Lesson 6
RADIO RECEIVERS
Basic Functions of a Receiver
➢ The receiver should be capable of changing the range of frequencies to be
received
➢ The receiver should be able to detect and convert the received RF signal
to the original source information
➢ The receiver should be able to provide sufficient amplification
6.1 Receiver Parameters
Selectivity
➢ A measure of the ability of a receiver to accept a given band of
frequencies and to reject all others
➢ A measure of the extent to which a receiver is capable of differentiating
between the desired information signals and the disturbances or
information signal at other frequencies
➢ Can be expressed as a bandwidth and as the ratio of the bandwidth at the
receiver at some predetermined attenuation factor (commonly –60dB) to
the bandwidth at the –3dB (half-power) points
SF (Shape Factor) =
0
0
0
0
BW-60dB
BW- 3dB
Selectivity = SF  100
BW
Selectivity = BW actual  100
minimum
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LESSON 6 Radio Receivers
Bandwidth Improvement
➢ The noise reduction ratio achieved by reducing the bandwidth
BI =
Noise Figure Improvement
BWRF
BWIF
NFimprovement = 10 log BI
Sensitivity
➢ The minimum RF signal level that can be detected at the input to the
receiver and still produce a usable demodulated information signal
Dynamic Range
➢ The difference in decibels between the minimum input level necessary
to discern a signal and the input level that will overdrive the receiver
and produce distortion
➢ The range of input power over which the receiver is useful
DR = 10 log
Pmax
Pmin
Fidelity
➢ A measure of the ability of a communications system to produce, at
the output of the receiver, an exact replica of the original source
information
Insertion Loss
➢ The ratio of the power transferred to the load to the power at the
source
IL = 10 log
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Pin
Pout
LESSON 6 Radio Receivers
6.2 Types of Radio Receivers
➢ Tuned Radio Frequency Receiver
Advantages
▪
Enhanced sensitivity
▪
Simple circuitry
Disadvantages
▪
Poor selectivity
▪
Instability
▪
Requires multistage tuning
➢ Superheterodyne Receiver
Heterodyning means mixing two frequencies together in a non-linear
device.
Advantages
▪
Good selectivity
▪
Sufficient image frequency rejection
Disadvantages
▪
Moderate sensitivity (relatively poor compared to the TRF)
▪
Complex circuitry
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LESSON 6 Radio Receivers
6.3 Sections of a Superheterodyne Receiver
RF Section
➢ Generally consists of a preselector and an amplifier stage
Preselector
➢ Provides enough initial bandlimiting to prevent a specific unwanted radio
frequency (image frequency) from entering the receiver
RF Amplifier
➢ Determines the sensitivity of the receiver
Advantages of including RF amplifiers in a Receiver
▪
Greater gain, thus better sensitivity
▪
Improved image-frequency rejection
▪
Better signal-to-noise ratio
▪
Better selectivity
104
LESSON 6 Radio Receivers
Mixer/Converter Section
➢ Includes a local oscillator and a radio-frequency oscillator stage.
Heterodyning takes place in the mixer stage and the radio-frequencies are
down-converted to intermediate frequencies (IF).
fs
fs+fo
MIXER
fs-fo
(IF)
fo
Image Frequency, fsi
▪
The frequency other than the desired which when mixed with the
local oscillator will produce the same intermediate frequency
fs
fsi
MIXER
(IF)
fo
f si = f s  2 f i
Where:
fs
=
desired signal frequency, (Hz)
fi
=
intermediate frequency, (Hz)
fsi
=
image frequency, (Hz)
fo
=
oscillator frequency, (Hz)
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LESSON 6 Radio Receivers
Image Frequency Rejection Ratio
▪
The ratio of the gain at the signal frequency to the gain at the
image frequency
IFRR = 1 + Q 2  2
IFRRdB = 20 log IFRR
where:
Q = quality factor of the tuned circuit.
=
f si − f s
f si
fs
IF Section
➢ Consists of a series of IF amplifiers and bandpass filters and is often called
the IF strip. Most of the receiver gain and selectivity is achieved in this
section
Detector
➢ Converts the IF signal back to the original source information
Audio Section
➢ Comprises several cascaded audio amplifiers and one or more speakers
6.4 Single Sideband Receivers
Beat Frequency Oscillator (BFO)
▪
Produces a frequency input to beat (mix) with the IF signal and results in
a difference and sum frequencies with the result being the original audio
▪
Should have at least the tuning range of 1.5 kHz above and below the
center of the IF passband
106
LESSON 6 Radio Receivers
➢ Non-Coherent BFO SSB Receiver
➢ Coherent BFO SSB Receiver
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LESSON 6 Radio Receivers
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