AM Transmitter
Prelab
In this experiment you will use what you learned in your previous lab sessions to make an
AM transmitter circuit. You will construct a chopper modulator to generate AM.
P1) Introduction
One of the simplest schemes for wireless communication is Amplitude Modulation (AM).
Modulation is necessary for a number of reasons:
 The attenuation of the channel, i.e. air, is high for low frequency voice signals, but the
attenuation is significantly lower for higher frequencies.
 If the voice signal is transmitted as is, i.e. baseband transmission, only one channel can
be broadcast at a time. Often it is needed to have many channels transmitting at the
same time, e.g. different radio and TV stations need to broadcast at the same time.
 Just think of it for a moment that the radio was broadcasted in baseband, you couldn’t
even sleep at night!
The idea behind AM is very simple: since high frequencies can propagate farther through
air, we make a high frequency sinusoid (the carrier) carry the low frequency signal that we
want to transmit. In AM, the signal to be transmitted, rides on the amplitude of the carrier,
thus forming the envelope of the modulated signal (Fig. P1). Mathematically this can be
represented as:
s (t ) = (1 + ms baseband (t ))cos ( f c t )
where s (t ) is the transmitted signal, s baseband (t ) is the original baseband signal that we want to
transmit., f c is the carrier frequency and m is referred to as the modulation index. The
modulation index defines how large the modulating signal will be compared to the carrier.
Envelope
Figure P1 – Amplitude Modulation (AM).
P2) Frequency Domain Representation
In order to better understand how amplitude modulation works, let’s analyze the
modulation in frequency domain. Basically, multiplying the input signal by the carrier signal
results in the frequency content of the input signal to be moved around the carrier frequency
(Fig. P2). As explained before, the original baseband signal may not be able to propagate well
through the channel, but frequencies near carrier frequency will. By multiplying the signal
with the carrier, its frequency content moves to around the carrier frequency and can thus
propagate through the channel efficiently.
Assume there is a frequency band, say 20 times the bandwidth of the original signal, that
can propagate through the channel efficiently. This frequency band can be divided between
10 different stations transmitting with AM. This can be done by assigning each of them a
unique carrier signal and sufficient spacing between the carrier frequencies (Fig P3). This
procedure is referred to as multiplexing.
f
fc
f
f
fc
Figure P2 – left – The original baseband input signal frequency content, middle – The
frequency content of the result of multiplying the input and carrier signal, right – The AM
transmitted signal, note that the constant carrier term appears as a delta function.
Station 1
Station 2
Station 3
Station 4
fc1
fc2
fc3
fc4
f
Figure P3 – left – The original baseband input signal frequency content of four stations,
right – Multiplexing the four stations using different carrier frequencies. The spacing between
carriers is chosen such that the frequency contents won’t overlap.
P3) AM Transmitter Circuit
There are different ways to perform amplitude modulation. In this experiment you will
use a chopper circuit to get the amplitude modulated signal. In this section you will learn how
the circuit functions.
Assume g (t )= 1 + mf (t ) , where f (t ) is the input modulating signal. If g (t ) is chopped
and then bandpass filtered at the carrier frequency, the result will be an AM signal (Fig. P4).
To see how this works assume S (t ) to be the switching function:
ìï 1,
S (t ) = ïí
ïïî 0,
cos (wc t )³ 0
cos (wc t )< 0
Then chopping the signal will be equivalent to multiplying it with S (t ). Hence, the chopped
signal will be:
g chopped (t ) = g (t )S (t )
Since S (t ) is an square wave with unit amplitude, it will have a Fourier series expansion as
follows:
S (t )=
g (t ) 
1 2
2
+ cos(wc t )cos(3wc t )+ K
2 p
3p
g chopped (t ) 
t
t
2p wc 
(a)
(b)
AM output
t
(c)
Figure P4 – (a) – Signal g(t), (b) – Signal g(t) chopped, (c) – Filtered chopped signal, as can be
seen, it is an AM signal. Note that the modulation index is greater than 1 here.
so,
g chopped (t ) =
g (t ) 2 g (t )
2 g (t )
+
cos(wc t )cos (3wc t )+ K
2
p
3p
As you can see the second term is the desired AM signal, which can be filtered out using a
bandpass filter.
In frequency domain the chopped signal will look like figure P5.
G(f)
Gchopped(f)
f
fc
3fc
f
Figure P5 – left – Frequency content of signal g(t), right – Frequency content of Signal g(t)
chopped.
This chopper modulator can be realized with a diode bridge (Fig. P6).
Assume the diodes are ideal and try to explain how the bridge functions as a voltage
controlled switch. The controlling voltage is v1 (t ) .
gchopped (t)
Figure P6 – Chopper circuit employing diode bridge.
v1(t)
Figure P7 – The switching signal v1(t).
1) The Transmitter Circuit
The AM transmitter circuit consists of 3 major parts:
1. The local oscillator
2. The chopper circuit
3. The tank circuit and output stage
To build the circuit proceed as follows:
1. Connect the oscillator circuit as in the oscillator design experiment.
2. Test your circuit and make sure it’s working properly.
3. Connect the circuit in figure 1.
4. Test the operation of the circuit by feeding a 1KHz sine wave from the signal
generator. Also feed a 10KHz sine wave with 8Vpp to the oscillator input of the
circuit.
5. Test if the circuit works properly by connecting the output of the bridge to the
oscilloscope and check if you get the chopped signal.
6. Hook-up the final stage and the tank circuit to the output of the bridge.
7. Connect collector voltage to the oscilloscope and view the output signal. Make sure
your circuit is working as it should.
Output
Figure 1 – The chopper circuit. The Oscillator shown in the circuit is the output stage of the
oscillator circuit you have built. The output of this stage is fed into the final stage and the
tank circuit.
Figure 1 – The final stage of the transmitter circuit. The tank circuit filters out the desired
component and the output is fed into the antenna for transmission.