Transcript Analog Communication

Analog Communication
Presented by
Dr.J.L Mazher Iqbal
FM and PM signals.
(a) Carrier. (b) Modulating signal. (c) FM signal. (d) PM signal
Phase modulation of a carrier
Modulation Index
and Sidebands
Modulation Index
and Sidebands
Modulation Index
and Sidebands
FM Signal Bandwidth
• Example:
If the highest modulating frequency is 3 kHz and the
maximum deviation is 6 kHz, what is the modulation
index?
mf = 6 kHz/3 kHz = 2
What is the bandwidth?
BW = 2fmN
Where N is the number of significant* sidebands
BW = 2(3 kHz)(4) = 24 kHz
*Significant
sidebands are those that have an amplitude of greater than 1% (.01) in the
Bessel table.
Noise-Suppression Effects of FM
• Noise is interference generated by lightning,
motors, automotive ignition systems, and power
line switching that produces transient signals.
• Noise is typically narrow spikes of voltage with
high frequencies.
• Noise (voltage spikes) add to a signal and
interfere with it.
• Some noise completely obliterates signal
information.
Continued
• FM signals have a constant modulated carrier
amplitude.
• FM receivers contain limiter circuits that
deliberately restrict the amplitude of the received
signal.
• Any amplitude variations occurring on the FM
signal are effectively clipped by limiter circuits.
• This amplitude clipping does not affect the
information content of the FM signal, since it is
contained solely within the frequency variations
of the carrier.
Continued
• FM signals have a constant modulated carrier
amplitude.
• FM receivers contain limiter circuits that
deliberately restrict the amplitude of the received
signal.
• Any amplitude variations occurring on the FM
signal are effectively clipped by limiter circuits.
• This amplitude clipping does not affect the
information content of the FM signal, since it is
contained solely within the frequency variations
of the carrier.
Continued
Fig. 1 An FM signal with noise
Nonlinear Distortion and Limiter
Nonlinear system to reduce envelop variations (AM)
Hard Limiter
• FM with unwanted amplitude variation A(t). Those variation
can be flattened out by an ideal hard limiter or clipper.
• Clipper circuit uses a comparator or high gain operational
amplifier such that any input voltages greater or less than
zero cause the output voltage to reach either positive or
negative power supply rails.
• Clipper circuit employing back to back Zener diode with break
down voltage at the output of a high gain amplifier.
Hard Limiter
Continued
The coefficients are then found from an
Continued
Continued
The Limiter plus BPF in (a) removes unwanted
amplitude variation from AM or PM wave and would
be used in receiver.
Deemphasis
• Noise can interfere with an FM signal and
particularly with the high-frequency components
of the modulating signal.
• Noise is primarily sharp spikes of energy and
contains a lot of harmonics and other highfrequency components.
• Detected FM interference is most severe at large
value of fi.
• Post detected filtering improves the performance
of the system.
Continued
Figure Deemphasis circuit
Continued
deemphasis
– A simple low-pass filter can operate as a deemphasis
circuit in a receiver.
– Demodulator is followed by a low pass filter having a
amplitude ratio that begin to decrease below W; this
will de-emphasis the high frequency portion of the
message band there by reduce the more serious
interference.
– A deemphasis circuit returns the frequency response
to its normal flat level.
Preemphasis
• Obviously de-emphasis filtering also attenuate
the high frequency component of the message
itself.
• But it is simple matter to compensate for
deemphasis distortion by preemphasizing the
modulating signal at the transmitter before
modulation.
Continued
– To overcome high-frequency noise, a technique
known as preemphasis is used.
– A simple high-pass filter can serve as a
transmitter’s pre-emphasis circuit.
– Pre-emphasis provides more amplification of only
high-frequency components.
Continued
Figure 2.Preemphasis and deemphasis. (a) Preemphasis circuit.
Filter frequency response
Filter function
deemphasis
Preemphasis
Continued
• The combined effect of preemphasis and deemphasis
is to increase the signal-to-noise ratio for the highfrequency components during transmission so that
they will be stronger and not masked by noise.
Capture Effect
• In referring to radio, capture effect, or FM
capture effect, is a phenomenon associated with
FM reception wherein only the stronger of two
signals at, or near, the same frequency will be
demodulated.
• Capture effect is defined as the suppression of
the weaker signal at the receiver limiter; where
the weaker signal is not amplified, but
attenuated.
Continued
Demodulated Signal with Interference
The presence of Фi(t) in the above equation indicates
potentially intelligent interference (or cross talk)
Continued
Contiued
• When more than one signal is nearly equal in
strength, or the signals are fading
independently, the receiver may switch from
one signal to the other and exhibit picket
fencing.
Picket Fencing
• Picket fencing is slang for the chopping effect
sometimes heard by cell phone users at the edge of a
cell's coverage area, or (more likely) by the landline
user to whom the cell phone is connected.
• "Picket fencing" refers to the way portions of speech
are stripped from the conversation, as if the listener
was walking by a picket fence, and hearing a
conversation on the other side that changes audibly
depending on the position of the pickets relative to the
listener.
How do we measure it?
Capture Effect is measured as the lowest ratio
of the power of two signals that will result in
the suppression of the smaller signal.