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1
Basic components of the. PLL
Many electronic devices usually need to synchronize external input signals with internal oscillating signals, which can be achieved through the phase-locked loop.
The phase-locked loop (PLL) is a feedback control circuit ). The Phase-Locked Loop uses external input reference signals to control the frequency and phase of internal oscillating signals in the loop.
Because the phase-locked loop can automatically track the input signal frequency by the output signal frequency, the phase-locked loop is usually used in the closed loop tracking circuit. When the frequency of the output signal is the same as that of the input signal, the phase difference between the output voltage and the input voltage remains fixed, that is, the phase between the output voltage and the input voltage is locked, which is the origin of the PLL name.
The phase-locked loop consists of three parts: Phase Detector (PD), loop filter (LF), and voltage controlled oscillator (VCO). The principle diagram of Phase-Locked Loop is 8-4-1.
The phase detector in the Phase-Locked Loop is also called a phase comparator. It detects the phase difference between the input and output signals and converts the detected Phase Difference signal to Ud.
(T) voltage signal output. The signal is filtered by the low-pass filter to form the control voltage UC of the voltage controlled oscillator.
(T) To control the frequency of the output signal of the oscillator.
2. Working Principle of the Phase-Locked Loop
The phase detector in the Phase-Locked Loop is usually composed of Analog Multiplier. The phase detector circuit consists of 8-4-2.
The working principle of the phase detector is: Set the signal voltage of the external input and the signal voltage output by the voltage controlled oscillator respectively:
(8-4-1)
(8-4-2)
ω 0 in Formula
It is the natural oscillation angle frequency of a voltage controlled oscillator when the input control voltage is zero or DC voltage. The output voltage of the analog multiplier is Ud.
Is:
Low-pass filter LF is used to filter out the sum of the preceding frequency, and the remaining difference frequency is used as the input control voltage UC of the voltage controlled oscillator.
(T ). That is, UC
(T:
(8-4-3)
ω I in Formula
Is the instantaneous oscillation angle frequency of the input signal, θ I
(T) and θ o
(T) are the instantaneous phases of the input and output signals respectively. The relationship between the instantaneous frequencies and the instantaneous phases can be obtained based on the relationship between the phase quantities:
That is
(8-4-4)
Then, the instantaneous phase difference is θ D.
Is
(8-4-5)
Find the difference between the two sides, and the relationship between the available frequency difference is
(8-4-6)
If the above formula is equal to zero, it indicates that the phase-locked loop enters the Phase Lock State. At this time, the frequency and phase of the output and input signals remain unchanged.
(T) is a constant value. When the formula is not equal to zero, it indicates that the phase of the Phase-Locked Loop is not locked, the frequency of the input and output signals is not equal to that of the UC
(T) changes with time.
As shown in Figure 8-4-3 of the voltage controlled oscillator, this characteristic describes the oscillation frequency ω U of the voltage controlled oscillator.
Take ω 0
Center, UC with input signal voltage
(T) changes. The expression for this feature is
(8-4-6)
When UC
(T) The oscillation frequency ω U of the voltage controlled oscillator varies with time
It also changes with time. The phase-locked loop enters the "Frequency traction" and automatically tracks and captures the frequency of the input signal, so that the phase-locked loop enters the locked state and maintains the ω 0
= ω I
.
8
4.2 Application of PLL
1
Application of. PLL in modulation and Decoding
(1) concepts of modulation and Decoding
In order to realize the long-distance transmission of information, the sender usually uses the modulation method to modulated the signal. After receiving the signal, the receiver must undergo Demodulation to restore the original signal.
The so-called modulation is to use the input signal UI carrying information
To control the carrier signal UC
So that a parameter of the carrier signal changes with the input signal. Carrier signal parameters include amplitude, frequency, and phase. Therefore, modulation involves three types: Am, FM, and PM.
Tune
The amplitude is characterized by the same frequency as the carrier signal, and changes with the change of the input signal amplitude. The frequency modulation wave is characterized by the same amplitude as the carrier signal, the frequency varies with the input signal amplitude.
The phase wave is characterized by the same amplitude as the carrier signal, and the phase changes with the change of the input signal amplitude. The 8-4-4 of the amplitude adjustment and frequency modulation are shown.
(A) is the input signal, also known as the modulation signal; (B) is the carrier signal, and (c) is the modulation wave and frequency modulation wave signal.
Demodulation is the inverse process of modulation.
Restore to original signal UI
.
2
Application of. PLL in frequency modulation and demodulation Circuits
The frequency of the Frequency Modulation changes with the amplitude of the modulation signal. According to the formula 8-4-6, the oscillation frequency of the voltage controlled oscillator depends on the amplitude of the input voltage. When the frequency of the carrier signal and the inherent oscillation frequency of the Phase-Locked Loop ω 0
When they are equal, the frequency of the output signal of the voltage controlled oscillator remains ω 0.
Unchanged. If the input signal of the voltage controlled oscillator contains the signal UC output by the low-pass filter of the Phase-Locked Loop
In addition, there is a modulation signal UI
, The frequency of the output signal of the voltage controlled oscillator is ω 0.
Frequency Modulation Signals that change with the amplitude of modulation signals. From this, the frequency modulation circuit can be composed of a phase-locked loop. The block diagram of the frequency modulation circuit consists of 8-4-5.
According to the working principle of the phase-locked loop and the characteristics of the frequency modulation, the structure of the demodulation circuit is 8-4-6.
3
Application of. PLL in frequency synthesis circuit
In modern electronic technology, in order to obtain high-precision oscillation frequency, a Z crystal oscillator is usually used. However, the frequency of the Z crystal oscillator is not easy to change. The multi-frequency and high-stability oscillation signal output can be obtained by using the phase-locked loop, frequency doubling, frequency division, and other frequency synthesis technologies.
The phase-locked frequency divider is called when the output signal frequency is higher than the crystal oscillator signal frequency. The phase-locked frequency divider is called when the output signal frequency is lower than the crystal oscillator signal frequency. The block diagram of the Phase-lock multiplier and Phase-lock divider circuits is 8-4-7.
When N is greater than 1, it is a frequency-division circuit; When 0 is less than 1, it is a frequency-doubling circuit.
