文档介绍：外文翻译
■ THE PHASE-LOCKED LOOP (PLL)
The PLL as an FM Demodulator
As you have seen, the VCO control voltage in a PLL depends on the deviation of the ing frequency. The PLL will produce a voltage proportional to the frequency of the ing signal which, in the case of FM, is the original modulating signal.
Figure 13-54 shows a typical connection for the LM565 as an FM demodulator. If the IF input is frequency modulated by a sinusoidal signal, you get a sinusoidal signal on the output as indicated. Since the maximum operating frequency is 500 kHz, this device must be used in double-conversion FM receiver is one in which essentially two mixers are used to first convert the RF to a MHz IF and then convert this to a 455 kHz IF.
The free-running frequency of the VCO is adjusted to approximately 455 kHz, which is the center of the modulated IF range, C₁ can be any value, but R₁ should be in the range from 2kΩ to 20kΩ,The input van be directly coupled as long as there is no dc voltage difference between pins 2 and VCO is connected to the phase detector by an external wire between pins 4 and 5.
Determine the values for R₁,C₁ and C₂ for the LM565 in Figure 13-54 for a free-running frequency of 455 kHz and a capture range of ± dc supply voltages are ± 6V.
Solution Use Equation (13-5) to calculate C₁.Choose R₁=.
■ COMMUNICATIONS CIRCUITS
The lock range and capture range must be determined before C₂ can be calculated. The lock range is
Use Equation (13-7) to calculate C₂.
Therefore,
Practice Exercise What can you do to increase the capture range from ±10 kHz to ± 15 kHz ?
13-8 REVIEW QUESTIONS
1. List the three ponents in a phase-locked loop.
2. What is another circuit used in some PLLs other than the three listed in Question 1?
3. What is the basic function of a PLL?
4. What is the difference between the lock range and the capture range lf a PLL?
5. Basically, how does a PLL track the ing frequency?
■ A SYSTEM APPLICATION
The DCE