ce- chapter2 am
TRANSCRIPT
-
8/3/2019 CE- Chapter2 AM
1/35
AMPLITUDEMODULATION
-
8/3/2019 CE- Chapter2 AM
2/35
MODULATION PRINCIPLES
The process of having the information to betransmitted alter a high-frequency signal forthe purpose of transmitting the information
somewhere in the electromagnetic spectrumvia radio, wire, or fiber optic.
Information signal (modulating signal): voice,
video, binary data Carrier: - higher frequency signal
- Sine wave
-
8/3/2019 CE- Chapter2 AM
3/35
AMPLITUDE MODULATION with SINEWAVES
Amplitude Modulation is the simplest andearliest form of transmitters. (535 1605 kHz)
AM applications include broadcasting in
medium- and high-frequency applications, CBradio, and aircraft communications.
-
8/3/2019 CE- Chapter2 AM
4/35
during the modulation process: carrierfrequency remain constant but its amplitudevaries accordance with the modulating signal.
Envelope showing it is same as themodulating signal.
Time domain signal variation of voltage over
time. Peak value of the modulating signal be less
than the peak value of the carrier.
AMPLITUDE MODULATION with SIGNWAVES (Cont.)
-
8/3/2019 CE- Chapter2 AM
5/35
Sine wave carrier can be expressed as below:
AMPLITUDE MODULATION with SIGNWAVES (Cont.)
tfVv ccc 2sin
Where
vc = instantaneous value of the sine voltage
Vc = the peak value of the sine wave asmeasured between zero and themax.amplitude.
fc= frequency of the carrier sine wave.
t = some particular points in time during the accycle.
*for modulating signal, signal can be expressed with similarexpression
-
8/3/2019 CE- Chapter2 AM
6/35
Complete modulated wave can be expressedas:
carriermodulation
tftfV
carrier
tfVvtfvv
envelopevVv
cmmcc
c
mc
)2(sin2sin2sin
2sin
)(?
2
12
1
modulator
vm v2
Vc
AMPLITUDE MODULATION with SIGNWAVES (Cont.)
-
8/3/2019 CE- Chapter2 AM
7/35
AMPLITUDE MODULATION WITH
DIGITAL SIGNAL
Binary 1= produce maximum carrier amplitude
Binary 0= produce a lower-value carrier.
Amplitude Shift Keying (ASK):
The carrier is switched between twodifferent carrier levels.
On-Off Keying (OOK):
The binary 1 turns the carrier on and thebinary 0 level turns the carrier off.
-
8/3/2019 CE- Chapter2 AM
8/35
MODULATION INDEX
Modulation Index - The ratio between theamplitudes between the amplitudes of themodulating signal and carrier, expressed by
the equation:
Should be between 0 and 1. Ideal condition: m=1
If more than 1: severe distortion of the
modulated waveform or overmodulation.
c
m
V
Vm =
-
8/3/2019 CE- Chapter2 AM
9/35
Modulation index can be computed from Vmaxand Vmin.
It can be expressed as:
The amount or depth of AM is normallyexpressed as percent modulation rather thanas a fractional.
Percent of modulation is 100 x m.
MODULATION INDEX
minmax
minmax=VV
VVm
-
8/3/2019 CE- Chapter2 AM
10/35
OVERMODULATION
The waveform is flattened near the zero line. A sine wave whose negative peaks have
been clipped off.
-
8/3/2019 CE- Chapter2 AM
11/35
SIDEBANDS AND THE FREQ.
DOMAIN
Sidebands is a modulating signal that containsmultiple frequencies such as voice, video ordigital signal.
Time domain information can be obtained usingan oscilloscope.
Frequency domain information can becalculated using Fouriermethods, but
trigonometric methods are simpler and valid.
-
8/3/2019 CE- Chapter2 AM
12/35
Sidebands are calculated using the formulasat below:
From the equation for an AM signal v2:
From trigonometric identity:
sin A sin B= [cos (A-B)/2 ] [cos (A+B)/2]
SIDEBANDS
mcLSB
mcUSB
fff
fff
)2(sin2sin2sin2 tftfVtfVv cmmcc
-
8/3/2019 CE- Chapter2 AM
13/35
By substituting the identity into the expression,the instantaneous amplitude of the signalsbecomes:
This means, an AM wave contains carrier and
sidebands frequencies. When modulation occurs, sidebands are
produced.
SIDEBANDS
USB
mc
m
LSB
mc
m
carrier
cc fftV
fftV
tfVe )(2cos
2
)(2cos
2
2sin2
-
8/3/2019 CE- Chapter2 AM
14/35
FREQUENCY DOMAIN
Plot the carrier and sideband amplitudes withrespect to frequency.
http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEETS-Module-12-1-41-1-50.htm
-
8/3/2019 CE- Chapter2 AM
15/35
BANDWIDTH
Signal bandwidth is an important characteristic ofany modulation scheme.
The difference between the upper and lowersidebands freq.
In general, a narrow bandwidth is desirable.
Bandwidth is calculated by:
B=2fm
Example:If the carrier frequency is 2.8MHz and the maximummodulating frequency for voice signals is 3kHz.Calculate the upper, lower sidebands and also thebandwidth.
(ans: fUSB= 2803kHz, fLSB=2797kHz and BW= 6kHz)
-
8/3/2019 CE- Chapter2 AM
16/35
COMMON AM APPLICATION
AM radio broadcasting
TV picture (video)
Two- way radio (aircraft, amateur radio (SSB),
military)
Digital data transmissions
Computer modems
-
8/3/2019 CE- Chapter2 AM
17/35
AM POWER DISTRIBUTION
Power in a transmitter is important, but themost important power measurement is that ofthe portion that transmits the information.
AM carrier power remain unchanged. So, it iswasted matter.
Power in an AM transmitter is calculatedaccording to the formula at the right:
PT= PC + PLSB + PUSB or
PT= PC (1+m2/2)
-
8/3/2019 CE- Chapter2 AM
18/35
SIDEBANDS POWERS
Depends upon the value of the modulationindex:
The greater the percentage of mod, the higher the
sideband power. The power in each sideband Ps is expressed
by:
The power in each sideband is 25% of thecarrier power if 100% modulation.
4
)(2
mPPPP
C
USBLSBS
-
8/3/2019 CE- Chapter2 AM
19/35
POWER CALCULATIONS
Total antenna current is given by:
2/1 2mIICT
The total AM power the is:RIP
TT
2
Modulation percentage then is given by:
1/2 2 CTIIm
-
8/3/2019 CE- Chapter2 AM
20/35
SINGLE SIDEBANDS
The two sidebands of an AM signal are mirrorimages of one another
As a result, one of the sidebands is redundant
Using single-sideband suppressed-carriertransmission results in reduced bandwidthand therefore twice as many signals may betransmitted in the same frequency range.
Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC.
Sidebands generated only during modulationprocess (ex: when someone speaks into mic).
-
8/3/2019 CE- Chapter2 AM
21/35
BENEFITS OF SINGLESIDEBANDS
The spectrum space occupied by the SSBsignal is only half that of AM and DSB signals.
All the power previously devoted to the carrier
and other sideband can be channeled into thesignal sideband.
Less noise on the signal because the SSBsignal has less BW than an AM or a DSB
signal. Less fading than an AM signal. The signal
alternately increases and decreases instrength as it is picked up by the receiver.
-
8/3/2019 CE- Chapter2 AM
22/35
TRANSMITTED SIGNAL OF AM
AND SSB
http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEETS-Module-17-2-11-2-20.htm
-
8/3/2019 CE- Chapter2 AM
23/35
The power advantage of SSB over AM is 3:1.
Peak Envelope Power (PEP) is expressed by:
PEP=V2/R
where V= root mean square (rms) output voltage
R= load resistance
OR
PEP=VsImaxwhere Vs= the amplifier supply voltage
Imax= the current peak
SSB POWER
-
8/3/2019 CE- Chapter2 AM
24/35
SSB
Two-way radio used in the military, CB radioand radio amateurs.
DSB
FM and TV broadcasting to transmit two-channel stereo signals.
Used in some types of phase-shift keying totransmit binary data.
DSB AND SSB APPLICATIONS
-
8/3/2019 CE- Chapter2 AM
25/35
AM CIRCUITS
Two basic ways to produce amplitudemodulation:
1- Multiply the carrier by a gain or attenuation
factor that varies with the modulating signal.2- Linearly mix or algebraically add the carrierand modulating signals and then apply the
composite signals to a nonlinear device orcircuit.
LOW LEVEL MODULATION
-
8/3/2019 CE- Chapter2 AM
26/35
LOW LEVEL MODULATIONSYSTEM
Low-level modulation systems use linear poweramplifiers
-
8/3/2019 CE- Chapter2 AM
27/35
LOW-LEVEL AMPLITUDE
MODULATOR
Differential Amplifier Modulator
DIFFERENTIAL AMPLIFIER
-
8/3/2019 CE- Chapter2 AM
28/35
DIFFERENTIAL AMPLIFIERMOD
The differential amplifier makes an excellentamplitude modulator.
Has high gain
Good linearity
Can be modulated 100%
Better frequency than this circuit is PIN Diode
Modulator.
-
8/3/2019 CE- Chapter2 AM
29/35
HIGH-LEVEL AMPLITUDE
MODULATION
High-level means the signals are generatedare relatively high voltage and poweramplitudes.
The modulator varies the voltage and power inthe final RF amplifier stage of the transmitter.
One of the HLAM is the collector modulator.
G
-
8/3/2019 CE- Chapter2 AM
30/35
Collectormodulator
HIGH-LEVEL AMPLITUDE
MODULATION
-
8/3/2019 CE- Chapter2 AM
31/35
AMPLITUDE DEMODULATOR
A circuit that accepts a modulated signal andrecovers the origin modulating information.
Available in any radio receivers.
The simplest and most widely used amplitudedemodulator is the diode detector.
-
8/3/2019 CE- Chapter2 AM
32/35
BALANCED MODULATOR
Generates a DSB signal.
The inputs are the carrier and a modulatingsignal and the output are the upper and lower
sidebands. The output can be further processed by filters
or phase-shifting circuitry to eliminate one ofthe sidebands to get SSB.
Examples of balanced modulator:1- Diode ring or lattice modulator
2- IC balanced modulator (1496/1596)
-
8/3/2019 CE- Chapter2 AM
33/35
AN SSB TRANSMITTER
Two primary method of generating SSBsignals:
1- Filter 2- Phasing
An SSB transmitter using the filter
method
-
8/3/2019 CE- Chapter2 AM
34/35
FILTER METHOD
A crystal oscillator provides the carrier signalto be applied to the balanced modulator.
The DSB signal is passed to the band-pass
filter to selects either the upper or the lowersideband.
The filter are usually designed with a
bandwidth of approximately 2.5 to 3kHz.
-
8/3/2019 CE- Chapter2 AM
35/35
The phasing method of SSB signalgeneration
AN SSB TRANSMITTER