Chapter 4: Radio and TV Broadcasting
Undergraduate Program
School of Electrical and Computer Engineering
Superheterodyne Receiver
• Most popular type of a radio receiver so far
• Used for AM/FM and TV broadcasting, cellular and satellite
systems, radars, GPS, etc
• Main idea
• Down convert RF signal to some fixed lower or intermediate
frequency (IF)
• Then amplify it and detect
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Superheterodyne Receiver …
• RF Amplifier
• Amplifies a weak RF signal coming out of the antenna
• Rejects the image frequency
• Bandwidth is much wider than the signal bandwidth
• Mixer
• Together with the local oscillator downconverts the RF signal to the
IF frequency band
• IF amplifier
• Amplifies the IF signal significantly (up to 106) and rejects adjacent
channel signals and interference (frequency selectivity)
• Its bandwidth is the same as the signal bandwidth
• Detector (demodulator)
• Demodulates (recovers) the message signal
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Superheterodyne Receiver …
• Automatic Gain Control (AGC)
• Adjusts the IF amplifier gain according to the signal level
• Keeps the average signal amplitude almost constant
• Local oscillator
• Allows tuning the receiver to a desired channel (frequency)
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Intermediate Frequency and Image Response
• IF must be such that the image response is rejected by RF
amplifier
• Image response: the same effect as that of the desired
signal -> must be rejected !
• Image frequency
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Example: AM Broadcast Receiver
• Incoming RF Signal: fc=850 kHz IF signal: fIF = 455 kHz
• Up-side conversion: fLO = fc + fIF = 1350 kHz
• Image frequency: fimage = fLO + fIF = fc + 2fIF 1760 kHz
Note: Image rejection
is due to RF amplifier
only! IF must be high
enough to reject the
image response
On the other hand, IF
must be low enough
to provide large gain
and adjacent channel
rejection
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Radio Transmitter
• Local oscillator (LO)
• Generates the carrier
• Modulator (Mod)
• Modulates the carrier using the message signal
• Power amplifier (PA)
• Amplifies the modulated signal to required power level
• Antenna (An)
• Radiates the modulated signal as an electromagnetic wave
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Radio Transmitter: Generalized Modulator
• Generalized transmitter using quadrature generation
techniques
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Radio Transmitter: Generalized Modulator
• Generalized transmitter using the AM-PM generation
technique
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Multiplexing
• Multiplexing
• Combining a number of message signals into a composite signal to
transmit them simultaneously over a wideband channel
• Two commonly used types
• Time-division multiplexing (TDM)
• Frequency division multiplexing (FDM)
• TDM
• Transmits different message signals in different time slots (mostly
digital)
• FDM
• Transmits different message signals in different frequency slots
(bands) using different carrier frequencies
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Frequency Division Multiplexing
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Time Division Multiplexing
• Similar to FDM -> the signals are separated in time
(instead of frequency)
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