Chapter Four
Topics discussed in this section:
AM radio transmitters
FM transmitters
Receiver & Performance parameters
AM receivers
FM receivers
Transceivers
4.1
AM transmitter
t requirements
• Transmitter allows transmission of audio to an AM radio. It
consists of an RF oscillator operating in the AM broadcast band,
together with a modulator stage, which mixes the incoming audio
and the RF. A signal appears on the output, which has an AM
component that can be picked up on a nearby AM radio receiver.
• A transmitter must generate a signal with the following
criteria:
- The correct modulation type
- Must have sufficient power
- Must generate at the correct carrier frequency
- Should be reasonably efficient
4.2
AM Transmitter
Block diagram
In Modulator the audio modulates the RF amplitude
4.3
SSB Transmitter
Block diagram
Modulator produces double-sideband suppressed-carrier
Sideband filter suppresses unwanted sideband
4.4
FM Transmitter
Block diagram
Audio used to modulate frequency of RF oscillator
4.5
FM stereo transmitter
4.6
Cont’d..
The FCC has assigned a frequency range 88 to 108MHz for FM
broadcasting, with a separation of 200KHz between adjacent
stations and a peak frequency deviation ∆f=75KHz.
Stereophonic FM broadcasting, in which two audio signal L(left
microphone) and R (right microphone) are used for more natural
effect.
Let assume the two message signal can have a signal frequency
of 500kHz and 5KHz
4.7
Cont’d..
4.8
Cont’d..
• It is possible to increase the out put of SNR by deliberately
distortion of the transmitted signal (Preemphasis ) and the
corresponding compensation (deemphasis) at the receiver.
•
4.9
Cont’d..
• Preemphasis is a high pass filter with amplifier circuit. The HPF
passes only high frequency component of the signal. The amplifier
boosts it up so as the noise in the channel has less effect on it.
• At the receiver the noise is deemphasised( attenuated) along with
the signal. its is a lowpass filter.
• As a result the signal power is restored and the noise power is
reduced
4.10
6.11
Radio Receivers
Radio receiver is an electronic equipment which pick ups the
desired signal, reject the unwanted signal and demodulate the
carrier signal to get back the original modulating signal.
Functions of radio receivers
•Select desired signal and reject unwanted signal
• Amplify the R.F. signal
• Demodulate the selected signal
4.12
Types of receivers
1.TRF (Tuned Radio frequency) receiver
Drawbacks of TRF Receiver
TRF receivers are simple to design and allow the broadcast
frequency 535 KHz to 1640 KHz. But at the higher frequency,
it produces difficulty in design.
It has poor audio quality.
4.13
Cont’d..
2.Super Heterodyne Receiver
•The shortcomings of the TRF receiver are overcome by the
invention of the super heterodyne receiver.
• A super heterodyne receiver converts all incoming radio
frequency (RF) signals to a lower frequency known as an
intermediate frequency (IF).
4.14
Cont’d..
This IF is constant, for AM receivers it is 456 to 465 KHz and
for FM receivers it is 9 MHz to 12 MHz.
The output of mixer will produce sum and difference
frequencies:
For all incoming fs IF is constant and constant frequency
difference is maintained between local oscillator and RF circuit
through capacitor tuning in which all capacitors are connected
together and operated by one control nob. It is called ganged
tuning.
4.15
Cont’d..
=
• Automatic Frequency Control (AFC) and Automatic Gain Control
(AGC) in Superheterodyne Receiver
AGC or AVC (Automatic Volume Control) is a system by means
of which the overall gain of radio receiver is varied
automatically with the variations in the strength of received
signals, to maintain the output constant.
AFC circuit is used to adjust and stabilize the frequency of
local oscillator.
4.16
Receiver Parameters
Important performance measures for receivers
Selectivity
Bandwidth
Sensitivity
Fidelity
Dynamic range
Signal-to-noise ratio (SNR)
Frequency stability
4.17
Cont’d..
Selectivity:-
- is the ability of a receiver to separate the wanted signal from
nearby unwanted signals (other stations)
4.18
Cont’d..
Selectivity is obtained by using tuned circuits, which are tuned to
desired frequency. The quality factor of these LC circuits
determines the selectivity. It is given by,
For better selectivity ‘Q’ should be high.
4.19
Cont’d..
Bandwidth
Band of frequencies which the receiver should accept
SSB uses 2.5 to 3kHz
AM uses typically 560 or 1600kHz
Usually 3dB BW specified, but not always!
4.20
Cont’d..
Sensitivity:-
Ability to amplify weak signals. Broadcast receivers/ radio
receivers should have reasonably high sensitivity so that it may
have good response to the desired signal but should not have
excessively high sensitivity otherwise it will pick up all undesired
noise signals. It is function of receiver gain and measures in
decibels.
• Sensitivity is determined by 2 factors:
Bandwidth of the receiver
- the wider the bandwidth, the more noise power it lets in
Noise figure of the receiver front-end
-a noisy receiver needs more signal to overcome the noise
4.21
Cont’d..
Fidelity:-
1. radio receiver should have high fidelity or accuracy.
2. it is determined by the high frequency response. Therefore it
should have high frequency response over entire audio frequency
range.
3. Ex. In an A.M. broadcast the maximum audio frequency is 5 KHz
hence receiver with good fidelity must produce entire frequency
up to 5KHz.
4.22
Cont’d..
Dynamic range is the range of signal levels between the smallest
and greatest a receiver can handle
Lower limit set by sensitivity
Upper limit set by distortion or AGC control range
In practice, we are more concerned about dynamic range to
handle unwanted out-of-band signals (AGC doesn’t apply).
How large an unwanted signal will it reject without affecting
sensitivity to wanted signals
Overload level may be specified for receiver front-end (RF
amplifier, mixer) as the 1dB compression point
4.23
Frequency Conversion Process
4.24
Image Frequency
Definition: In radio reception using heterodyning in the tuning
process, an undesired input frequency that is capable of
producing the same intermediate frequency (IF) that the
desired input frequency produces.
It is given by signal frequency plus twice the intermediate
frequency
4.25
Cont’d..
Image frequency is also received
Image frequency not a problem.
4.26
Image Frequency Rejection
The rejection of an image frequency by a single tuned circuit
is the ratio of the gain at the signal frequency to the gain at
the image frequency. It is given by,
Where,
4.27
Tracking
The process of tuning circuit to get the desired
output is called tracking.
Any error that exists in the frequency difference will
result in an incorrect frequency being fed to the IF
amplifier. Such errors are known as tracking errors.
To avoid tracking errors ganged capacitors are used.
4.28
AM Radio Receiver
6.29
Block Diagram of FM Receiver
6.30
FM Receivers
FM receivers, like AM receivers, utilize the super
heterodyne principle, but they operate at much higher
frequencies (88 - 108 MHz).
A limiter is often used to ensure the received signal is
constant in amplitude before it enters the discriminator
or detector.
6.31
FM stereo receiver
4.32
Transceiver
4.33
In the transceiver, the transmitter and receiver parts are
found at the same station. Here transmission and reception
is simultaneously processed.
When there is signal transmission the transponder will
processes the transmitter, at the same time there is also
signal reception at the receiver.
6.34
