Wireless Communications

1
Weve seen that an envelope detector is at the
heart of a full-carrier AM receiver. Now lets
see how DSB-SC and SSB could be demodulated.
Radio Frequency Current Waves
Antenna
Radio Waves
Microphone
Transmitter
Audio Frequency Voltage Waves
Antenna
Receiver
Audio Frequency Voltage Waves
Radio Frequency Current Waves
Speaker
2
Unfortunately, the simple envelope detector will
not work for DSB SC. Heres the reason A simple
full-carrier AM waveform is shown below, with the
envelope overlayed in red. As weve seen, the
envelope detector recovers a very good
approxmation of the modulating audio waveform.
3
Heres a DSB-SC waveform, with the audio
modulation waveform overlayed in red. It looks
quite different from the full-carrier AM waveform.
4
And heres the result of envelope-detecting the
DSB-SC waveform, in red, with the DSB waveform
shown in blue. Its badly distorted, a full-wave
rectified version of the modulating audio.
5
Fortunately, theres a simple way of demodulating
DSB-SC that does work. Here again is the
time-domain view of the DSB-SC waveform.
6
And heres the frequency-domain view. As with the
full-carrier AM waveform, it appears that all we
need to do is
Energy
-fcarrier
0 Hz.
fcarrier
7
Translate both sidebands downward
Energy
-fcarrier
0 Hz.
fcarrier
8
Translate both sidebands downward
Energy
-fcarrier
0 Hz.
fcarrier
9
translate both sidebands downward As it turns
out, to demodulate DSB-SC, it really is that
simple. Downconversion and demodulation are the
same process.
Energy
-fcarrier
0 Hz.
fcarrier
10
The simplest DSB-SC receiver looks like this. The
LO frequency is equal to the frequency of the
(suppressed) carrier. This receiver architecture
is very similar to the superheterodyne
architecture. The difference is that the
intermediate frequency is zero.
Antenna
Local Oscillator (LO) fLO f carrier
Audio Output
RF Amplifier (optional)
AF Amplifier
Mixer
11
This architecture is known by several
names. Among radio amateurs, it is usually called
direct conversion, sometimes abbreviated as
DC. Other names include synchrodyne, homodyne,
and zero-IF. It became popular among radio
amateurs as a low-cost receiver architecture in
the late 1960s, replacing the regenerative
receiver.
Antenna
Local Oscillator (LO) fLO f carrier
Audio Output
RF Amplifier (optional)
AF Amplifier
Mixer
12
This architecture can have disadvantages, which
are beyond the scope of this course. Although
its disadvantages can be reduced and in some
cases eliminated, the superheterodyne
architecture is still more common. In a
superheterodyne receiver, DSB-SC signals are
demodulated by, in effect, replacing the envelope
detector with a DC receiver tuned to the IF
frequency.
Second Local Oscillator
First Local Oscillator
Antenna
fRF
fIF
fLO1
fLO2
AF Output
RF Amplifier (wideband)
IF Amplifier
AF Amplifier
First Mixer
Second Mixer
13
Closer inspection shows that this architecture is
identical to the full-carrier AM superheterodyne
receiver, except that the envelope detector is
replaced by a second LO and mixer. These two
blocks are sometimes referred to as a product
detector. When the second LO is part of a
product detector, it is sometimes called a Beat
Frequency Oscillator, or BFO.
First Local Oscillator
BFO
Antenna
fRF
fIF
fLO1
fLO2
AF Output
IF Amplifier
AF Amplifier
RF Amplifier (wideband)
First Mixer
Product Detector
14
Direct conversion can also be used for SSB
demodulation Heres the frequency-domain view of
an upper-sideband (USB) signal, which was
generated using a carrier frequency of fcarrier.
The carrier is suppressed, so it is not
shown. The lower-sideband (LSB) is also
suppressed, but is shown for reference.
Energy
Upper Sideband
Lower Sideband (Suppressed)
-fcarrier
0 Hz.
fcarrier
15
Here it is again, but without the lower sideband.
Energy
Upper Sideband
-fcarrier
0 Hz.
fcarrier
16
A direct-conversion receiver tuned to fcarrier
shifts both the positive-frequency and
negative-frequency upper sidebands directly to
baseband, resulting in recovery of the audio
signal which was used (along with the suppressed
carrier) to generate the SSB waveform.
Energy
-fcarrier
0 Hz.
fcarrier
17
There is a problem with direct conversion of SSB
signals. The bandwidth of the receiver is the
same as a DSB receiver, twice the bandwidth of an
SSB waveform. This wider-than-needed bandwidth
makes the DC receiver more susceptible to
interference than it should be. Still, because of
the simplicity and low cost of DC receivers,
radio amateurs often use them.
Energy
-fcarrier
0 Hz.
fcarrier
18
Heres why theres a problem. Shown below is a
frequency-domain view of the RF and IF signals
which would be found in a DC receiver tuned to a
USB waveform at the fcarrier. Everything is as it
should be.
USB RF
Energy
Recovered Baseband
-fcarrier
0 Hz.
fcarrier
19
Now lets retune the receiver, setting the LO
frequency to fcarrier2, about 3 KHz. (one voice
bandwidth) above fcarrier.
Energy
Recovered Baseband
-fcarrier
0 Hz.
fcarrier
fcarrier2
fcarrier2
20
As you can see, the positive-frequency upper
sideband is shifted to the negative-frequency
baseband position while the negative-frequency
upper sideband is shifted to the
positive-frequency baseband position. As a
result, the baseband signal has its high
frequency components and low frequency components
reversed. This is called frequency inversion,
and was used in telephone scramblers during world
war II.
Energy
Recovered Baseband
-fcarrier
0 Hz.
fcarrier
fcarrier2
fcarrier2
21
Suppose we want to recover a USB signal at
fcarrier2, but there is another transmitter
generating a USB signal at fcarrier. This
situation is shown below.
Desired Signal
Energy
Adjacent Signal
-fcarrier
0 Hz.
fcarrier
fcarrier2
fcarrier2
22
A DC receiver shifts both the desired signal and
the signal adjacent to it to baseband. At
baseband, the adjacent signal is
frequency-inverted and overlapping the desired
signal. Both signals are heard simultaneously,
which means the overlapping signal interferes
with the desired signal.
Energy
-fcarrier
0 Hz.
fcarrier
fcarrier2
fcarrier2
23
The most common solution to this interference
problem is to use the superheterodyne receiver
architecture, with a product detector. There is
one little change...
First Local Oscillator
BFO
Antenna
fRF
fIF
fLO1
fLO2
AF Output
IF Amplifier
AF Amplifier
RF Amplifier (wideband)
First Mixer
Product Detector
24
A bandpass filter, usually a high-quality
crystal or mechanical filter, is added to the IF
section. The IF bandpass filter is usually
preceeded and followed by IF amplifiers, to
isolate it from the mixers. The IF bandpass
filters bandwidth is equal to the bandwidth of a
voice signal, which is the same as the bandwidth
of the desired SSB signal.
First LO
BFO
Antenna
fRF
fIF
fLO1
fLO2
AF Output
IF Amplifier
AF Amplifier
RF Amplifier (wideband)
IF Amplifier
First Mixer
Bandpass Filter
Product Detector
25
To illustrate how this solves the problem, lets
start with the same situation we considered a
moment ago. We want to recover a USB signal at
fcarrier2, but there is another transmitter
generating a USB signal at fcarrier.
Desired Signal
Energy
Adjacent Signal
-fcarrier
0 Hz.
fcarrier
fcarrier2
-fcarrier2
26
The First LO is tuned to fLO fcarrier2 - fIF.
This shifts both the desired signal and the
adjacent signal to IF, as shown below
Desired Signal
Energy
Adjacent Signal
-fcarrier
0 Hz.
fcarrier
-fIF
fcarrier2
-fcarrier2
fIF
27
The bandpass filter passes the desired signal to
the product detector, but rejects the adjacent
signal
Desired Signal
Energy
Bandpass Filter
-fcarrier
0 Hz.
fcarrier
-fIF
fcarrier2
-fcarrier2
fIF
28
so the input to the product detector looks like
this Its the desired USB signal, shifted to IF,
without the adjacent signal.
Energy
-fcarrier
0 Hz.
fcarrier
-fIF
fcarrier2
-fcarrier2
fIF
29
The product detector completes the process by
shifting the IF USB signal to baseband. By the
way, this works very well for DSB-SC demodulation
as well. One sideband is simply discarded.
Energy
-fcarrier
0 Hz.
fcarrier
-fIF
fcarrier2
-fcarrier2
fIF
30
Before leaving this topic, lets compare the
block diagram of a superheterodyne SSB receiver,
shown below
First LO
BFO
Antenna
fRF
fIF
fLO1
fLO2
AF Output
IF Amplifier
AF Amplifier
RF Amplifier (wideband)
IF Amplifier
First Mixer
Bandpass Filter
Product Detector
31
to the block diagram of an SSB transmitter,
which was discussed in an earlier section. The
transmitter block diagram is shown here The
similarities are striking. The IF chain (the
bandpass filter and the two IF amplifiers in the
transmitter are identical to those in the
receiver. The receivers product detector is
identical to the carrier oscillator and first
mixer in the transmitter. The LO and 2nd mixer in
the transmitter are identical to the LO and 1st
mixer in the receiver.
AF Amplifier
LO
IF Amplifier
Microphone
To Antenna
Carrier Oscillator
RF PA
IF Bandpass Filter
1st mixer
2nd mixer
32
Its almost as if an SSB transmitter were an SSB
receiver, operated in reverse! This means that
its a relatively simple matter, once an SSB
receiver is designed, to turn it into a
transceiver, a combination transmitter and
receiver in a single package.
AF Amplifier
LO
IF Amplifier
Microphone
To Antenna
Carrier Oscillator
RF PA
IF Bandpass Filter
1st mixer
2nd mixer