Transcript No Slide Title

Radio Broadcasting System
Transmitter
Receiver
1 What are the features of AM Radio Broadcasting ?
Different audio sources have different bandwidths “W”
AM radio limits “baseband” bandwidth W to 5kHz
FM radio uses “baseband” bandwidth W to 15kHz
AM Radio Spectrum
2
Draw a block diagram of a AM receiver and explain its
operation
Receiver Block Diagram
Antenna
RF
IF
IF
Amplifier
Mixer
Amplifier
Audio
Envelope
Amplifier
Detector
Speaker
1 Antenna
• The antenna captures electromagnetic energyits output is a small voltage or current.
• In the frequency domain, the antenna output is
Undesired Signals
0
Carrier Frequency
of desired station
Desired Signal
frequency
•
The signal from the antenna is usually very weak. Amplification
is, therefore, necessary.
2 RF Amplifier
• RF stands for radio frequency.
• RF Amplifier amplifies small signals from the
antenna to voltage levels appropriate for
transistor circuits.
• RF Amplifier also performs a bandpass filter
operation on the signal
– Bandpass filter attenuates the frequency
components outside the frequency band
containing the desired station
RF Amplifier-Frequency Domain
• Frequencies outside the desired frequency band
are attenuated
• Frequency domain representation of the output:
Undesired Signals
0
Carrier Frequency
of desired station
Desired Signal
frequency
3 IF Mixer
• The IF Mixer shifts its input in the frequency
domain from the carrier frequency to an
intermediate frequency of 455kHz:
Desired Signal
Undesired Signals
0
455 kHz
frequency
4 IF Amplifier
• The IF amplifier bandpass filters the output of
the IF Mixer, eliminating essentially all of the
undesired signals.
Desired Signal
0
455 kHz
frequency
5 Envelope Detector
• Computes the envelope of its input signal
Input Signal
Output Signal
6 Audio Amplifier
• Amplifies signal from envelope detector
• Provides power to drive the speaker
3
•
Draw a block diagram of a AM superherodyne receiver
and explain its operation.
Superhetrodyne receiver is the type used in most modern radio
and TV receivers. This receiver was designed by Armstrong
•
The first stage is a standard RF amplifier.
•
The next stage is the mixer, which accepts two inputs,
the output of the RF amplifier and a steady sine wave
from the local oscillator (LO).
The function of the mixer is to mix the AM signal with a
sine wave to generate a new set of sum and difference
frequencies. It can be shown that the mixer output is an
AM signal with a constant carrier frequency regardless of
the transmitter frequency.
•
The next stage is the intermediate-frequency (IF)
amplifier, which provides signal amplification at a fixed
frequency.
•
Following
the
IF
amplifier
stage
is
the
envelope
detector, which extracts the message signal from the
intermediate radio frequency signal.
•
A DC level proportional to the received signal's strength
is extracted from the detector stage and fed back to
the IF amplifiers and sometimes to the mixer and/or
the RF amplifier. This is the Automatic Gain Control
(AGC) level, which allows relatively constant receiver
output for widely variable received signals.
•
Output of the detector is amplified by audio amplifiers
to drive the speaker.
Frequency Conversion
•
Mixer
process.
performs
a
frequency
translation/conversion
•
Consider a 1000-kHz carrier that has been modulated by a
1-kHz sine wave (AM signal into the mixer), thus producing
side frequencies at 999 kHz and 1001 kHz.
•
Suppose that the LO input is a 1455-kHz sine wave. mixer,
being a nonlinear device, will generate the following
components:
•
Frequencies at all of the original inputs: 999 kHz, 1000
kHz, 1001 kHz, and 1455 kHz.
•
Sum and difference components of all the original
inputs: 1455 kHz ±(999 kHz, 1000 kHz, and 1001 kHz).
This means outputs at 2454 kHz, 2455 kHz, 2456 kHz,
454 kHz, 455 kHz, and 456 kHz.
•
Harmonics of all the frequency components listed in 1
and 2 and a dc component.
•
The IF amplifier has a tuned circuit that only accepts
components near 455 kHz, in this case 454 kHz, 455
kHz, and 456 kHz.
•
Since the mixer maintains the same amplitude proportion
that existed with the original AM signal input at 999
kHz, 1000 kHz, and 1001 kHz, the signal now passing
through the IF amplifiers is a replica of the original AM
signal.
•
The only difference is that now its carrier frequency is
455 kHz.
Its envelope is identical to that of the
original AM signal. A frequency conversion or translation
has occurred that has translated the carrier from 1000
kHz to 455 kHz
•
A frequency intermediate to the original carrier and
intelligence frequencies-which led to the terminology
"intermediate frequency amplifier," or IF amplifier.
Tuned-Circuit Adjustment
•
Now consider the effect of changing the tuned circuit
at the front end of the mixer to accept a station at
1600 kHz.
This means a reduction in either its
inductance or capacitance (usually the latter) to change
its center frequency from 1000 kHz to 1600 kHz.
•
The capacitance in the local oscillator's tuned circuit is
simultaneously
reduced
so
that
oscillation goes up by 600 kHz.
its
frequency
of
•
The mixer's output still contains a component at 455
kHz (among others), as in the previous case when we
were tuned to a 1000-kHz station.
Of course, the
other frequency components at the output of the mixer
are not accepted by the frequency selective circuits in
the IF amplifiers.
•
Thus, the key to superheterodyne operation is to make
the LO frequency "track" with the circuit or circuits
that are tuning the incoming radio signal such that their
difference is a constant frequency (the IF).
•
For a 455-kHz IF frequency, the most common case for
broadcast AM receivers, this means the LO should
always be at a frequency 455 kHz ABOVE the incoming
carrier frequency.
•
The receiver's "front-end" tuned circuits are usually
made to track together by mechanically linking (ganging)
the capacitors in these circuits on a common variable
rotor assembly.
Image Frequency
•
•
•
•
•
•
•
Example: Incoming carrier frequency
1000 kHz,
Local oscillator = 1000+455=1455 kHz
Consider another carrier at 1910 kHz
If this is passed through the same oscillator, will have a 19101455=455 kHz component
Therefore, both carriers will be passed through IF amplifie
RF filter should be designed to eliminate image signals
The frequency difference between a carrier and its image signal
is:
2 fIF
•
RF filter doesn’t have to be selective for adjacent stations, have to
be selective for image signals
Therefore,
T
RF
IF
B  B 2f
Example
2
Question:
Determine the image frequency for a standard
broadcast band receiver using a 455-kHz IF and tuned to a
station at 620 kHz.
•
The first step is to determine the frequency of the LO
•
The LO frequency minus the desired station's frequency of 620
kHz should equal the IF of 455 kHz.
Hence,
fLO - 620 kHz = 455 kHz
fLO = 620 kHz + 455 kHz
fLO = 1075 kHz.
Now determine what other frequency, when mixed with 1075
kHz, yields an output component at 455 kHz.
X - 1075 kHz = 455 kHz
X = 1075 kHz + 455 kHz
•
Thus, 1530 kHz is the image frequency in this situation.
Automatic Gain Control (AGC)
•
The AGC help to maintain a constant output voltage level over a wide
range of RF input signal levels.
•
Tuning the receiver would be a nightmare. So as to not miss the weak
stations, you would have the volume control (in the non-AGC set) turned
way up. As you tune into a strong station, you would probably blow out
your speaker while a weak station may not be audible.
•
The received signal from the tuned station is constantly changing as a
result of changing weather and atmospheric conditions. The AGC
allows you to listen to a station without adjusting the volume control.
4
•
Compare AM radio broadcasting with FM
Broadcasting
FM radio stations have better quality sound than AM radio stations.
Reasons
1 Noise immunity introduced by the non-linear modulation.
2 Bandwidth of FM stations are 15kHz, whereas AM stations are
only 5kHz.
•
FM receivers can have aerials (antennas) which are half the
wavelength of the transmitted carrier (due to the higher frequency of
operation). This allows more signal power to be received than the
AM.
FM Transmitter
FM signal in Time Domain
FM Radio
• The FM band extends from 88 to 108 MHz.
• The maximum information frequency fm is specified as 15 kHz.
(high fidelity)
• The minimum bandwidth is to be at least 200 kHz (0.2 MHz).
• Therefore, carrier frequencies are separated by 200 kHz.
5 Explain the operation of the FM Superheterodyne
Receiver.
•
The FM Superheterodyne Receiver has many similarities to that of
the AM Superheterodyne receiver.
•
The only apparent differences are the use of the presence of
Limiter-discriminator circuit in place of envelope detector
and
the addition of a de-emphasis network
•
RF stage, mixer, local oscillator, and IF amplifiers are basically
similar to those discussed for AM receivers and do not require further
elaboration.
•
The universally standard IF frequency for FM is 10.7 MHz, as
compared to 455 kHz for AM.
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A limiter is a circuit whose output is a constant amplitude for all
inputs above a critical value. Its function in a FM receiver is to
remove any unwanted amplitude variations due to noise.
AGC
•
In addition to the limiting function also provides AGC action, since
signals from the critical minimum value up to some maximum value
provide a constant input level to the detector.
FM discriminator
•
The FM discriminator (detector) extracts the intelligence that has
been modulated onto the carrier via frequency variations.
•
It should provide an intelligence signal whose amplitude is dependent
on instantaneous carrier frequency deviation.
•
the response is linear in the allowed area of frequency deviation and
that the output amplitude is directly proportional to carrier frequency
deviation.
Pre-emphasis and De-emphasis.
•
Despite the fact that FM has superior noise rejection qualities, noise
still interferes with an FM signal. This is particularly true for the
high-frequency components in the modulating signal.
•
These high frequencies can at times be larger in amplitude than the
high-frequency content of the modulating signal. This causes a form
of frequency distortion that can make the signal unintelligible.
•
To overcome this problem Most FM system use a technique known
as Pre-emphasis and De-emphasis.
•
At the transmitter the modulating signal is passing through a simple
network which amplifies the high frequency component more the
low-frequency component.
•
The simplest form of such circuit is a simple high pass filter.
•
To return the frequency response to its normal level, a de-emphasis
circuit is used at the receiver.
•
This is a simple low-pass filter
•
The de-emphasis circuit provides a normal frequency response.
•
The combined effect of pre-emphasis and de-emphasis is to increase
the high-frequency components during the transmission so that they
will be stronger and not masked by noise.
•
This improves the signal-to-noise ratio.
6 Briefly explain the operation of a FM Stereo
Broadcasting system
•
All new FM broadcast receivers are being built with
provision for receiving stereo, or two-channel broadcasts.
•
The left (L) and right (R) channel signals from the program
material are combined to form two different signals, one of
which is the left-plus-right signal and one of which is the
left-minus-right signal
• An ordinary mono signal consists of the summation of the
two channels, i.e. L + R.
• If a signal containing the difference between the left and
right channels ( L - R) is transmitted then it is possible to
reconstitute the left (L) and right (R) signals.
• Adding (L + R) + (L - R) gives 2L i.e. left signal and
subtracting (L + R) - (L - R) gives 2R, i.e. the right signal.
•
The (L - R) signal is double-sideband suppressed carrier
(DSBSC) modulated about a carrier frequency of 38 kHz,
with the LSB in the 23 to 38 kHz slot and the USB in the
38 to 53kHz slot.
•
The (L + R) signal is placed directly in the 0 to 15 kHz slot,
and a pilot carrier at 19 kHz is added to synchronize the
demodulator at the receiver.
FM Stereo Transmitter
FM Stereo Receiver
•
The output from the FM detector is a composite audio
signal containing the frequency-multiplexed (L + R) and (L R) signals and the 19-kHz pilot tone. This composite signal
is applied directly to the input of the decoding matrix.
•
The composite audio signal is also applied to one input of a
phase-error detector circuit, which is part of a phase
locked loop 38-kHz oscillator.
•
The output drives the 38-kHz voltage-controlled oscillator,
whose output provides the synchronous carrier for the
demodulator.
•
The oscillator output is also frequency divided by 2 (in a
counter circuit) and applied to the other input of the phase
comparator to close the phase locked loop.
•
The phase-error signal is also passed to a Schmitt trigger
circuit, which drives an indicator lamp on the panel that
lights when the error signal goes to zero, indicating the
presence of a synchronizing input signal (the 19-kHz pilot
tone).
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The outputs from the 38-kHz oscillator and the filtered
composite audio signals are applied to the balanced
demodulator, whose output is the (L - R) channel.
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The (L + R) and (L - R) signals are passed through a matrix
circuit that separates the L and R signals from each other.
•
These are passed through de-emphasis networks and lowpass
filters
to
remove
unwanted
high-frequency
components and are then passed to the two channel audio
amplifiers and speakers.
•
On reception of a monaural signal, the pilot-tone indicator
circuit goes off, indicating the absence of pilot tone, and
closes the switch to disable the (L - R) input to the matrix.
•
The (L + R) signal is passed through the matrix to both
outputs. An ordinary monaural receiver tuned to a stereo
signal would produce only the (L + R) signal, since all
frequencies above 15 kHz are removed by filtering, and no
demodulator circuitry is present. Thus the stereo signal is
compatible with the monaural receivers.