Date: …../…../……..
Experiment 1
FM generation and demodulation using PLL
Objective
To design and set up a Frequency modulator and a demodulator circuits using PLL IC.
Outcome
● Design and test a FM modulator
● Analyses the modulator for different values of modulation indices.
Components Required
Sl No.
Component Name
Specification
Qty
Theory
The phase locked loop, PLL is a very useful RF building block. The PLL uses the concept of
minimising the difference in phase between two signals: a reference signal and a local oscillator to
replicate the reference signal frequency. Using this concept it is possible to use PLLs for many
applications from frequency synthesizers to FM demodulators, and signal reconstitution.
In FM frequency of the carrier signal is varied in accordance with the instantaneous amplitude of
the modulating signal. PLL(Phase Locked Loop) is widely used for FM generation. In PLL,
frequency generated by VCO is varied by the input signal applied
FM Detector
Suppose the center frequency of the FM signal is fc; and it lies within the hold-in range of PCL the
VCO is locked to fc, by applying an demodulated carrier at the input of the phase detector When VCO
is locked to fc, the error signal is zero, and therefore, the control signal that changes the VCO
frequency is also equal to zero. If an FM signal is applied to the phase detector, there will be a
difference in the phases of the VCO output and the input FM signal. The control signal is produced in
proportion to the phase difference at an instance of time. This control voltage will modify the VCO
frequency, which is again compared with the incoming frequency. In this way, the current incoming
frequency is compared with the previously attained value of the VCO frequency, which is the
previously attained frequency of the FM Signal.
The VCO; therefore tries to track the instantaneous frequency of the applied FM signal. The control
signal is produced in proportion to the difference between the VCO frequency and the instantaneous
frequency of FM signal. In other words, the control signal so produced is proportional to the frequency
deviation in the FM signal. Since the frequency deviation si proportional to the modulating signal, the
control signal appearing at the output of LPF is the modulating signal. Therefore, the FM signal is
demodulated by PLL.
Design
Modulator
Let V+ =10 V and V- = -10
Let the center frequency of the FM be fo =(1.2/4*R1*C1)=2.5Khz
Take C1=0.01μF
Then R1= -------------Take CC =10μF, R2=R1
Detector
Let V+ =10 V and V- = -10
Let the center frequency of the FM be fo =(1.2/4*R1*C1)=2.5Khz
Take C1=0.01μF
Then R1= -------------Take C2=1μF, C3=0.01μF
Internal Block Diagram of IC 565
Circuit Diagram
Procedure:
Modulator
1.
2.
3.
4.
5.
6.
Check the component using multi meter an setup the circuit on the breadboard
Note down the amplitude and frequency of modulating and carrier wave
Feed the modulating signal of 5V p-p ,1Khz sine wave
observe the FM output on the CRO
calculate modulation index
plot the output wave form on the graph sheet
Demodulator
1. Set the input to the PLL to be a FM signal from the FM Modulator, with a center frequency equal
to your fo
2. Connect the scope and CRO to the output (pin 7) of the PLL through the coupling capacitor.
3. At this point, the output should be a demodulated signal, at 1kHz. If the low frequencies seem to
be overpowered by feedthrough of the higher frequencies (i.e. if your output seems to be at fo
rather than at 1 kHz), build a low-pass filter and add it to the output (Be sure to add the LPF after
the coupling capacitor at Pin 7).
4. Plot or sketch this signal in the frequency and time domain. (Don’t worry about how messy it
might look in the time domain.)
Draw observed waveforms:
Modulator
Modulating Signal
FM Output
Demodulator
FM Input
Demodulated Output
Post lab Questions
1. A system having input impedance X and connected with variable power supply 0-30V/2A. What
will be the range of values of X that can be connected without any overload condition if power
supply shows 5V? Explain?
Inference