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)