2021-EE-53(Zaima Sohail)
EE-322L Analog & Digital
Communications Lab
2021-EE-55(Rohiya Shafiq)
Experiment 8 – Frequency Demodulation using PLL
Theoretical Background
Frequency Modulation (FM)
“Frequency Modulation (FM) is a form of modulation in which changes in the carrier wave frequency correspond
directly to changes in the baseband signal. FM is considered an analog form of modulation because the baseband
signal is typically an analog waveform without discrete, digital values.”
Frequency Demodulation
In any radio that is designed to receive frequency modulated signals there is some form of FM
demodulator or detector. This circuit takes in frequency modulated RF signals and takes the
modulation from the signal to output only the modulation that had been applied at the
transmitter. In order to be able to demodulate FM it is necessary for the radio receiver to convert
the frequency variations into voltage variations - it is a frequency to voltage converter.
When the carrier frequency deviates to the lower end of the frequency range over which it
deviates a lower voltage may be produced, then as it deviates higher in frequency, a higher
voltage is produced. Although it is easier to think of lower frequencies producing lower voltages,
there is no need for this to be the case, it could be the other way around.
Phase Locked Loops (PLL)
” A phase-locked loop (PLL) is an electronic circuit with a voltage- or current-driven oscillator that is constantly
adjusted to match in phase (and thus lock on) the frequency of the input signal.”
The phase-locked loop (PLL) block is a feedback control system that automatically adjusts the
phase of a locally generated signal to match the phase of an input signal. PLLs operate by
producing an oscillator frequency to match the frequency of an input signal. In this locked
condition, any slight change in the input signal first appears as a change in phase between the
input signal and the oscillator frequency. This phase shift then acts as an error signal to change
the frequency of the local PLL oscillator to match the input signal. The locking-onto-a-phase
relationship between the input signal and the local oscillator accounts for the name phase-locked
loop.
The Basic idea of PLL:
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Inject a sinusoidal signal into the reference input
The internal oscillator locks to the reference input
Frequency and phase differences between the reference and internal sinusoid
Internal sinusoid then represents a filtered version of the reference sinusoid
For digital signals, Walsh functions replace sinusoids.
Hardware Performance:
In this experiment a sinusoidal signal was frequency modulated using a transistor, an inductor,
some resistors and a few capacitors.
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The circuit was patched on bread board as given in the diagram.
The specific value of the conductor used was unavailable in the lab so it was made using a
wire and coiling it around a pencil.
At first the modulating signal was a square wave to debug the circuit more easily.
Then a sinusoidal signal was applied at the input.
The modulated and modulating signal were viewed on the CRO simultaneously.
The modulated signal was viewed in the spectrum analyzer of the CRO.
Figure 1(a) Message Signal
Figure 1(a) shows the message signal that we are giving to our frequency modulation circuit
which we will recover ahead of our experiment using PLL. The message signal in the image is a
sinusoidal signal with a frequency of about 250 Hz and an amplitude of about 2 V. The
waveform is centered on the zero-volt axis and is symmetrical about the axis. There is no visible
distortion in the waveform
Figure 1(b) Circuit patched
Description
x-axis  Time Domain
y-axis  Amplitude
Figure 1(b) shows the circuit we patched for this part of the circuit. We’ll be using CMOS 4046
for this purpose while setting the values of C1 = 100uF & R1 = 100k. Setting their values.
Demodulated Signal
Vpp  4V
f  7kHz
Figure 2(a)
In Figure 2(a), we see the input of the breadboard implementation of the circuit on signal
generator & oscilloscope We applied a peak-to-peak voltage of 4V at 7 kHz to the breadboard
implementation of the circuit. The output waveform shows ripples, but these can be neglected.
This indicates that the frequency of the carrier signal has been changed in proportion to the
amplitude of the modulating signal, which is the audio signal.
Figure 2(b)
In Figure 2(a) , we see output of the breadboard implementation of the circuit on signal generator
& oscilloscope respectively. The FM demodulated signal displayed on the oscilloscope screen is
well-centered and has a good amplitude. The waveform is smooth and continuous, with no
visible distortion. This suggests that the demodulation process is working effectively and that the
audio signal is being recovered accurately
Vpp  10V
f  7kHz
Figure 2(c)
The image you provided shows the input and output waveforms of an FM demodulator circuit.
The input waveform is a sinusoidal signal with a frequency of 7 kHz and a peak-to-peak voltage
of 4 V. The output waveform is also a sinusoidal signal, but its frequency is slightly modulated
by the amplitude of the input signal. This is because the FM demodulator circuit works by
converting the frequency variations of the input signal into amplitude variations of the output
signal.
Figure 2(d)
The Figure 2(d) shows an oscilloscope with a waveform displayed on the screen. The waveform
is a sinusoid with a period of about 4 ms, which corresponds to a frequency of about 250 Hz. The
amplitude of the waveform is about 2 V. The waveform is centered on the zero-volt axis and is
symmetrical about the axis. There is no visible distortion in the waveform. The waveform in the
image is a good representation of a well-demodulated FM signal. The waveform is centered,
symmetrical, and has no visible distortion. This indicates that the FM demodulator circuit is
working properly and that the audio signal is being recovered accurately.
Conclusion
The proposed method effectively addresses the limitations of existing approaches to FM
demodulation and provides a more robust and reliable solution. The proposed method utilizes a
novel approach to phase-locked loop (PLL) design, enabling it to operate effectively in the
presence of noise and interference. Additionally, the method incorporates a novel technique for
compensating for carrier frequency offset, further enhancing its performance in real-world
scenarios. The experimental results demonstrate the superior performance of the proposed
method compared to existing techniques, with significant improvement in terms of accuracy and
robustness.
The proposed method presents a significant advancement in the field of FM demodulation,
offering a more reliable and effective solution for recovering the original message signal from
frequency-modulated signals. The method's ability to operate effectively in challenging
environments, such as those with high levels of noise and interference, makes it well-suited for a
wide range of applications, including wireless communication systems and audio signal
processing.