NHE2484: FM Demodulation Report

This engineering report details an experiment on FM demodulation using a Phase-Locked Loop (PLL) circuit. The report covers the theoretical background of PLL FM demodulators, including their advantages like linearity and low manufacturing costs. The experimental procedure involves determining PLL component parameters for a center frequency of 470 kHz, selecting loop filter components for audio demodulation, building the circuit, and verifying the center frequency. The report also investigates the effects of changing loop components on the lock-in and capture range, and explores the demodulation of both narrowband FM (NBFM) and wideband FM (WBFM). The results section includes observations on the output waveform and the effects of component changes. The discussion section addresses questions about demodulating NBFM and WBFM, the impact of altering loop components, and modifications needed for WBFM demodulation. Finally, the conclusion summarizes the findings, noting that a low-pass filter is needed to improve the output waveform.

NHE2484: Communication System
FM Demodulation

NHE2484: Communication System
Abstract
The PLL CD4046 IC is used as a demodulator circuit for frequency demodulation

NHE2484: Communication System
Table of Contents
Introduction...................................................................................................................................3
Procedure...................................................................................................................................... 4
Theory............................................................................................................................................9
Conclusion................................................................................................................................... 13

NHE2484: Communication System
Introduction
The Phase-Locked Loop (PLL) circuit is used in control and communication systems. A phase
locked loop circuit consists of three main components; a voltage-controlled oscillator (VCO), a
phase comparator, and a low-pass filter . When used as a FM demodulator, the input to the PLL
circuit is a FM signal and the output (demodulated signal) is the output from the low-pass filter.
The operation of this demodulator is described as follows: The phase comparator detects the
difference between the phase (or frequency) of the VCO output and that of the input signal.
The function of low-pass filter is to filter phase error and then it is used to adjust the VCO .
That is the phase of the VCO phase locks on the phase of the input signal.
Procedure

NHE2484: Communication System
Preliminary works
1. Determine the PLL component parameters for a centre frequency of 470 kHz by
carefully examining the data sheet.
Given the value of fo = 470KHz
From the Data sheet
fo =1/2 fmax
fmax =2*fo
= 2 x 470 KHz
= 940 KHz
fmax = fo + fL
Therefore
fL= fmax –fo
= 940 KHz – 470 KHz
= 470 KHz
fmin= fo-fL
= 470 KHz – 470 KHz
= 0 KHz
Lock frequency range,2fL=fmax-fmin = 940KHz
Capture range ,2fc
fc<fL
2.Select component values for the loop filter bearing in mind it will be used to
demodulate audio.
Cut off frequency ,fcut off=19.5KHz (fL)

NHE2484: Communication System
from data sheet
R2=R3=10KΩ
C1=220pF
C2=10μF
fcutoff =1/2πR1C3
if select R1=100KΩ
C3=1/2πR1C3
=1.25μF
So we can use 1μF capacitor
1. Build the PLL circuit. Verify the centre frequency as being close to 470 kHz.
R1=10KΩ
C1=220pF
R2=100KΩ
C2=10μF
R3= 10KΩ
C3=1μF
We can verify the output using Pspice simulation
Here by doing the calculations
Centre frequency is given by fo=455KHz

NHE2484: Communication System
2. Measure the lock-in and capture range of the loop.
Remove the potential divider and close the loop. Give the PLL the centre
frequency you found in pin 4 and display the input and output on the
oscilloscope. Now we can see two square waves both in phase then the loop is
locked
2fL= 940KHz lock range frequency
Fc=470KHz
3. Check o peration as an FM demodulator.

NHE2484: Communication System
.
4. Determine the effects of changing loop components
Replacing C1 with a different capacitance affects the PLL's lock range
exponentially. The higher the capacitance, the lower and narrower the capture
and lock ranges become. We can observe this by graphing the input frequency
(KHz) versus voltage (mV DC) acrossC3. Frequencies beyond the ranges
displayed in the graphs for each respective capacitor will break the phase lock,
and output an inaccurate voltage reading across C3.
5. Check operation if wide-band FM is applied
Much higher levels of deviation are used for broadcasting. This is called wide band FM
(WBFM) and for broadcasting deviation of +/- 75 kHz is used.

NHE2484: Communication System
In wide band FM the circuit should be able to process multiple signal.
The center frequency resulting in distortion.
Alter on the capture range of the PLL and the high-frequency ripple in vo. You can
change fp simply by changing one of the

NHE2484: Communication System
Theory
Phase locked loop, PLL FM demodulator or detector is a form of FM demodulator that
has gained widespread acceptance in recent years.
PLL FM detectors can easily be made from the variety of phase locked loop integrated
circuits that are available, and as a result, PLL FM demodulators are found in many
types of radio equipment ranging from broadcast receivers to high performance
communications equipment.
The PLL FM demodulation integrated circuits started to appear when integrated circuit
technology developed to the degree to allow RF analogue circuits to be manufactured.
Although high frequencies are not normally needed, for PLL FM demodulators, the
circuit must be capable of operating at the intermediate frequency of the receiver, and
for receivers using FM this was often 10.7 MHz. Although by today's standards, this is
not high, it was necessary for the technology to reach this state before PLL FM
demodulators became available.
The way in which a phase locked loop, PLL FM demodulator works is relatively
straightforward. It requires no changes to the basic phase locked loop, itself, utilising the
basic operation of the loop to provide the required output.
PLL FM demodulator performance
The PLL FM demodulator is normally considered a relatively high performance form of
FM demodulator or detector. Accordingly they are used in many FM receiver
applications.
The PLL FM demodulator has a number of key advantages:
 Linearity: The linearity of the PLL FM demodulator is governed by the voltage
to frequency characteristic of the VCO within the PLL. As the frequency deviation
of the incoming signal normally only swings over a small portion of the PLL
bandwidth, and the characteristic of the VCO can be made relatively linear, the
distortion levels from phase locked loop demodulators are normally very low.
Distortion levels are typically a tenth of a percent.
 Manufacturing costs: The PLL FM demodulator lends itself to integrated
circuit technology. Only a few external components are required, and in some
instances it may not be necessary to use an inductor as part of the resonant

NHE2484: Communication System
circuit for the VCO. These facts make the PLL FM demodulator particularly
attractive for modern applications.
PLL FM demodulator design considerations
When designing a PLL system for use as an FM demodulator, one of the key
considerations is the loop filter. This must be chosen to be sufficiently wide that it is able
to follow the anticipated variations of the frequency modulated signal. Accordingly the
loop response time should be short when compared to the anticipated shortest time
scale of the variations of the signal being demodulated.
A further design consideration is the linearity of the VCO. This should be designed for
the voltage to frequency curve to be as linear as possible over the signal range that will
be encountered, i.e. the centre frequency plus and minus the maximum deviation
anticipated.
In general the PLL VCO linearity is not a major problem for average systems, but some
attention may be required to ensure the linearity is sufficiently good for hi-fi systems.

NHE2484: Communication System
Result
Square wave out put