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Bài giảng Electronics circuits for communications - Chapter 3: RF oscillator and frequency synthesizer - Đinh Quốc Hùng

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CHAPTER 3:  
RF OSCILLATOR AND  
FREQUENCY SYNTHESIZER  
RF Oscillator  
Oscillator Fundamentals  
• 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 ().  
RF Oscillator  
Oscillator Fundamentals  
RF Oscillator  
Sửa lại Z1 thành Z2 và  
Z2 thành Z1 cho thống  
nhất trong sách và phía  
dưới  
Three-Point Oscillators  
RF Oscillator  
Three-Point Oscillators  
RF Oscillator  
Three-Point Oscillators  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Colpitts Oscillator  
• Example of Colpitts circuit (with bias),  
oscillating frequency.  
1
f =  
C1C2  
2L  
C1 + C2  
RF Oscillator  
Colpitts Oscillator  
RF Oscillator  
Hartley Oscillator  
• The Hartley oscillator is almost identical to the Colpitts oscillator.  
• The primary difference is that the feedback network of the  
Hartley oscillator uses tapped inductors (L1 and L2) and a single  
capacitor C.  
RF Oscillator  
Hartley Oscillator  
• The analysis of Hartley oscillator is identical  
to that Colpitts oscillator.  
• The frequency of oscillation  
1
o =  
(
L + L2 C  
)
1
RF Oscillator  
Hartley Oscillator  
• Example:  
RF Oscillator  
Hartley Oscillator  
RF Oscillator  
Hartley Oscillator  
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