Analog Communication

Module I

Jerry Kuriakose Analog Communication 1

Review of AM & FM

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 FM & PM

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AM Spectrum

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 AM Wave

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 FM Wave
• For FM wave, modulation Index,

• m is greater than or equal to 1

• For AM, m is always between 0 and 1

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 AM-FM Comparison
AM FM
Modulating Carrier wave is modulated in amplitude Carrier wave is modulated in frequency by
differences by the signal that is to be transmitted. the signal that is to be transmitted. The
The freq and phase remain the same. amplitude and phase remain the same.
Frequency 540 to 1650 KHz 88 to 108 MHz
Range
Bandwidth Twice the highest modulating frequency. Twice the sum of the modulating signal
Requirement In AM radio broadcasting, the frequency and the frequency deviation. If
s modulating signal has bandwidth of the frequency deviation is 75kHz and the
15kHz, and hence the bandwidth of an modulating signal frequency is 15kHz, the
amplitude-modulated signal is 30kHz. bandwidth required is 180kHz.
Pros and cons AM has poorer sound quality compared FM is less prone to interference than AM.
with FM, but is cheaper and can be However, FM signals are impacted by
transmitted over long distances. It has a physical barriers. FM has better sound
lower bandwidth so it can have more quality due to higher bandwidth.
stations available in any frequency range.

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 Radio receivers
• Several parameters are commonly used to evaluate the ability
of a receiver to successfully demodulate a radio signal. The
most important parameters are Selectivity, sensitivity and
fidelity
• Sensitivity indicates the ability of a radio receiver to amplify
weak signals
• Selectivity is a parameter used to measure the ability of a
receiver to accept a given desired band of frequencies, while
rejecting others.
• Fidelity is a measure of the ability of a communication system
to produce at the output of the receiver an exact replica of the
original source information.

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Superheterodyne AM Receiver

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 Superheterodyne AM Receiver
• Heterodyne means to mix two frequencies together in a non
linear device or to translate one frequency to another using non
linear mixing.

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 Superheterodyne AM Receiver
• RF Section
• Consists of preselector and an amplifier stage
• Preselector-broad tuned bandpass filter with an adjustable
centre frequency that’s tuned to the desired carrier frequency.
• Purpose of Preselector
• a) to provide initial band limiting to prevent a specific
unwanted frequency from entering the receiver(image
frequency)
• b) to reduce the noise bandwidth of the receiver, thus
providing initial step towards reducing the overall receiver
bandwidth required to pass the desired radio frequency signals.

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 Superheterodyne AM Receiver
• RF Amplifier
• Determines the sensitivity of the receiver. A receiver can have
one or more amplifiers depending on the desired sensitivity.

• Advantages of including amplifiers at the RF stage

• Greater gain.
• Better selectivity.
• Improved image frequency rejection ratio.
• Better signal to noise ratio.

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 Superheterodyne AM Receiver
• Mixer/Converter Section
• This includes a RF oscillator called Local Oscillator & a mixer
• The Local Oscillator can be any of the oscillator circuits
(Hartley, Colpitts, Crystal etc)
• Mixer is a non linear device which converts the radio
frequency to intermediate frequency (RF to IF translation).
Heterodyning takes place in the mixer stage.
• Although carrier & sidebands are converted from RF to IF, the
shape of AM envelope remains the same, and therefore the
original information contained in the envelope remains
unchanged. Most common IF for AM receiver is 455KHz.

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 Superheterodyne AM Receiver
• IF Section
• Consists of a series of IF amplifiers & band pass filters
• Most of the gain & selectivity is achieved in this section
• IF centre freq & bandwidth is constant for all stations

• Need for an IF section

•  it is easier & less expensive to construct high gain stable
amplifiers for low frequency signals
•  low freq IF amplifiers are less likely to oscillate compared
to their RF counterparts

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 Superheterodyne AM Receiver
• Audio Detector Section
• To convert IF signals back to the original source information
• Can be as simple as a diode detector or as complex as a
balanced detector.
• Audio Amplifier section
• Comprises of several cascaded audio amplifiers & one or more
speakers
• The number of audio amplifiers used depends on the audio
signal power desired

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 Superheterodyne AM Receiver
• Ganged Tuning
• The converted freq from mixer is wIF. Whatever be the input
from RF filter, output of mixer is always wIF.
• We have to make sure that the RF filter & Local oscillator
should be tuned together so that if RF filter is tuned to a
different frequency, Local oscillator should also generate that
freq ±wIF. If RF filter changes tuning from wc to w2, Local
oscillator should produce w2 ±wIF instead of wc ±wIF . As RF
Filter is tuned, LO should also be tuned. The tuning is
typically ganged up. Tuning of the two happens together
through a ganged up set of capacitors.

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 AM Receiver - RF Amplifier
• RF Amplifiers

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 AM Receiver - RF Amplifier
• RF Amplifier is a small signal tuned amplifier with tuned
circuits at input & output side. Both tuned circuits are tuned to
the desired incoming signal freq. so the tuned circuits select
the desired signal freq & reject all other unwanted freq
including the image freq.
• Gain provided by the RF amplifiers results in improved SNR
at the output of receiver
• Signal received at received antenna is transformer coupled to
the base of transistor. Input tuned circuit is tuned to the desired
carrier freq with the help of ganged tuning. Amplified signal
developed across the collector of transistor is coupled through
a step down transformer to the next stage
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 AM Receiver - RF Amplifier
• Advantages of including RF Amplifiers
•  greater gain (so better sensitivity)
•  improved image frequency rejection (so better selectivity)
•  improved SNR
•  improved coupling of signals from antenna to later stages
of receiver.

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AM Receiver - Mixer

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 AM Receiver - Mixer
• Mixer
• To down convert incoming RF signals to IF signals
• A mixer is similar to a non linear amplifier except that its
output is tuned to the difference frequency.
• RF signal from the antenna are filtered by the preselector
tuned circuit & then transformer coupled to the base of Q1. Q1
provides amplification for the local oscillator (LO). This
configuration is called self excited mixer because the mixer
excites itself by feeding energy back to the LO tank circuit (C2
& L2) to sustain oscillations

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 AM Receiver - Mixer
• The LO frequency is the resonant freq of the tank circuit. A
portion of the resonant tank circuit energy is coupled through
L2 & L5 to the emitter of Q1. This signal drives Q1 into its non
linear operating region & consequently produces sum &
difference frequencies at its collector. The difference freq is
the IF. The output tank circuit (C3 & L3) is tuned to the IF
band. The IF signal is transformer coupled to the input of first
IF amplifier

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AM Receiver - Detector

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 AM Receiver - Detector
• Diode is the common device used for AM demodulation or
detection.
• The parallel combination of R & C is the load resistance across
which the rectified output voltage Vo is developed. At each
positive peak of input cycle C charges up to a potential almost
equal to the peak signal voltage Vs.the time constant of RC
combination must be slow enough to keep the RF ripple as
small as possible, but sufficiently fast for detector circuit to
follow the fastest modulation variations.

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AM Receiver – AGC Circuits

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 AM Receiver – AGC Circuits
• The AGC circuit automatically increases the receiver gain for
weak RF input levels & automatically decreases the receiver
gain when a strong RF signal is received. Weak signals can be
buried in receiver noise and may be impossible to detect. An
excessively strong signal can overdrive the RF/IF amplifiers
and produce excessive non linear distortion & even saturation.
• The AGC circuit monitors the received signal level & sends a
signal back to the RF & IF amplifiers to adjust their gain
automatically. The purpose of AGC is to allow a receiver to
detect & demodulate, signals that are transmitted from
different stations whose output power & distance from
receiver vary
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 AM Receiver
• An AGC circuit is essentially a peak detector. The circuit
shows a negative peak detector. The greater the amplitude of
input carrier, the more negative the output voltage. The
negative voltage from the AGC detector is fed back to the IF
stage, where it controls the bias voltage on the base of Q1.
When the carrier amplitude increases voltage on the base of Q1
becomes less positive, causing the emitter current to decrease
which in turn decreases the gain.
• When the carrier amplitude decreases, AGC voltage becomes
less negative, emitter current increases and amplifier gain
increases.

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 FM Reactance Modulator (JFET)
• s

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 FM Reactance Modulator (JFET)
• The circuit is called reactance modulator, because the JFET
looks like a variable reactance load to the LC tank circuit. The
modulating signal varies the reactance of Q1, which causes a
corresponding change in the resonant frequency of the
oscillator tank circuit.
• When a modulating signal is applied to the bottom of R3, the
gate to source voltage is varied accordingly, causing a
proportional change in circuit impedance. So the resonant
frequency of the oscillator tank circuit is a function of the
amplitude of the modulating signal.

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 FM Reactance Modulator (BJT)
• s

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 FM Reactance Modulator (BJT)
• The circuit is a standard class A common emitter amplifier
with two external inputs – modulating signal input and
external carrier input.
• The transistor emitter to collector resistance is a part of the
phase shifter consisting of C1 in series with Rt & emitter
resistor RE.
• Output is taken across series combination of Rt & RE.
• The circuit is designed such that at the carrier input frequency,
sum of Rt & RE equals capacitive reactance of C1, the carrier
input signal is shifted 45o.

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 FM Reactance Modulator (BJT)
• When modulating signal is applied, its voltage adds and
subtracts from the dc base bias, producing corresponding
changes in collector current. Changes in collector current
changes emitter to collector resistance, producing changes in
phase shift that the collector undergoes as it passes through the
phase shifting network. The phase shift is directly proportional
to the amplitude of the modulating signal.
• Higher the amplitude of the modulating input signal, greater
the change in emitter to collector resistance and greater the
phase shift.

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Armstrong FM Modulator

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 Armstrong FM Modulator
• With indirect FM, the modulating signal directly deviates the
phase of the carrier, which indirectly changes the frequency.
• A relatively low frequency subcarrier is phase shifted 90o &
fed to a balanced modulator, where it is mixed with the input
modulating signal.
• Output of modulator is a DSBSC wave which is combined
with the original carrier to produce a phase modulated
waveform.
• Since the DSBSC wave is 90o out of phase with the carrier
(Vc), the lower & upper sidebands combine to produce a
component (Vm) that is always in quadrature with Vc

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Armstrong FM Modulator

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 Armstrong FM Modulator
• The peak phase deviation can be calculated as
• Ɵ = m = arctan(Vm/Vc)
• For very small angles, tangent of angle is equal to the angle.
• So Ɵ = m = (Vm/Vc)
• Change in modulating voltage can cause change in the phase,
which in turn causes frequency variation.

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 Armstrong FM Modulator
• For a modulating signal with freq of 15KHz & amplitude of
0.0048V, the Armstrong modulator works as follows.
• The maximum phase deviation is
• Ɵ = m = arctan(Vm/Vc) = 1.67 milli radians
• Freq deviation, Δf = mfm
• Δf max = 0.00167 * 15000 = 25Hz
• A combination of multiplying & mixing is necessary to
develop the desired transmit carrier freq with 75KHz freq
deviation.

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 Armstrong FM Modulator
• The output of a combining network is multiplied by 72
producing
• f1 = 72*200 KHz = 14.4MHz
• m = 72*0.00096 = 0.06912 rad
• Δf = 72 *14.4Hz = 1036.8Hz
• The output from multiplier is mixed with a 13.15MHz crystal
controlled freq to produce a diff signal having
• f2 = 14.4MHz – 13.15MHz =1.25 MHz
• m = 0.06912 rad
• Δf = 1036.8Hz

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 Armstrong FM Modulator
• The output from mixer is once again multiplied by 72 to
produce a transmit signal with following characteristics

• ft = 72*1.25 MHz = 90MHz

• m = 72*0.00096 = 4.98 rad
• Δf = 72 * 1036.8Hz = 74650Hz

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 FM Superheterodyne Receiver
• s

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 FM Receiver – Simplified Block
Diagram
• s

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 FM Superheterodyne Receiver
• The preselector, RF amplifier & the two mixer circuits perform
the same function as they did in AM receivers.
• The preselctor rejects the image freq, the RF Amplifier
establishes SNR & noise figure, the mixer converts RF to IF,
IF amplifier provides most of the gain & selectivity of the
receiver.
• AGC is not used with FM receivers because with FM
transmission, there is no information contained in the
amplitude of the received signal.
• The final IF amplifier is specially designed for ideal saturation
characteristics and is called a limiter, or sometimes passband
limiter, if the output is filtered.

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 FM Superheterodyne Receiver
• The envelope (peak) detector in AM receivers is replaced by a
limiter, freq discriminator & de-emphasis network. The
frequency discriminator extracts the information from the
modulated wave, while the limiter circuit & de-emphasis
network contribute to an improvement in the signal to noise
ratio.
• For broadcast band FM receivers, the first IF is relatively high
freq, (often 10.7MHz) for good image freq rejection & the
second IF is a relatively low freq (often 455KHz) that allows
the IF amplifiers to have a relatively high gain & still not
susceptible to oscillating. With a first IF of 10.7MHz, the
image freq for even the lowest possible FM station (88.1MHz)
is 109.5MHz, which is beyond the FM broadcast range.
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FM Demodulators - Slope Detector

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 Slope Detector
• The tuned circuit (La & Ca) produces an output voltage that is
proportional to the input frequency. The maximum output
voltage occurs at the resonant frequency of the tank circuit (fo)
& its output decreases as the input frequency deviates above or
below fo. The circuit is designed so that the IF centre
frequency (fc) falls in the centre of the most linear portion of
the voltage versus frequency curve. When the intermediate
freq deviates above fc the output voltage increases & when the
intermediate freq deviates below fc the output voltage
decreases. So the tuned circuit converts freq variation to amp
variations. D,C & R makes up a peak detector that converts
amp variations to an output voltage that varies at a rate equal
to input frequency changes

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Balanced Slope Detector

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 Balanced Slope Detector
• The circuit consists of two slope detector circuits connected in
parallel & fed 1800 out of phase. The tuned circuits(La, Ca and
Lb, Cb) perform the FM to AM conversion, and the balanced
peak detectors (D1,C1,R1 & D2,C2,R2 ) remove the information
from the AM envelope.
• The top tuned circuit (La, Ca )is tuned to a frequency (fa) that is
above IF centre freq & lower tuned circuit (Lb, Cb) is tuned to
a frequency (fb) that is below IF centre freq by an equal
amount.
• The output voltage from each tuned circuit is proportional to
the input frequency & each output is rectified by its respective
peak detector
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 Balanced Slope Detector
• Therefore the closer the input freq is to the tank circuit
resonant freq, the greater the tank circuit output voltage. The
IF centre freq falls half way between the resonant freq’s of the
two tuned circuits.
• So at IF centre freq, the output voltages from the two tuned
circuits are equal in amp, but opposite in polarity. So the
rectified output voltage when added produce a differential
output voltage, Vout = 0V.
• When the IF deviates above resonance, the top tuned circuit
produces a higher output voltage than lower tank circuit and
Vout goes positive. & vice versa.

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Ratio Detector

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 Ratio Detector
• A ratio detector has a single tuned circuit in the transformer
secondary. After several cycles of input signal, the capacitor Cs
charges to approximately the peak voltage across the
secondary winding of T1.
• Rs simply provides a dc path for diode current. So the time
constant for Rs & Cs is sufficiently long so that rapid changes
in the amplitude of input signal due to thermal noise or other
interfering signals are shorted to ground & have no effect on
the average voltage across Cs. C1 & C2 charge & discharge
proportional to freq changes in input signal.

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 Ratio Detector
• At resonance the output voltage is divided equally between C1
& C2 and redistributed as the input freq is deviated above or
below resonance.
• So changes in Vout are due to the changing ratio of the voltage
across C1 & C2, while the total voltage is clamped by Cs.
• At resonance Vout is not equal to 0V, but rather to one half of
the voltage across the secondary windings of T1.

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