How phase-locked loops work

PLL is a feedback circuit, the full name of the PLL is phase-locked Loop, referred to as the PLL. The function is to synchronize the clock on the circuit with the phase of an external clock. Because the PLL can realize the output signal frequency to the input signal frequency automatic tracking, so the phase-locked loop is usually used for the closed-loop tracking circuit. Phase-locked loop during operation, when the output signal frequency and the frequency of the input signal is equal, the output voltage and the input voltage remains fixed phase difference value, that is, the output voltage and the input voltage
Phase is locked, which is the origin of the name of the phase lock loop.

In the data acquisition system, PLL is a very useful synchronization technology, because through the phase-locked loop, the different data acquisition board can share the same sampling clock. As a result, the phase of the respective local 80MHz and 20MHz time bases on all boards is synchronous, so the sampling clock is also synchronous. Since the sampling clocks of each board are synchronized, data acquisition can be carried out strictly at the same time. PLL is a phase feedback automatic control system. It consists of the following three basic components: Phase detector (PD), Loop Filter (LPF), and voltage controlled oscillator (VCO).
How phase-locked loops work:
1. The output of the voltage-controlled oscillator is collected and divided;
2. And the reference signal simultaneously input phase detector;
3. Phase detector by comparing the frequency difference between the two signals, and then output a DC pulse voltage;
4. Control the VCO so that its frequency changes;
5. In such a short time, the output of the VCO will stabilize to a certain expectation.

The PLL can be used to achieve phase synchronization between the output and input two signals. When there is no reference (reference) input signal, the output of the loop filter is 0 (or a fixed value). At this point, the voltage-controlled oscillator is free-oscillating at its natural frequency FV. When a reference signal with a frequency of FR is input, UR and UV are simultaneously added to the phase detector for phase discrimination. If the difference between FR and FV, phase detector to ur and UV phase of the results, output a with ur and UV phase difference proportional to the error voltage UD, and then through the loop filter to remove the high-frequency components of UD, output a control voltage UC,UC will make the voltage-controlled oscillator frequency FV (and phase) change, The frequency of the reference input signal is closer, and finally the fv= FR, the loop lock. Once the loop enters the lock state, there is only one fixed steady phase difference between the output signal of the voltage-controlled oscillator and the input signal of the loop (reference signal), and no frequency difference exists. Then we call the loop locked.

The lock state of the loop is the frequency and phase of the input signal is constant, if the loop input is the frequency and phase of the constantly changing signal, and the loop can make the voltage control oscillator frequency and phase continuously tracking the frequency and phase changes of the input signal, then the state of the loop is called the tracking state.

After locking the PLL, not only can the output signal frequency and the input signal frequency strictly synchronous, but also has the frequency tracking characteristics, so it in the electronic technology in all fields have a wide range of applications

． The basic components of phase-locked loop many electronic equipment to work normally, the external input signal is usually required to synchronize with the internal oscillation signal, the use of phase-locked loop can achieve this purpose. A phase-locked loop is a feedback control circuit called a phase-locked loop (PLL). Phase-locked loops are characterized by the use of external input reference signals to control the frequency and phase of the internal oscillation signal in the loop. Because the PLL can realize the output signal frequency to the input signal frequency automatic tracking, so the phase-locked loop is usually used for the closed-loop tracking circuit. PLL in the process of operation, when the output signal frequency and the frequency of the input signal is equal, the output voltage and the input voltage to maintain a fixed phase difference value, that is, the output voltage and the input voltage phase is locked, this is the origin of the lock loop name. Phase-locked loops are usually composed of phase detector (PD), Loop Filter (LF) and voltage controlled oscillator (VCO), and the schematic diagram of the PLL is shown in Fig. 8-4-1. Phase detector in Phase-locked loop is also referred to as phasing comparator, its function is to detect the phase difference between the input signal and the output signal, and convert the detected phase difference signal to the U D (t) voltage signal output, the signal is filtered by the low-pass filter to form a voltage controlled oscillator control voltage U C (t), the frequency of the oscillator output 2. The phase-locked loop works The phase discriminator in the PLL is usually composed of analog multipliers, and a phase discriminator circuit consisting of analog multipliers is shown in Fig. 8-4-2. The phase detector works by: Set the signal voltage of the external input and the output signal voltage of the voltage control oscillator are:        (8-4-1)       The ω0 in   (8-4-2) is the oscillating angular frequency of a voltage-controlled oscillator when the input control voltage is zero or DC voltage, which is called the intrinsic oscillation angle frequency of the circuit. The analog multiplier output voltage U D is: With the low-pass filter LF the upper and the frequency component is filtered out, the remaining differential component as the voltage-controlled oscillator input control voltage U C (t). That is, u C (t) is: The Ωi in the        (8-4-3) is the instantaneous oscillation angular frequency of the input signal, θi (t) and Θo (t) are the instantaneous phase of the input signal and the output signal, respectively. The relationship between instantaneous frequency and instantaneous phase can be got by the relation of phasor:         ie                              (8-4-4), the instantaneous phase difference θd for            (8-4-5) on both sides of differential, can get the difference between the formula          (8-4-6) is equal to zero, indicating the phase locked loop into the state of lock, At this point the frequency and phase of the output and input signals remain constant, and u C (t) is a constant value. When the upper type is not equal to zero, the phase of the PLL is not locked, and the frequency of the input signal and the output signal are different, and U C (t) changes with time. As shown in Fig. 8-4-3, the voltage-controlled oscillator is characterized by the oscillation frequency of the voltage-controlled oscillator ωu is centered on the ω0 and varies with the input signal voltage u C (t). The expression for this attribute is        (8-4-6) that when u C (t) changes with time, the oscillation frequency of the voltage-controlled oscillator ωu also changes with time, the phase-locked loop enters "frequency traction", Automatic tracking captures the frequency of the input signal, so that the PLL enters the locked state and keeps the ω0 =ωi state intact. 8. 4. 2 application of phase-locked loop 1. Application of phase-locked loop in modulation and demodulation (1) The concept of modulation and demodulation in order to realize the long-distance transmission of information, the signal is usually modulated by the modulation method at the sender end, and the signal must be demodulated to recover the original signal. The so-called modulation is to carry the information of the input signal U i to control the parameters of the carrier signal U C, so that a certain parameter of the carrier signal with the input signal changes. The parameters of the carrier signal have amplitude, frequency and phase, so the modulation has amplitude modulated (AM), FM (FM) and phase modulation (PM) three kinds. Amplitude modulation wave is characterized by the frequency and the frequency of the carrier signal is equal, the amplitude varies with the amplitude of the input signal, the amplitude is the same as the amplitude of the carrier signal, the frequency varies with the amplitude of the input signal, and the amplitude of the phase modulation wave is the same as the amplitude of the carrier signal, which changes with the amplitude of the input The schematic diagram of the amplitude modulated wave and the FM wave is shown in Fig. 8-4-4.

(a) above is the input signal, also known as the modulation signal, the figure (b) is the carrier signal, the figure (c) is the amplitude modulation wave and the FM wave signal. Demodulation is the inverse process of modulation, which can restore the modulated wave u O to the original signal U i. 2. Application of phase-locked loop in FM and demodulation circuit the characteristic of FM wave is the change of frequency tiaogan signal amplitude. By 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 is equal to the intrinsic oscillation frequency of the phase-locked loop, the frequency of the output signal of the ω0 oscillator will remain ω0 unchanged. If the input signal of the voltage-controlled oscillator is in addition to the signal U C of the PLL low-pass filter output and the modulated signal u I, then the frequency of the output signal of the voltage-controlled oscillator is the FM-wave signal which is centered on the ω0 and changes with the modulation signal amplitude. Thus the FM circuit can be composed of a phase-locked loop, the frequency modulation circuit composed of phase-locked loop block diagram as shown in Figure 8-4-5.

According to the working principle of phase-locked loop and the characteristic of FM wave, the circuit block diagram of Demodulation is shown in Figure 8-4-6. 3. Application of phase-locked loop in frequency synthesis circuit in modern electronic technology, quartz crystal oscillator is usually used in order to obtain high precision oscillation frequency. However, the frequency of quartz crystal oscillator is not easy to change, using PLL, octave, frequency synthesis technology, can obtain multi-frequency, high stability of the oscillation signal output. The output signal frequency is higher than the crystal oscillator signal frequency is called the phase-locked multiplier circuit, the output signal frequency is smaller than the crystal signal frequency is called the phase-locked divider circuit. The block diagram of the PLL frequency and phase-locked circuit is shown in Fig. 8-4-7.

n is greater than 1 o'clock in the figure and is a crossover circuit; when 0

How phase-locked loops work August 16, 2007 Thursday 11:20

The most basic structure of the phase-locked loop is shown in Figure 6.1. It consists of three basic components: Phase detector (PD), Loop Filter (LPF), and voltage controlled oscillator (VCO). The phase detector is an aspect comparison device. It compares the phase of the output signal so (t) of the input signal si (t) and the voltage-controlled oscillator to produce an error voltage of SE (t) corresponding to the phase difference of two signals.
The function of the loop filter is to filter out the high frequency components and noises in the error voltage SE (t) to ensure the performance required by the loop and to increase the stability of the system.
The voltage controlled oscillator is controlled by the control voltage SD (T), so that the frequency of the voltage-controlled oscillator moves closer to the frequency of the input signal until the frequency difference is eliminated and locked. Phase-locked loop is an error control system. It compares the phase difference between the input signal and the output signal of the voltage-controlled oscillator, thus producing an error control voltage to adjust the frequency of the voltage-controlled oscillator to achieve the same frequency as the input signal. When the loop starts to work, if the input signal frequency and the voltage control oscillator frequency is different, because of the inherent frequency difference between the two signals, the phase difference between them will always be changing, the error voltage of the phase detector output varies within a certain range. Under the control of this error voltage, the frequency of the voltage-controlled oscillator is also changing. If the frequency of the voltage-controlled oscillator can change to the same frequency as the input signal, it stabilizes at this frequency under the condition of satisfying the stability. When the stability is reached, the frequency difference between the input signal and the voltage-controlled oscillator output signal is zero, the difference no longer changes with the time, the error voltage is a fixed value, then the loop enters the "lock" state. This is the general process of phase-locked loop operation.
The above analysis is for the frequency and phase invariant input signal. If the frequency and phase of the input signal are constantly changing, it is possible to make the frequency and phase of the voltage control continuously track the change of the input frequency through the action of the Loop.
The phase-locked loop has good tracking performance. If the input FM signal, let the Loop pass band is sufficiently wide, so that the signal modulation spectrum falls within the bandwidth, then the voltage control oscillator frequency tracking input modulation changes.
      for a detailed analysis of phase-locked loops, refer to the book on lock-in technology. Only the phase-locked loop frequency discrimination principle is described here. It is easy to think that the tracking error between the voltage-controlled oscillator frequency and the input signal frequency can be neglected. Thus any instantaneous, frequency ωv (t) of the voltage-controlled oscillator is equal to the instantaneous frequency ΩFM (t) of the FM wave. The instantaneous angular frequency of an FM wave can be expressed as a    assuming that the VCO has a linear control characteristic, its slope kv (voltage-controlled sensitivity) is (radians/sec, volts), while the VCO at SD (t) =0 oscillation frequency is ωo ', when the control voltage, the VCO's instantaneous angular frequency of   The   is equal to the upper two, that is, Ωv (t) ≈ωfm (t), can get    ωo is the frequency of the FM wave, Ωo ' is the intrinsic oscillation frequency of the voltage-controlled oscillator, both are constants. Therefore, the first item of the above is the DC term, can be eliminated by the isolation element, or the voltage-controlled oscillator frequency is adjusted to Ωo=ωo ' at the beginning. Therefore, the above formula can be further written