University of Pennsylvania
ESE206: Electrical Circuits and Systems II – Lab
Amplitude Modulated Radio Frequency Transmission System
Mini-Project
Part 1: Modulator and Transmitter
Introduction
An Amplitude Modulated Radio Frequency Transmission system consists of a modulator
in which a sinusoidal high frequency carrier waveform cos(2fc t) is amplitude modulated
(AM) by a lower frequency signal vm(t), containing the information to be transmitted. In
case of an AM radio transmission, the signal v m(t) is the voice or music while the carrier
frequency f c is the frequency at which one tunes the radio, e.g. 1060 kHz. The expression
of the AM signal is then be written as,
vam(t) = [(A + vm(t)] cos(2fc t)
(1)
in which A is large enough so that [(A + vm(t)] never becomes negative. Let us assume
that the modulating signal is a sinusoidal signal of frequency fm. In that case the AM
signal can be expressed as,
vam(t) = [(A + Vm cos(2f mt)] cos(2f ct) = A[1 + m cos(2fmt)] cos(2fc t)
(2)
In this expression m is called the "modulation index" that is always less or equal to 1. An
example of a modulated signal vam(t) is shown in Fig. 1 below. Notice that the envelope
of the high frequency signal corresponds to the modulating signal v m(t).
Figure 1: Am Modulated Signal vam(t) with (f m=500 Hz, f c=5kHz and m=0.5)
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The modulation process requires a multiplication of the two signals. In case of two
sinusoids this results in a frequency spectrum that is composed of the carrier frequency,
fc, and two sideband frequencies at (f c - fm) and (fc + f m). Note that the low frequency
information-bearing signal, fm, has been translated to a much higher frequency range for
greater transmission efficiency. Either sideband can be used to recover the modulation
signal. The theoretical frequency spectrum of the AM signal of Fig. 1 is shown in Fig. 2.
Figure 2: Spectrum of an AM signal consisting of a carrier of frequency fc and a
sinusoidal modulating signal of frequency f m.
An example of a measured FFT spectrum of the AM signal is given in Fig. 3. The carrier
frequency is centered at 5kHz and the the two sidebands are at a frequency of 4.5 and 5.5
kHz as one would expect. The signal of the sidebands is about 12dB below that of the
carrier signal. This corresponds to a modulation index m of 0.5.
Figure 3: Measured spectrum of an AM modulated sinusoid of 5kHz and modulating
frequency of 500Hz with a modulation index m=0.5.
AM Radio Transmission System
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In this lab exercise, you will design, built and test an AM radio modulation and
transmission system (part 1). The receiver and demodulator will be the topic of the next
lab assignment (part 2).
The AM system block diagram is shown in Figure 4.
Figure 4: Block diagram of the transmitter and receiver.
The mini-project will demonstrate some basic principles of AM radio transmission
systems. The system elements consist of:
1. AM modulator/transmitter
2. Tuned circuit receiver
3. RF amplifier
4. Peak (or envelope) detector
5. Audio frequency transistor amplifier
6. Frequency spectrum of a AM signal
7. Total Harmonic Distortion (THD)
During the first lab you will build and test the modulator with the tuned filter. The second
part will be done in the next lab
Pre-lab assignment
1. Read the first part of section 12.3 in the textbook by D. Irwin (8th Edition) on
Resonance circuits.
2. Design a parallel LC circuit so that the resonance frequency f is about 100 kHz (90 < f
< 110 kHz). This will be used as the tuned filter that detects the career frequency. You
can use one of the following values for the inductance L:
47 H
82 H
100 H
330 H
680 H
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3. Assume that the frequencies of the carrier and modulating signals are fc = 100kHz and
fm = 5kHz, respectively. The value of the modulation index m=0.7 and of the amplitude
A=1V (see expression (2)).
a. What is the maximum and minimum amplitude of the modulated waveform
vam(t)?(see Fig. 1 for the definition of Vam,max and Vam,min)
b. Prove that the frequency components of the modulated signal are 95kHz, 100kHz
and 105kHz (hint: re-write the product of sinusoids in exp. (2) as a sum of
sinusoids. What is the amplitude of each frequency component? How much smaller
(in dB) is the amplitude of the sideband signals as compared to that of carrier
frequency component for a modulation index m=0.7? Express your answer in dB.
4. Consider the spectrum of an AM signal, shown in Fig. 5 below. What is the frequency
of the modulating signal vm(t)? Can you determine the value of the modulation index
m of the AM signal? Show how you determined the value of m.
Figure 5: Measured spectrum of an AM modulated sinusoid. The vertical scale is
10dB/div and the horizontal one 2.44kHz/div.
In-lab experiments
Parts:
1 – 2N5457 n-channel JFET (used as a variable resistor) (datasheet is available
from the Fairchild website:
http://www.fairchildsemi.com/ds/2N/2N5459.pdf)
1 – LF356 Op-Amp
1 – Inductor (TBD)
1 – Capacitor (TBD)
2 – 0.1 microFarad capacitors (for power supply decoupling)
1 – Resistor of 15kOhm
1 – Resistor of 1kOhm
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Note: Construct this circuit in one corner of your protoboard and save it. In the second
part of this project an AM receiver will be designed. After completion of the two circuits
the combination will be tested to see if you have a working AM transmit/receive system.
Procedure:
1. Tuned LC circuit
a. Build the LC circuit that has a resonance of about 100 kHz. Use the values of
L and C that you calculated as part of the prelab.
b. To test the circuit, connect a resistance of 1 kOhm between the function
generator and the LC circuit as shown in the Fig. 6 below.
Figure 6: LC circuit.
Apply a sinusoidal signal to the circuit with amplitude of 5Vpeak-to-peak and
vary the frequency from 10kHz to about 500 kHz. Measure the voltage over the
LC circuit as a function of the frequency. Find the frequency at which the voltage
is a maximum (resonant frequency). Find also the 3dB bandwidth and quality
factor of this resonant circuit. The quality factor Q is defined as the ratio of the
center frequency and the bandwidth (see also the "Active Filter lab"); the
bandwidth is the frequency range f between the two 3dB points of the frequency
characteristic.
2. Modulator/Transmitter
The circuit of Fig. 7 shows how to build a modulator. It consists of a non-inverting
amplifier in which the carrier vc (t) ,with frequency fc , is applied to the non-inverting input
terminal. The gain of the amplifier (1 + R1/Rds) can be varied through a voltage-controlled
resistor Rds. This resistor is implemented with a JFET (Junction Field Effect Transistor)
transistor. This JFET can be considered as a variable resistor, as is shown in Fig. 8. The
voltage applied to the gate input terminal, VG, determines the value of the resistor. The
resistance is proportional to the applied gate voltage. The gate terminal is connected to
the modulating signal v m(t), with frequency fm. This signal will thus change the
amplification of the amplifier in relation to the value of the modulating signal, as
schematically indicated in Fig. 7. This result in an AM modulated signal.
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Figure 7: A Simple AM Modulation/Transmitter Circuit
Figure 8: A JFET used as a voltage-controlled resistor.
a. Assemble the circuit shown in the Fig. 7. You should add a 0.1 uF decoupling
capacitor between each of the pins of the power supply V+ and V- of the op-amp and
ground. For the power supply of the op-amp you can use +12V and -12V (you can go
up to +18 and -18V if needed). Also, connect all ground wires to a single point on the
proto-board. This will reduce the noise of the output signal.
The signal applied to the non-inverting input of the amplifier will be the carrier signal
that should be in the 90 - 110 kHz range (supplied from a function generator). The
exact frequency will be determined in the design of the tuned LC circuit at the
receiver end (see previous section). Use the HP 33120A generator for the carrier
waveform.
vc(t) = 0.5sin (fc t)
The modulation waveform, vm(t), requires approximately a -1V off-set voltage to
provide a quiescent operating point in the linear range of the R ds vs. V g characteristic.
Use the “Krohn-Hite 1200A” function generator to provide this off-set to the
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modulation waveform. Check the waveform on the oscilloscope before connecting it
to your circuit.
vm(t) = 0.2 sin (f mt) - 1
b. Using a modulation frequency of 5000 Hz, observe the modulator output Vam as the
amplitude Vm of the modulating signal is increased. Make adjustments to the
amplitude and off-set voltage of v m(t) to produce a maximum linear output waveform
(approximately 2 Vp-p of modulation.). You will notice that the modulated signal Vam
is pretty sensitive to the adjustments of the offset and the amplitude of the modulating
signal Vm. Take a snapshot (use Hypersnap) of the modulator's output Vam and
determine the minimum and maximum amplitude of the modulated signal (Vam,max and
Vam,min). Record the corresponding settings of the modulating signal V m: amplitude
and offset voltage.
3. Frequency spectrum of the transmitted AM signal
Use the built-in FFT function of the oscilloscope to display the spectrum of the
transmitted AM signal (see: FFT using the 54600 Oscilloscope or open the link:
http://www.seas.upenn.edu/~ese206/labs/FFTScopeTutorialPart1.pdf)
Use the cursors to find the exact values of the frequencies. Write down the frequencies of
the carrier signal and the sidebands. How many dB is the amplitude of sideband signal
below that of the carrier signal (use the cursors to find the levels in dB)?
4. Modulator/Receiver
The goal is to transmit an AM signal and receive it using the tuned LC circuit.
a. After connecting a 1-ft antenna to the output of the transmitter, connect also a 1-ft
antenna to the LC circuit (you do not need the 1kOhm resistor or function generator).
Since the transmission efficiency at this frequency is very low, you will get better results
when you twist the two antennas together. The antenna will receive the transmitted waves
and convert them in a voltage over the LC circuit. Make sure that you ground one side of
the LC circuit.
b. Measure the voltage over the LC circuit using an oscilloscope probe. This votage is
pretty small. Display the received AM modulated signal on the oscilloscope. Measure the
amplitude of the transmitted and received signals. Take a snapshot of the transmitted and
received signals.
Report
Under the Experimental Results section, discuss:
a. Tuned filter:
i. Design, component values
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ii. Measured characteristics: resonant frequency, bandwidth, quality
factor
b. Modulator:
i. Show the measured modulated output signal and give the value of the
amplitude Vm and off-set voltage of vm(t) that gives you a maximum
linear output waveform.
ii. What is the modulation index m of your signal?
c. Frequency spectrum of the AM signal:
i. Give the spectrum of the AM signal.
ii. What is the measured frequency of the carrier and the sideband?
iii. How many dB is the amplitude of the sideband below that of the
carrier frequency? Can you find the value of the modulation index
from this graph (see also prelab question 3b)?
d. Modulator/Receiver:
Show an oscilloscope trace of a typical received signal. Indicate the
amplitude of the overall signal and compare it with the amplitude of the
transmitted signal
March 21, 2006.
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