Name of The Expt: Construct a Phase-Shift Oscillator and Determine its Frequency of
Oscillation.
Objectives:
After completing this experiment we will able to know1. How does an A.C. signal is produced by phase shift oscillator.
2. How a RC network does is used to phase shift.
3. Where can we use this circuit.
4. The designing procedure of a phase shift oscillator.
Theory:
Phase Shift Oscillator:
An oscillator is a circuit, which generates ac output signal without giving any input ac signal.
This circuit is usually applied for audio frequencies only. The basic requirement for an oscillator
is positive feedback. The operation of the RC Phase Shift Oscillator can be explained as follows.
The starting voltage is provided by noise, which is produced due to random motion of electrons
in resistors used in the circuit. The noise voltage contains almost all the sinusoidal frequencies.
This low amplitude noise voltage gets amplified and appears at the output terminals. The
amplified noise drives the feedback network which is the phase shift network. Because of this
the feedback voltage is maximum at a particular frequency, which in turn represents the
frequency of oscillation. Furthermore, the phase shift required for positive feedback is correct at
this frequency only.
Phase shift oscillators are most commonly used in tremolo circuits in guitar amplifiers. They are
used as the low-frequency oscillator (LFO) that generates the sinusoidal waveform which
amplitude modulates the guitar signal to produce the characteristic tremolo amplitude variations.
Rc Phase-Shift Network:
Figure 1: Phase angle at various stage of RC network
The circuit on the left shows a single resistor-capacitor network and whose output voltage
"leads" the input voltage by some angle less than 90o. An ideal RC circuit would produce a
phase shift of exactly 90o. The amount of actual phase shift in the circuit depends upon the
values of the resistor and the capacitor, and the chosen frequency of oscillations with the phase
angle ( Φ ) being given as:
Phase Angle
In our simple example above, the values of R and C have been chosen so that at the required
frequency the output voltage leads the input voltage by an angle of about 60o. Then the phase
angle between each successive RC section increases by another 60o giving a phase difference
between the input and output of 180o (3 x 60o) as shown by the following vector diagram.
Then by connecting together three such RC networks in series we can produce a total phase shift
in the circuit of 180o at the chosen frequency and this forms the bases of a "phase shift
oscillator" otherwise known as a RC Oscillator circuit.
We know that in an amplifier circuit either using a Bipolar Transistor or an Operational
Amplifier, it will produce a phase-shift of 180o between its input and output. If a RC phase-shift
network is connected between this input and output of the amplifier, the total phase shift
necessary for regenerative feedback will become 360o, ie. the feedback is "in-phase". Then to
achieve the required phase shift in an RC oscillator circuit is to use multiple RC phase-shifting
networks such as the circuit below.
Basic Rc Oscillator Circuit:
Figure 2: Theoretical Circuit For Phase shift oscillator.
The RC Oscillator which is also called a Phase Shift Oscillator, produces a sine wave output
signal using regenerative feedback from the resistor-capacitor combination. This regenerative
feedback from the RC network is due to the ability of the capacitor to store an electric charge,
(similar to the LC tank circuit). This resistor-capacitor feedback network can be connected as
shown above to produce a leading phase shift (phase advance network) or interchanged to
produce a lagging phase shift (phase retard network) the outcome is still the same as the sine
wave oscillations only occur at the frequency at which the overall phase-shift is 360o. By
varying one or more of the resistors or capacitors in the phase-shift network, the frequency can
be varied and generally this is done using a 3-ganged variable capacitor.
If all the resistors, R and the capacitors, C in the phase shift network are equal in value, then the
frequency of oscillations produced by the RC oscillator is given as:
•
•
•
•
•
Where:
ƒ is the Output Frequency in Hertz
R is the Resistance in Ohms
C is the Capacitance in Farads
N is the number of RC stages. (in our example N = 3)
Since the resistor-capacitor combination in the RC Oscillator circuit also acts as an attenuator
producing an attenuation of -1/29th (Vo/Vi = β) per stage, the gain of the amplifier must be
sufficient to overcome the losses and in our three mesh network above the amplifier gain must
be greater than 29. The loading effect of the amplifier on the feedback network has an effect on
the frequency of oscillations and can cause the oscillator frequency to be up to 25% higher than
calculated. Then the feedback network should be driven from a high impedance output source
and fed into a low impedance load such as a common emitter transistor amplifier but better still
is to use an Operational Amplifier as it satisfies these conditions perfectly.
Apparatus:
1.
2.
3.
4.
5.
6.
7.
8.
Transistor(C828)
Oscilloscope.
Power breadboard
Some Resistor(2.2,3.3,4.7,5.6,1.8,10,1,)KΩ
Capacitor (4.7n,23µ,10n)F
Trainer with bread board.
Connecting Wires.
Voltmeter
Practical Circuit:
Figure 3: Practical Circuit For Phase shift oscillator(where+V=14V,Re=110Ω,Ce=10nF)
Procedure:
a. Firstly we Cheek the transistor, power button of the trainer calibrate the oscilloscope.
b. Then we arrange the all the components according to the practical circuit.
c. Biasing voltage is fixed at 14V and we vary the variable resistor up to suitable signal
appeared on the screen of the oscilloscope.
d. Then we take readings at various stages by fixing capacitance and changing resistance
and vice versa.
e. These readings are taken at the following table-01 and table-02.
Calculations:
Here R1=R2=R3=2.2KΩ & time period Tmea=.34µs
So Measured Frequency Fmea=1/Tmea
= 2.94 KHz
Fcal =1/ (2πRC ) KHz
=1/ (2π×2.2×1000×.01×10-6
= 2.93 KHz
Error(%) = (Fcal- Fmea)/ Fcal
)
=.34%
Table-01: For measuring the frequency of a oscillator sin phase shift arrangement when
resistor is fixed.
(R1=R2=R3=2.2KΩ
No Capacitor Time period
of
C
Tmea
Obs
(F)
µs
Frequency
Fcal
=1/(2πRC )kHz
Frequency
Fmea=1/T
kHz
Error (%)=
{(Fcal-Fmea)/Fcal}
01
.01µ
.34
2.93
2.94
.34
02
03
4.7n
35n
.14
1
6.23
.843
7.14
1.0
14
18.48
Table-02: For measuring the frequency of a oscillator in phase shift arrangement when
capacitor is fixed
C=10nF.
Time
Period
Tmea
µs
.34
Frequency
Fcal
=1/(2πRC )
kHz
2.93
Frequency
Fmea=1/T
kHz
01
Resistor
(R1=R2=R3
=R)
kΩ
2.2
2.94
.34
02
03
5.6
6.8
.81
.57
1.16
1.81
1.25
1.75
7.75
3.64
No. Of
Obs.
Error (%)=
{(Fcal- Fmea)/Fcal}
Result & Discussion:
From the above data table-01 we can observe that when resistance is fixed then there is
maximum of 18.48% error is occurred. Again when capacitance is fixed then we can see that
from data table-02 the maximum error is 7.75%.
Its essential behavior is that it does not require transformers or inductors. It can be used to
produce very low frequencies. This circuit provides good frequency stability.
But in the arrangement is that the starting voltage is provided by noise, which is produced due to
random motion of electrons in resistors used in the circuit. The noise voltage contains almost all
the sinusoidal frequencies these may be not perfectly sinusoidal.
These variations is also occurred due to following reasons
• Aging problem of the instruments
• Eye estimation.
• Di-electric loss in capacitor.
• Heating Problem.
If these variations will be removed we will get error less phase shift oscillator.
Precautions:
a. We have to cheek all the connection as to practical circuit otherwise components may be
destroyed.
b. Supply voltage is fixed at not more than 15V.Connections is made rigidly.
c. Connections are made rigidly.
d. Calibrate the oscillator more accurately.
e. Readings are taken attentively.
Prepared by
Sharat Chandra+Bhajan Saha
Sess: 2007-2008
Dept. of AECE.
Islamic University,Kushtia,
Bangladesh.