radio broadcasting36
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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
Receiver Block DiagramReceiver Block Diagram
RF
Amplifier
IF
Mixer
IF
Amplifier
Envelope
Detector
Audio
Amplifier
Antenna
Speaker
2 Draw a block diagram of a AM receiver and explain its operation
1 Antenna1 Antenna
• The antenna captures electromagnetic energy-its output is a small voltage or current.
• In the frequency domain, the antenna output is
0 frequency
Undesired SignalsDesired Signal
Carrier Frequencyof desired station
2 RF2 RF AmplifierAmplifier
• 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 DomainRF Amplifier-Frequency Domain
• Frequencies outside the desired frequency bandare attenuated
• Frequency domain representation of the output:
0 frequency
Undesired SignalsDesired Signal
Carrier Frequencyof desired station
• The IF Mixer shifts its input in the frequency domain from the carrier frequency to an intermediate frequency of 455kHz:
3 IF3 IF MixerMixer
0 frequency
Undesired Signals
Desired Signal
455 kHz
• The IF amplifier bandpass filters the output of the IF Mixer, eliminating essentially all of the undesired signals.
4 IF4 IF AmplifierAmplifier
0 frequency
Desired Signal
455 kHz
5 Envelope5 Envelope DetectorDetector• Computes the envelope of its input signal
Input Signal
Output Signal
6 Audio6 Audio AmplifierAmplifier
• Amplifies signal from envelope detector
• Provides power to drive the speaker
• Superhetrodyne receiver is the type used in most modern radio and TV receivers. This receiver was designed by Armstrong
3 Draw a block diagram of a AM superherodyne receiver and explain its operation.
• 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.
• 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 its
frequency of oscillation goes up by 600 kHz.
• 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 1910-
1455=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:
• RF filter doesn’t have to be selective for adjacent stations, have to be selective for image signals
Therefore,
IFf2
IFRFT fBB 2
Example 2Question: 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.
• 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.
4 Compare AM radio broadcasting with FM Broadcasting
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.
• 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 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).
• 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.
• 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
low-pass 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.