COMMUNICATION SYSTEM EECB353
Chapter 2 Part IV
AMPLITUDE MODULATION
Dept of Electrical Engineering
Universiti Tenaga Nasional
Superhetrodyne Receiver
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Superhetrodyn receivers convert all incoming signals to
a lower frequency, known as the intermediate
frequency (IF), at which a single set of amplifiers is
used to provide a fixed level of selectivity and
sensitivity.
The key circuit is the mixer – act as a simple amplitude
modulator to produce sum and difference frequencies.
Heterodyne means to mix two frequencies together in a
nonlinear device or to translate one frequency to
another using nonlinear mixing.
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Superhetrodyne Receiver
AM Non-Coherent Superhetrodyne Receiver Block Diagram
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Superhetrodyne Receiver
fLO should always be at a freq 455kHz
above the incoming carrier freq., fc
When the freq of LO is tuned above RF : High side injection  fLO = fRF + fIF
When the freq of LO is tuned below RF : Low side injection  fLO = fRF - fIF
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Superhetrodyne Receiver
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RF Section
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Consist of pre-selector and RF amplifier
Pre-selector is a broad-tuned BPF with an adjustable centre
frequency that is tuned to the desired carrier frequency. Main
purpose of the pre-selector is by selectivity, it accepts only the
desired frequency or prevent unwanted radio frequency (Image
Frequency) from entering the receiver and all others are
rejected.
RF Amplifier – to amplify the signal before entering the mixer
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Superhetrodyne Receiver
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Mixer/Converter Section
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This section includes a radio frequency oscillator stage (LO) and
a mixer/converter stage (the first detector) which is a nonlinear
device used to convert RF’s to IF’s (RF to IF frequency
translation). IF signal in commercial AM broadcast are between
450kHz and 460kHz. The most common IF used is 455kHz.
Reason to translate RF ->IF is to obtain adequate selectivity.
-RF signal are down converted to IF.
-Shape of the envelope remains the
same, although the carrier and SBs
freq are translated from RF->IF, the
original info contained in the
envelope remain unchanged.
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Superhetrodyne Receiver
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Mixer/Converter Section
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LO is always maintained at a frequency which is higher, by a
fixed amount, than the incoming RF signals.
The mixer in the receiver combines the signal from the RF
amplifier and the frequency input from the local oscillator to
produce three frequencies:
 A ‘difference’ frequency of local oscillator frequency - RF
signal frequency.
 A ‘sum’ frequency equal to local oscillator frequency + RF
signal frequency.
 A component at the local oscillator frequency.
Mixing two signals to produce such components is called a
‘heterodyne’ process.
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Frequency Conversion
Concept of a mixer.
Frequency Conversion Process
Difference component:
1455-(999,1000,1001)kHz
All original inputs
Sum component:
1455+(999,1000,1001)kHz
Consider a situation shown above.
The AM signal into the mixer is a 1000kHz carrier that has been modulated by a 1kHz
sinewave, thus producing side freqs at 999 kHz and 1001 kHz.
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Frequency Conversion Process
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The IF amplifier has a tuned circuit that accepts
components only near 455kHz, in this case 454kHz,
455kHz and 456kHz.
Since the mixer maintains the same amplitude
proportion that existed with the original AM signal input
at 999kHz, 1000kHz and 1001kHz, the signal now passing
through the IF amplifiers is a replica of the original AM
signal. The only difference is that now its carrier freq is
455kHz.
A frequency conversion has occurred that has translated
the carrier from 1000kHz to 455kHz – a frequency
intermediate to the original carrier and intelligence
frequencies.
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Mixer Output
460kHz
455kHz
450kHz
545kHz
550kHz
555kHz
1005kHz
1550kHz
1555kHz
1560kHz
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Superhetrodyne Receiver
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How LO tracks the RF amplifier so that the difference between the
two frequencies is maintained at a constant value?
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Example – the radio is tuned to received a broadcast station which
transmit at 800kHz.
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The LO will be running at 1255khz. The difference frequency is
1255kHz – 800kHz = 455kHz.
A Superhetrodyne Receiver Tuned to 800kHz
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Superhetrodyne Receiver
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Similarly if the radio is tuned to received a station transmitted at
700kHz
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The LO will be running at 1155kHz and the difference is still
maintained at the required 455kHz.
This frequency difference therefore remain constant regardless of the
frequency to which the radio is actually tuned and is called the
intermediate frequency (IF).
When the freq of LO is tuned above RF : High side injection  fLO = fRF + fIF
When the freq of LO is tuned below RF : Low side injection  fLO = fRF - fIF
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Example 1
AM superhetrodyne receiver using high-side
injection, with LO freq of 1355kHz. Determine
the IF carrier, upper side freq and lower side
freq for an RF wave that is made up of a carrier
and upper and lower side freq of 900kHz,
905kHz and 895kHz respectively.
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Image Frequency
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Previously, for a 700kHz broadcast station, the LO of 1155kHz
giving the difference (IF) frequency of the required 455kHz.
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What happen if we were to receive another station broadcasting
on a frequency of 1610kHz?
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This would also mix the LO freq of 1155kHz to produce the IF freq of
455kHz. i.e this station also received the same time as one at 700kHz.
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Station 1: Frequency 700kHz, LO 1155kHz, IF = 455kHz
Station 2: Frequency 1610kHz, LO 1155kHz, IF = 455kHz
An Image Frequency is an unwanted frequency that can also
combine with the LO output to create the IF frequency.
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Image Frequency (fim)
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An image freq is any freq other than the selected radio freq carrier
that if allowed to enter a receiver and mix with the LO, will
produce a cross-product freq that is equal to the IF.
An image freq is equivalent to a second RF that will produce an IF
that will interfere with the IF from the desired RF.
Once an image freq has been mixed down to IF, it cannot be
filtered out or suppressed. For RF to produce a cross product equal
to the IF, it must be displaced from the LO freq by a value equal to
the IF:
f im  f LO  f IF
 For high-side injection:
 Giving:
f
RF 
f IF  f LO
f im  f RF  2 f IF
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Image Frequency (fim)
Figure : Relative Frequency Spectrum for the RF, IF, LO and Image
Frequencies for a superhetrodyne receiver using High-side Injection
Example 2- Determine the image frequency for a
standard broadcast AM receiver tuned to a station at
1320kHz.
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Example 3
For a citizen band receiver using high side injection
with an RF carrier of 27 MHz and IF center frequency
of 455 kHz, determine :
i- Local oscillator frequency
ii- Image frequency
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Superhetrodyne Receiver
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IF Section
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Consists of a series of IF amplifiers and BPFs
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These stages contain most of the amplification in the receiver as
well as the filtering that enables signals on one frequency to be
separated from those on the next.
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Most of the receiver gain and selectivity is achieved in the IF
section.
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Detector Section
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Convert the IF signals back to the original source information.
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Audio Amplifier Section
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Comprises several cascaded audio amplifiers and one of more
speakers.
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The number of amplifiers used depends on the audio signal power
desired.
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Automatic Gain Control Circuit (AGC)
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The AGC circuit is used to prevent very strong signals from
overloading the receiver. It can also reduce the effect of
fluctuations in the received signal strength.
The AGC circuit makes use of the mean DC voltage level present
at the output of the diode detector.
If the signal strength increase, the mean DC voltage level also
increase.
If the mean DC voltage level exceeds a predetermined threshold
value, a voltage is applied to the RF and IF amplifiers in such a
way as to decrease their gain to prevent overload.
As soon as the incoming signal strength decreases, the mean DC
voltage level is reduced below the threshold, the RF and IF
amplifiers return to their normal operation.
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Automatic Gain Control Circuit (AGC)
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Exercise – Midterm Sem II 07/08
A radio station transmits using AM at a carrier frequency
of 600 kHz. The AM band frequencies are allocated with a
separation of 9 kHz. The AM signal is received by means
of a superhetrodyne receiver with an intermediate
frequency of 455 kHz. Calculate:
i. The highest possible modulating frequency.
ii. The local oscillator frequency.
iii. The image frequency.
iv.
Draw the block diagram of the receiver used
at the station and list all the frequency components
before and after the heterodyning process.
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