3 Frequency demodulation circuit composed of CD4046

Figure 4 Characteristic curve of CD4046 under different external component parameters

3 Practical Circuit Introduction

The following is a frequency modulation infrared talker, which is divided into two parts: the transmitting and receiving circuits. As shown in Figure 5 and Figure 6, the weak signal emitted by the microphone MIC is amplified by the inverting amplifier composed of 5G28 (the op amp is powered by a single power supply, R1 and R2 bias the op amp's in-phase input at 1/2 of the power supply voltage, so that its output is also 1/2 of the power supply voltage, and the amplifier has a large dynamic range) and is coupled to the control pin 9 of the internal voltage-controlled oscillator of CD4046 through C3. The output pin 4 of the voltage-controlled oscillator outputs a modulated signal modulated by the audio signal at pin 9. The center frequency of CD4046 is determined by R4 and C4, which is about 100KHZ. The parameters of R4 and C4 are obtained by looking up the table from the selected f0 and Figure 4 during design. When there is no audio signal, the phase-locked loop works at the center frequency f0. This makes the CMOS phase-locked loop have the advantages of good frequency modulation linearity, large modulation index, and low power consumption. The frequency modulated signal drives the light-emitting diodes VD1-VD4 to radiate infrared pulses. In the receiving circuit, VT is a photosensitive transistor, which is directly connected to the inverting input terminal of the op amp. After being amplified by the op amp, it is coupled to the input terminal 14 of the phase comparator of CD4046 through C1. The center frequency of the demodulation circuit is consistent with the center frequency of the transmitter, which is also 100KHZ (C3, R4 are the same as C4, R4 of the transmitter). Pin 10 of CD4046 is the control terminal of the voltage-controlled oscillator, that is, the output terminal of the demodulation signal, and pin 2 is the output terminal of the phase comparator. The comparator signal is connected to pin 9 after being formed into a low-pass filter by R5 and C4. The demodulated signal of pin 9 is buffered by the follower formed by the internal op amp of CD4046 and output from pin 10. Pin 4 of CD4046 is the output terminal of the voltage-controlled oscillator, and pin 3 is the comparison input terminal of the phase comparator, which are connected together. The demodulated audio signal output by CD4046 is coupled to VT2 through C4, and VT2 drives the speaker to produce sound. In the transmitter, MIC uses a dynamic microphone, VT uses CS9013, and VD1-VD4 uses HIR405B infrared light-emitting diodes; in the receiver, VT1 uses 3U31 phototransistor and VT2 uses CS8050.

Figure 5 Transmitter circuit

Figure 6 Receiving circuit

4. Conclusion

The circuit designed by using the modulation tracking characteristics of the phase-locked loop has the characteristics of few peripheral components, stable performance and powerful functions. With the development of microelectronics technology, computer technology and the improvement of chip production technology, it is believed that more advanced, more powerful and cheaper (even with intelligent) phase-locked loop products will continue to appear.

References

1. Zhang Juesheng, Zheng Jiyu, Wan Xinping: Phase-locked Technology Xi'an: Xi'an University of Electronic Science and Technology Press 1994
2. Alan Blanchard: Phase-locked Loop and Its Application in Coherent Receiver Design Beijing: People's Posts and Telecommunications Press 1980
3. Wan Xinping, Zhang Juesheng: Integrated Phase-locked Loop - Principle, Characteristics, Application Beijing: People's Posts and Telecommunications Press 1990

Reference address：Application of Phase-Locked Loop in Frequency Modulation and Demodulation Circuit

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