Chapter 8 - Chapter 8 Phase Locked Loops 1 Low noise...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Chapter 8 Phase Locked Loops
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Duplexer Phase Locked Loop Oscillator Low noise amplifier Power amplifier Up- converter Down- converter Rx IF Tx IF Ctrl Bus
Background image of page 2
3 What’s Phase Locked Loop A phase locked loop (PLL) is a feed back loop, which can lock the frequency of a voltage-controlled oscillator (VCO) to the stability of a reference crystal oscillator. The basic PLL configuration will be considered is shown in Figure 1. If the frequency of the crystal oscillator is F r , the VCO’s frequency will be R N F F r p = . FIGURE 1. Basic Charge Pump Phase Locked Loop
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4 C B E 1 2 3 GBJT3 ID= GP1 CAP ID= C1 CAP ID= C2 IND ID= L1 CAP C= ID= 100000 pF C3 C B E 1 2 3 GBJT3 ID= GP1 CAP ID= C1 CAP ID= C2 CAP C= ID= 100000 pF C3 CRYSTAL Crystal Oscillator In a typical Colpitts oscillator design, is shown in Figure 2. Its oscillating frequency is L C C F ) ( 2 1 2 1 0 + = π (1) If the inductor is replaced by a crystal resonator, the oscillator is called crystal oscillator (See Figure 3). A crystal acts as an inductor in Figure3. However, the inductor is not an inductor with inductance value fixed, but an inductor with inductance value varies swiftly with the frequency around its resonating frequency, which is shown in Figure 4. Figure 3. The inductor in the Colpitts oscillator replaced by a crystal resonator.
Background image of page 4
5 It can be seen from Figure 4 that the inductance of the crystal can change from zero to infinite around the resonating frequency F 0 . This means that when the oscillator’s oscillating frequency is shifted from the resonating frequency F 0 to higher or lower frequency F 0 ±∆ F due to some external varieties such as temperature, bias voltage, and etc. the fast increase of decrease of inductance value will pull it back to F 0 , means the oscillating frequency of the crystal oscillator is trapped around F 0 . That’s why a crystal oscillator has very good stability of frequency. Figure 4. The inductance versus frequency characteristic of a crystal resonator Inductance Frequency L 0 F 0
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
6 Why PLL There are three main reasons for using a PLL as a frequency synthesizer: 1. To translate the high frequency stability of a low frequency crystal oscillator to a high frequency tunable oscillator. There is no high frequency crystal resonator, so that we can’t construct crystal oscillator at high frequency. PLL is used to attain the stability of a crystal oscillator at high frequency. 2. To translate the high quality noise characteristics of a low frequency crystal oscillator to a high frequency tunable oscillator. 3. To enable the high frequency tunable oscillator in a programmable way so that its operating frequency can jump to difference channels according to the program. The frequency crystal oscillator is fixed according to the resonating frequency of the crystal.
Background image of page 6
Image of page 7
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 05/01/2011 for the course ELECTRICAL EE5602 taught by Professor Xuequan during the Spring '11 term at City University of Hong Kong.

Page1 / 29

Chapter 8 - Chapter 8 Phase Locked Loops 1 Low noise...

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online