RF Oscillators Engr. Edelito A. Handig
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
RF Oscillators Engr. Edelito A. Handig
Introduction
• An oscillator is a circuit capable of continuously generating a repetitive waveform of a desired frequency.
• The waveform can be sinusoidal or rectangular.
• It can be self-sustaining or free running.
• It can be triggered or one-shot.
Oscillator: Block diagram
Barkhausen Criterion
LC Tank Circuit
Self-sustaining Oscillator
• Four requirements for a self sustaining oscillator:
1. DC power source
2. Frequency determining components
3. Amplification
4. Positive feedback
Sample problem:
• Assume that the feedback fraction is 0.01. Calculate the required value of amplification in order to sustain oscillations.
• B = 0.01;
• AvB = 1 (Barkhausen Criterion)
• Av = 100
LC Oscillators: Hartley
LC Oscillators: Hartley • Assume that it is desired to design an
oscillator with 4 MHz output frequency. If the C3 is adjusted to 300 pF and the desired feedback fraction is 1/20, calculate the ff:
a. L1 and L2
b. Av to sustain oscillation.
Answer: L1 = 5.029 µH; L2 = 0.251 µH;
Av = 20
LC Oscillators: Hartley
In a Hartley oscillator, C3=100 pF, L1=1mH, and L2=10 µH. Calculate the ff:
a. Frequency of oscillation
b. Feedback fraction and Av to sustain oscillations.
Answer: Freq = 500.79 KHz; B = 0.01
Av = 100
LC Oscillators: Colpitts
LC Oscillators: Colpitts
• Determine the feedback fraction, Av, and operating frequency for the Colpitts oscillator with L1=1µH, C1=10pF, and C2=100pF.
• Answer: B=0.1 ; Av=10; Freq=52.8MHz
LC Oscillators: Colpitts • Assumed that it is desired to design a Colpitts
oscillator so that its output frequency is 4 MHz. If L1 = 50 µH and the feedback fraction is 1/20. Calculate the ff:
a. C1 and C2
b. Av to sustain oscillations.
Answer: C1 = 33.24 pF; C2 = 666.06 pF
Av = 20
LC Oscillators: Clapp
LC Oscillators: Clapp
• In a Clapp oscillator, L1 = 1 µH, C1=100pF, C2=1000 pF, and C3=10pF. Calculate the ff:
a. Frequency of Oscillation.
b. Feedback fraction and Av to sustain oscillations.
Answer: F = 53.02 MHz; B = 0.1; Av = 10
Seatwork
• In a Clapp oscillator, L1 = 5 µH, C1=75 pF, C2=500 pF, and C3=30pF. Calculate the ff:
a. Frequency of Oscillation.
b. Feedback fraction and Av to sustain oscillations.
c. Suppose the value of C2 doubles, by what percentage does this change the operating frequency?
RC Oscillator: Phase Shift Oscillator
RC Oscillator: Phase Shift Oscillator
RC Oscillator: Wien Bridge Oscillator
Crystal Oscillator
• A crystal oscillator is an electronic oscillator circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency.
Crystal Oscillator
• A radio transmitter has to operate at temperature from – 5ºC to 35ºC. If its signal is derived from a crystal oscillator with a temperature coefficient of 1ppm/ºC and it transmits at exactly 146MHz at 20ºC, find the transmitting frequency at the two extremes of the operating temperature range.
• fmax = 146.00219 MHz
• fmin = 145.99635 MHz
Crystal Oscillator
• A crystal oscillator applied to Hartley Oscillator.
Crystal Oscillator
• A crystal oscillator applied to Colpitts Oscillator.
Crystal Oscillator
• Variation of crystal oscillator applied to Colpitts Oscillator.
Crystal Oscillator: Pierce Oscillator
• If B = 1/25 and the equivalent capacitance of C1 and C2 is 250 pF, find the values of C1 and C2.
Voltage controlled Oscillator: VCO
• It is a free running oscillator whose frequency of operation is controlled by an external dc bias voltage. The natural frequency is equal to the output frequency with a dc input of zero volt.
• K = Δf / ΔV Hz/Volt
(Conversion gain or transfer function)
Related Documents
