›
›

Bài giảng Mạch điện tử thông tin - Chapter 5: RF Oscillator and Frequency Synthesizer - Đỗ Hồng Tuấn

Download
Chapter 5:  
RF Oscillator and  
Frequency Synthesizer  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
Oscillator Fundamentals (1)  
An oscillator is a circuit that converts energy from a power source  
(usually a DC power source) to AC energy (periodic output signal). In  
order to produce a self-sustaining oscillation, there necessarily must be  
feedback from the output to the input, sufficient gain (amplifier) to  
overcome losses in the feedback path, and a resonator (filter).  
The block diagram of an oscillator with positive feedback is shown  
below. It contains an amplifier with frequency-dependent forward gain  
a() and a frequency-dependent feedback network ().  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
2
Oscillator Fundamentals (2)  
The output voltage is given by:  
It gives the closed loop gain as:  
For an oscillator, the output Vo is nonzero even if the input signal Vi is zero.  
This can only possible if the closed loop gain A is infinity. It means:  
This is called the Barkhausen criterion for oscillation and is often described  
in terms of its magnitude and phase separately. Hence oscillation can occur  
when  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
3
Oscillator Fundamentals (3)  
Osillator type: (DC and bias circuit not shown)  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
4
Oscillator Fundamentals (4)  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
5
Oscillator Fundamentals (5)  
Example: This example illustrates the design method. The transistor is in  
CB configuration, then there is no phase inversion for the amplifier.  
The circuit analysis can be simplified with the assumption:  
1
REhib  
|| RE || h   
ib  
C2  
RE h  
ib  
It is also assumed that the quality factor of load impedance is high. Then,  
the circuit reduces to:  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
6
Oscillator Fundamentals (6)  
where  
and  
The forward gain:  
and the feedback transfer function:  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
7
Oscillator Fundamentals (7)  
where  
According to Barkhausen criterion for phase:  
and in this example does not depend on frequency, then the phase shift  
of or ZL must be 360o (or 0o). This only occurs at the resonant frequency  
of the circuit:  
At this frequency:  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
8
Oscillator Fundamentals (8)  
and  
The Barkhausen criterion for magnitude is:  
Three-reactance oscillators: Instead of using block diagram formulation  
using Barkhausen criterion, a direct analysis based on circuit equations is  
frequently used (particular for single ended amplifier), as shown in next  
slide.  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
9
Oscillator Fundamentals (9)  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
10  
Oscillator Fundamentals (10)  
Omitting hoe, the loop equations are then:  
For the amplifier to oscillate, the current Ib and I1 must be nonzero even  
Vin = 0. This is only possible if the system determinant:  
is equal to 0. That is:  
which reduces to:  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
11  
Oscillator Fundamentals (11)  
Assumed that Z1, Z2, Z3 are purely reactive impedance. Since both real and  
imaginary parts must be zero, then  
and  
Since hfe is real and possitive, Z2 and Z3 must be of opposite sign. That is:  
Since hie is nonzero, then  
or  
Thus, since hfe is positive, then Z1 and Z2 will be reactances of same kind.  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
12  
Oscillator Fundamentals (12)  
If Z1 and Z2 are capacitors, Z3 is an inductor, then it is referred as Colpitts  
oscillator. If Z1 and Z2 are inductors, Z3 is a capacitor, then it is referred as  
Hartley oscillator.  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
13  
Oscillator Fundamentals (13)  
Example of Colpitts circuit  
Example of Hartley circuit  
(with bias), oscillating frequency:  
(with bias), oscillating frequency:  
1
1
f  
f   
C1C2  
2L  
2LC  
C1 C2  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
14  
Oscillator Fundamentals (14)  
An oscillator known as the Clapp circuit (or Clapp-Gourier circuit):  
This circuit has practical advantage of being able to provide another  
degree of design freedom when making Co much smaller than C1 and C2.  
The Co can be adjusted for the desired oscillating frequency o, which is  
determined from:  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
15  
Crystal Oscillators (1)  
One of the most important features of an oscillator is its frequency  
stability, or in other words its ability to provide a constant frequency  
output under varying conditions. Some of the factors that affect the  
frequency stability of an oscillator include: temperature, variations in the  
load and changes in the power supply.  
Frequency stability of the output signal can be improved by the proper  
selection of the components used for the resonant feedback circuit  
including the amplifier but there is a limit to the stability that can be  
obtained from normal LC and RC tank circuits. For very high stability a  
quartz crystal is generally used as the frequency determining device to  
produce another types of oscillator circuit known generally as crystal  
oscillators.  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
1
Crystal Oscillators (2)  
When a voltage source is applied to a small thin piece of crystal quartz, it  
begins to change shape producing a characteristic known as the piezo-  
electric effect. This piezo-electric effect is the property of a crystal by  
which an electrical charge produces a mechanical force by changing the  
shape of the crystal and vice versa, a mechanical force applied to the  
crystal produces an electrical charge. Then, piezo-electric devices can be  
classed as transducer as they convert energy of one kind into energy of  
another. This piezo-electric effect produces mechanical vibrations or  
oscillations which are used to replace the LC circuit.  
The quartz crystal used in crystal oscillators is a very small, thin piece or  
wafer of cut quartz with the two parallel surfaces metallized to make the  
electrical connections. The physical size and thickness of a piece of quartz  
crystal is tightly controlled since it affects the final frequency of  
oscillations and is called the crystals "characteristic frequency".  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
2
Crystal Oscillators (3)  
A mechanically vibrating crystal can be represented by an equivalent  
electrical circuit consisting of low resistance, large inductance and small  
capacitance as shown below:  
fp  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
3
Crystal Oscillators (4)  
The impedance of the equivalent circuit is  
or  
where s is series resonant frequency and p is parallel resonant frequency  
(or anti-resonant frequency). The series and parallel resonant frequencies are  
very stable and not affected by temperature variations.  
At series resonant frequency, the crystal has a low impedance (ideally, zero  
impedance). At parallel resonant frequency, the crystal has a high impedance.  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
4
Crystal Oscillators (5)  
A quartz crystal has a resonant frequency similar to that of a electrically  
tuned tank circuit (LC circuit) but with a much higher Q factor due to its  
low resistance, with typical frequencies ranging from 4kHz to 10MHz.  
In a crystal oscillator circuit the oscillator will oscillate at the crystals  
fundamental series resonant frequency when a voltage source is applied to  
it. However, it is also possible to tune a crystal oscillator to any even  
harmonic of the fundamental frequency, (2nd, 4th, 8th etc.) and these are  
known generally as harmonic oscillators (while overtone oscillators  
vibrate at odd multiples of the fundamental frequency, 3rd, 5th, 11th etc.).  
Colpitts crystal oscillator: The design of a crystal oscillator is very  
similar to the design of the Colpitts oscillator, except that the LC circuit  
has been replaced by a quartz crystal as the example shown below:  
Dept. of Telecomm. Eng.  
Faculty of EEE  
CSD2020  
DHT, HCMUT  
5
Tải về để xem bản đầy đủ
pdf 37 trang myanh 17440 Free
Download
Bạn đang xem 20 trang mẫu của tài liệu "Bài giảng Mạch điện tử thông tin - Chapter 5: RF Oscillator and Frequency Synthesizer - Đỗ Hồng Tuấn", để tải tài liệu gốc về máy hãy click vào nút Download ở trên

File đính kèm:

  • pdfbai_giang_mach_dien_tu_thong_tin_chapter_5_rf_oscillator_and.pdf