18-12-2012, 04:56 PM
EXPERIMENT NUMBER 7 Phase Locked Loop (PLL)
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AIM:
To study IC 565 PLL and find the following parameters.
1. Lock Range
2. Capture Range.
LEARNING OBJECTIVE:
To compare the two modes of operation of timer IC 555.
To classify the types of pulse modulation.
THEORY:
Phase-locked loop (PLL) is a feedback loop which locks two waveforms with same frequency but
shifted in phase. The fundamental use of this loop is in comparing frequencies of two waveforms and then
adjusting the frequency of the waveform in the loop to equal the input waveform frequency. The heart of the
PLL is a phase comparator which along with a voltage controlled oscillator (VCO), a filter and an amplifier
forms the loop. If the two frequencies are different the output of the phase comparator varies and changes the
input to the VCO to make its output frequency equal to the input waveform frequency. The locking of the two
frequencies is a nonlinear process but linear approximation can be used to analyse PLL dynamics. The range
over which the loop system will follow changes in the input frequency is called the lock range. On the other
hand, the frequency range in which the loop acquires phase-lock is the capture range, and is never greater than
the lock range.
PROCEDURE:
1. Connections are made as shown in the circuit diagram.
2. Measure the free running frequency of IC565 at pin 4 using display with the input signal (say 0V) from
the function generator or by shorting pin 2 to ground.
3. Set the input signal say 1V, 1 KHz to pin no. 2 using function generator and observe the corresponding
waveform.
4. The frequency is varied till the output signal is 180˚ out of phase with the input. This is the upper end
of the lock range.
5. The frequency is reduced till the output is 90˚ out of phase with the input. This is the upper end of the
capture range.
6. The frequency is varied till a 90˚ phase shift is obtained in the output with reference to the input once
again. This is the lower end of the capture range.
7. As the frequency is decreased further, output goes to 180˚ out of phase with the input once again. This
is the lower end of the lock range.
8. The lock range ΔfL = (f2-f1)
The capture range ΔfC = (f3-f1)
9. Compare these values with the theoretical values.
CIRCUIT DESCRIPTION:
Operating frequency range of the IC: 0.001 Hz to 500 kHz.
Operating voltage range: ± 6 to ± 12 V.
Input level required for tracking: 10 mVrms minimum to 3 V peak-to-peak maximum.
As shown in the figure, the PLL system consists of a phase detector or comparator (PC), a voltage-controlled
oscillator (VCO), an amplifier and R-C combination forming low-pass filter circuit. The input signals are fed to
the phase detector through pins 2 and 3 in differential mode. The input signals can be direct-coupled provided
that the dc level at these two pins is made same and dc resistances seen from pins 2 and 3 are equal. By shorting
pins 4 and 5 output of VCO is supplied back to the phase comparator (PC). The output of PC is internally
connected to amplifier, the output of which is available at pins 6 and 7 through a resistor of 3.6 k Q connected
internally. A capacitor C2 connected between pins 7 and 10 forms a low-pass filter with 3.6 k Q resistor. The
filter capacitor C2 should be large enough so as to eliminate the variations in demodulated output and stabilize
the VCO frequency. Voltage available at pin 7 is connected internally to VCO as a control signal. At pin 6 a
reference voltage nominally equal to voltage at pin 7 is available allowing both the differential stages to be
biased. Pins 1 and 10 are supply pins.