2.9 : AM Receiver




AM demodulation is the reverse process of AM modulation.
A conventional double sideband AM receiver converts the amplitudemodulated waveform back to the original source by receiving, amplifying and
demodulating the wave.
The receiver also functioning to bandlimit the total RF spectrum to a specific
desired band of frequency – tuning the receiver
Simplified block diagram of typical AM receiver
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2.9 : AM Receiver

RF section (Receiver front end)


Mixer/converter



Used for amplification and selectivity.
AM detector


Down-converts the received RF frequencies to intermediate frequencies (IF).
Intermediate frequencies are the frequencies that fall somewhere between the RF
and the information frequencies.
IF section


used to detect, bandlimit and amplifying the received RF signal.
Demodulates the AM wave and converts it to the original information signal.
Audio section

Used to amplify the recovered signal
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2.9.1 : Receiver Parameters
2.9.1.1 : Selectivity

Selectivity – parameter used to measure the ability of the receiver to accept a
given band of frequencies and reject all others.



Ex : for the commercial AM broadcast band, each stations transmitter is allocated a
10 kHz bandwidth. For a receiver to select only those frequencies assigned in a
single channel, the receiver must limit its bandwidth to 10 kHz.
A method to describe the selectivity of the receiver is to give the receiver a
bandwidth at 2 levels of attenuation (e.g. -3 dB and -60 dB).
The ratio of these 2 bandwidths is called as shape factor (SF),
SF 

B( 60dB)
B( 3dB)
(31)
In ideal, both bandwidth would be equal and the value of the shape factor
would be 1. But this is impossible to be achieve in practical circuit.

Ex : AM broadcast-band radio receiver : SF = 2
satellite, microwave & 2-way radio receivers: SF = closer to 1
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2.9.1.1 : Selectivity

A radio receiver must be capable of separating the desired channel’s signal
without allowing interference from an adjacent channel to spill over into the
desired channel’s passband.
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2.9.1.2 : Bandwidth Improvement



Thermal noise is one form of noise occurs in communication system that is
proportional to a bandwidth.
As signal propagates from the antenna through the RF section, mixer/converter
section and IF section, the bandwidth of signal is reduced thus reducing the
noise.
Noise reduction ratio achieved by reducing the bandwidth is called bandwidth
improvement (BI) expressed as follow,
BRF
BI 
BIF
(32)
where BI = bandwidth improvement
BRF = RF bandwidth
BIF = IF bandwidth
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2.9.1.3 : Bandwidth Improvement

The corresponding reduction in noise due to reduction in bandwidth is called as
noise figure improvement
NFimprovement  10 log 10 BI

(33)
Ex 5-1
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2.9.1.4 : Sensitivity
Sensitivity of the receiver is defined as - the minimum RF signal level that
can be detected at the input to the receiver and still produce a usable
demodulated information signal.
Signal-to-noise ratio (SNR) and the power of signal at the output of the audio
section are used to determine the quality of the received signal and whether it
is usable.


Typical AM broadcast-band receivers, a 10 dB or more SNR with approximately
0.5W of signal power at audio section is considered usable.

Sensitivity of a receiver is expressed in microvolts of the received signal.

Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.

Sensitivity of the receiver depends on :

1.
2.
3.
4.

Noise power present at the input to the receiver
Receiver noise figure
Sensitivity of the AM detector
Bandwidth improvement factor of the receiver
The best way to improve the sensitivity is to reduce the noise level
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2.9.1.5 : Dynamic range

Dynamic range of a receiver is defined as - the difference in decibels between
the minimum input level necessary to recognize a signal and the input level that
will overdrive the receiver and produce distortion.




The minimum received level is a function of the desired signal quality, front-end
noise and the noise figure of the receiver : X
The level that will produce overload distortion is a function of the net gain of the
receiver (total gain of all stages in the receiver) : Y
A dynamic range of 100 dB (between X and Y) is considered about the highest
possible.
A low dynamic range can cause severe intermodulation distortion.
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2.9.1.6 : Fidelity



Fidelity is defined as – a measure of the ability of a communication system to
produce an exact replica of the original source information at the output of the
receiver.
Any variations in the demodulated signal that are not in the original
information signal is considered as distortion.
3 forms of distortions :




Phase distortion
Amplitude distortion
Frequency distortion
Phase distortion



Filtering is the predominant cause of phase distortion
Frequencies at or near the break frequency of a filter undergo varying the values of the
phase shift (i.e. the phase is shifted/delayed).
If all the frequencies are not delayed by the same amount of time, the frequency-versusphase relationship of the received signal is not consistent with the original signal and the
recovered signal is distorted.
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2.9.1.6 : Fidelity

Amplitude distortion



Occurs when the amplitude-versus-frequency characteristics of the output signal of a
receiver differs from those of the original signal.
It is the result of nonuniform gain in amplifiers and filters
Frequency distortion


Occurs when frequencies that are present in a received signal are not present in the original
source information.
It is a result of harmonic and intermodulation distortion and caused by nonlinear
amplification
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2.9.1.7 : Insertion Loss


Insertion loss – ratio of the power transferred to a load with a filter in the
circuit to the power transferred to a load without a filter in the circuit
Filters are generally constructed from lossy components such as resistorand
imperfect capacitor that tend to attenuate (reduce the magnitude) the signal
Pout
IL ( dB)  10 log 10
Pin
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2.9.1.8 : Noise Temperature/Equivalent Noise
Temperature

Thermal noise is directly proportional to temperature and can be expressed in
degress as well as watts and volts.
T 
N
KB
where T = environmental temperature (kelvin)
N = Noise power (watts)
K = Boltsmann’s constant (1.38 x 10-23 J/K)
B = bandwidth (Hz)
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2.9.2 : Types of receiver

2 basic types of receiver



Coherent receiver – the frequencies generated in the receiver and used for
demodulation are synchronized to oscillator frequencies generated in the transmitter.
Noncoherent receiver – frequencies that are generated in the receiver or the
frequencies that are used for demodulation are completely independent from the
transmitter’s carrier frequency
For AM DSBFC scheme, the noncoherent receivers are typically used.


Tuned Radio Frequency receiver (TRF)
Superheterodyne Receiver
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2.9.2.1 : Tuned Radio Frequency Receiver (TRF)

Block diagram of 3-stages TRF receiver that includes an RF stage, a detector
stage and an audio stage :




Two or three RF amplifiers are required to filter and amplify the received signal to a
level sufficient to drive the detector stage.
The detector converts RF signals directly to information.
An audio stage amplifies the information signals to a usable level
Simple and have a relatively high sensitivity
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2.9.2.1 : Tuned Radio Frequency Receiver (TRF)

3 distinct disadvantages :
1. The bandwidth is inconsistent and varies with the center frequency when tuned over a
wide range of input frequencies.


As frequency increases, the bandwidth = f/Q increases. Thus, the selectivity of the input
filter changes over any appreciable range of input frequencies.
Ex 5-2
2. Instability due to large number of RF amplifiers all tuned to the same center
frequency

High frequency, multi stage amplifiers are susceptible to breaking into oscillation.
3. The gains are not uniform over a very wide frequency range.

The nonuniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers.
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2.9.2.2 : Superheterodyne Receiver


Heterodyne – to mix two frequencies together in a nonlinear device or to
transmit one frequency to another using nonlinear mixing.
Block diagram of superheterodyne receiver :
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2.9.2.2 : Superheterodyne Receiver

1. RF section




Consists of a pre-selector and an amplifier
Pre-selector is a broad-tuned bandpass filter with an adjustable center frequency
used to reject unwanted radio frequency and to reduce the noise bandwidth.
RF amplifier determines the sensitivity of the receiver and a predominant factor in
determining the noise figure for the receiver.
2. Mixer/converter section




Consists of a radio-frequency oscillator and a mixer.
Choice of oscillator depends on the stability and accuracy desired.
Mixer is a nonlinear device to convert radio frequency to intermediate frequencies
(i.e. heterodyning process).
The shape of the envelope, the bandwidth and the original information contained in
the envelope remains unchanged although the carrier and sideband frequencies are
translated from RF to IF.
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2.9.2.2 : Superheterodyne Receiver

3. IF section





Consists of a series of IF amplifiers and bandpass filters to achieve most of the
receiver gain and selectivity.
The IF is always lower than the RF because it is easier and less expensive to
construct high-gain, stable amplifiers for low frequency signals.
IF amplifiers are also less likely to oscillate than their RF counterparts.
4. Detector section
Gambar


To convert the IF signals back to the original source information (demodulation).
Can be as simple as a single diode or as complex as a PLL or balanced demodulator.
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2.9.2.2 : Superheterodyne Receiver

5. Audio amplifier section

Comprises several cascaded audio amplifiers and one or more speakers
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2.9.3 : Receiver Operation
2.9.3.1 : Frequency Conversion

Frequency conversion in the mixer stage is identical to the frequency
conversion in the modulator except that in the receiver, the frequencies are
down-converted rather that up-converted.





In the mixer, RF signals are combined with the local oscillator frequency
The local oscillator is designed such that its frequency of oscillation is always above
or below the desired RF carrier by an amount equal to the IF center frequency.
Therefore the difference of RF and oscillator frequency is always equal to the IF
frequency
The adjustment for the center frequency of the pre-selector and the local oscillator
frequency are gang-tune (the two adjustments are tied together so that single
adjustment will change the center frequency of the pre-selector and at the same time
change the local oscillator)
when local oscillator frequency is tuned above the RF – high side injection
when local oscillator frequency is tuned below the RF – low side injection
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2.9.3.2 : Frequency Conversion

Mathematically expressed :
High side injection
flo  fRF  fIF
(33)
Low side injection
flo  fRF  fIF
(34)
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2.9.3.2 : Frequency Conversion

Illustration of the frequency conversion process for an AM broadcast-band
superheterodyne receiver using high side injection :
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2.9.3.2 : Frequency Conversion

Ex 5-3
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2.9.3.3 : Local oscillator tracking

Local oscillator tracking – the ability of the local oscillator in a receiver to
oscillate either above or below the selected radio frequency carrier by an
amount equal to the intermediate frequency throughout the entire radio
frequency band.


With high side injection- local oscillator should track above the incoming RF carrier
by a fixed frequency equal to fRF + fIF
With low side injection- local oscillator should track below the incoming RF carrier
by a fixed frequency equal to fRF - fIF
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2.9.3.4 : Image frequency


Image frequency – any frequency other than the selected radio frequency
carrier that will produce a cross-product frequency that is equal to the
intermediate frequency if allowed to enter a receiver and mix with the local
oscillator.
It is equivalent to a second radio frequency that will produce an IF that will
interfere with the IF from the desired radio frequency.


if the selected RF carrier and its image frequency enter a receiver at a same time,
they both mix with the local oscillator frequency and produce different frequencies
that are equal to the IF.
Consequently, 2 different stations are received and demodulated simultaneously
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2.9.3.4 : Image frequency

The following figure shows the relative frequency spectrum for the RF, IF,
local oscillator and image frequencies for a superheterodyn receiver using high
side injection.



For a radio frequency to produce a cross product equal to IF, it must be displaced
from local oscillator frequency by a value equal to the IF.
With high side injection, the selected RF is below the local oscillator by amount
equal to the IF.
Therefore, the image frequency is the radio frequency that is located in the IF
frequency above the local oscillator as shown above, i.e.
fm  flo  fIF  fRF  2 fIF
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2.9.3.4 : Image frequency


The higher the IF, the farther away the image frequency is from the desired radio
frequency. Therefore, for better image frequency rejection, a high IF is preferred.
However, the higher the IF, it is more difficult to build a stable amplifier with high
gain. I.e. there is a trade-off when selecting the IF for a radio receiver (image
frequency rejection vs IF gain and stability)
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2.9.3.5 : Image frequency rejection ratio


Image frequency rejection ratio (IFRR) – a numerical measure of the ability of
a pre-selector to reject the image frequency
Mathematically expressed as,
IFRR  1  Q 2  2
(36)
where ρ= (fim/fRF) – (fRF/fim)
Q = quality factor of a pre-selector

Once an image frequency has down-converted to IF, it cannot be removed. In
order to reject the image frequency, it has to be blocked prior to the mixer stage.
I.e. the bandwidth of the pre-selector must be sufficiently narrow to prevent
image frequency from entering the receiver.
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2.9.3.5 : Image frequency rejection ratio

Ex 5-5
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2.9.4 : Double Conversion Receivers


For good image rejection, relatively high IF is desired. However, for a high
gain selective amplifiers that are stable, a low IF is necessary.
The solution fro above constrain is to use 2 intermediate frequencies, i.e. by
using double conversion AM receiver.


The 1st IF is a relatively high frequency for good image rejection.
The 2nd IF is a relatively low frequency for good selectivity and easy amplification.
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2.9.5 : Net Receiver Gain



Net receiver gain is simply the ratio of the demodulator signal level at the
output of the receiver to the RF signal level at the input to the receiver.
In essence, net receiver gain is the dB sum of all gains to the receiver minus the
dB sum of all losses.
Gains and losses found in a typical radio receiver :
Net Receiver Gain GdB = gainsdB – lossesdB
where gains = RF amplifier gain + IF amplifier gain + audio amplifier gain
losses = pre-selector loss + mixer loss + detector loss
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2.9.5 : Net Receiver Gain

Ex 5-8
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