Transcript AM Principles_Lecture2

Chapter 3
Amplitude Modulation
Circuits
AMPLITUDE MODULATORS
• AM is the process of multiplying
the carrier by the modulating
signal.
• Two ways:multiply with gainor
attenuation factor that varies with
modulating signal or linearly mix
/algebraically ad the carrier and the
modulating signal,then apply the
composite signal to a non liner
device
Cont’d
• Non-linear mixing is another name
for AM.
• An amplitude modulator must
multiply analog signals or mix them
together in a non-linear device like a
diode.
DIODE
MODULATOR
Information
Carrier
Diode provides
multiplication
Envelope matches
information signal
AM signal
HOW THE
DIODE MODULATOR WORKS
• The resistive network
adds the carrier and
modulating signals.
• The diode rectifies the
result producing nonlinear mixing.
• The parallel tuned
circuit selects the
carrier and sidebands
and rejects the
modulating signal.
DIFFERENTIAL AMPLIFIER
MODULATOR
• A differential amplifier makes a good
amplitude modulator. The modulating
signal is used to vary the common
mode current in the transistors that
amplify the carrier.
• Most integrated circuit (IC) amplitude
modulators are based upon the
differential amplifier modulator.
DIFFERENTIAL AMPLIFIER(CONT’D)
• ADVANTAGES:
• MAKES EXCELLENT AMPLITUDE
MODULATOR.
• HIGH GAIN AND GOOD LINEARITY,
CAN BE MODULATED 100%
PIN DIODE MODULATOR
• A PIN diode acts as a voltage variable
resistor at very high frequencies.
• PIN diodes are special diodes made to be
used for frequencies above 100MHz
• An amplitude modulator can be made with
PIN diodes if the carrier frequency is VHF,
UHF, or microwaves.
• The PIN diodes are connected as resistive
attenuators for the carrier. The attenuation
is varied by the modulating signal
producing AM.
PIN DIODE MODULATOR
• Disadvantages:
-produces attenuation, thus needs of
amplifying the signal back before ready
for transmission.
-but widely used, because the only
modulator for microwave frequencies.
AM LEVEL
• Low level AM produces the AM signal
at a very low power level. High
power amplifiers increase the power
to the desired level. Less efficient
linear amplifiers must be used to
amplify the AM signal.
• High level AM is produced by
amplitude modulating the final
amplifier stage in a transmitter. More
efficient class C amplifiers can be
used.
AM(HIGH LEVEL)
• Produce best type of AM, but needs
extremely high power modulator
circuit.
• For 100% modulation, the modulator
must have at least half the total power
of class c amplifier. Say if class C
amplifier has input power of 1000W, the
modulator must be able to deliver 500W
AM DEMODULATOR
• A demodulator is a circuit that recovers
the original information signal from the
carrier that is received.
• The simplest AM demodulator is a half
wave rectifier circuit called a diode
detector.
• The diode detector is also the simplest AM
receiver. It is called a crystal radio.
AM DIODE DEMODULATOR
(DETECTOR)
Information out
AM signal in
Diode current
(no filter)
Filtered waveform
BALANCED MODULATOR
• A balanced modulator is an amplitude
modulator that suppresses the carrier but
produces both upper and lower
sidebands.
• A balanced modulator produces DSB.
• Diode ring or diode lattice modulators are
widely used.
• IC balanced modulators based upon
differential amplifiers are very popular.
DIODE LATTICE BALANCED MODULATOR
Modulating
signal
DSB
output
Carrier oscillator
When When
the modulating
signal signal
is 0, there
is no
the modulating
is non-zero,
outputthe
since
the currents
in the output
diodes
are unbalanced
and sotransformer
are the
are equal
and currents
opposite which
(flux cancellation).
carrier
produce an output.
GENERATING SSB
• A balanced modulator eliminates
the carrier and provides DSB.
• A filter removes the undesired
sideband producing SSB.
• Quartz crystal filters are the most
widely used sideband filters since
they are very selective and
inexpensive.
TYPICAL SSB
TRANSMITTER
Antenna
DSB
signal
Carrier
oscillator
Balanced
modulator
SSB
signal
Sideband
filter
Linear
amplifier
Microphone
Audio
amplifier
Filter
response
curve
Lower fC Upper
sidebands sidebands
THE PHASING METHOD OF SSB
• Another way to produce SSB uses
a phase shift method to eliminate
one sideband.
• Two balanced modulators driven by
carriers and modulating signals 90º
out of phase produce DSB.
• Adding the two DSB signals
together results in one sideband
being cancelled out.
DEMODULATING SSB AND DSB
• The basic diode detector will not recover the
information from a DSB or SSB signal.
• A balanced modulator is used to recover the
SSB or DSB signal.
• A SSB/DSB demodulator is often called a
product detector.
• The incoming SSB or DSB signal is mixed with
the carrier generated in the receiver to
reproduce the original modulating signal.
DSB SIGNAL WITH
SINE WAVE MODULATION
Note: the envelope is not a sine wave
The basic diode detector will produce this waveform
which is not the original information signal.
TYPICAL DSB
OR SSB DETECTOR
Carrier
oscillator
Also called a
product detector
Balanced
modulator
Information out
DSB (or SSB) signal
Example of how product detector
works..
• Carrier=9MHz
• Modulating signal=2kHz=0.002MHz
• After modulation, and SSB generated, only
upper sideband=9.002MHz transmitted.
• In receiver, add with local oscillator at
9MHz, output is sum and
difference:18.002MHz, and
0.002MHz=modulating signal.
• Use filter to filter everything else