EE12: Laboratory Project (Part-2)
AM Transmitter
ECE Department, Tufts University
Spring 2008
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Objective
This laboratory exercise is the second part of the EE12 project of building an
AM transmitter in medium-wave band (550kHz-1700kHz). This part of the
project involves in using the LC based oscillator (Colpitt Oscillator) already
designed and design an Amplitude Modulator using voice-band signal from
the line-out port of the PC to transmit voice.
Vdd= 9V
L1
500µH
R1
CVdd
Q1
10µF
2N3704
RE1
CB
1nF
R2
RE2
C1
CT
1-10K
470Ω
vrf
6-60pF
C2
Figure 1: Circuit Diagram of the Modified Colpitt Oscillator
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The Modified Colpitt Oscillator
The circuit shown in Figure-1 is the modified Colpitt oscillator. The only
modification being the emitter resistor RE . It’s divided into two resistors
RE1 and RE2 and the LC tank is fed at the voltage divider instead of the
emitter directly.
The purpose is to give an extra degree of freedom to adjust the gm of the
feedback by changing the ratio of RE1 to RE2 . By adjusting the gm we can get
an oscillation with reduced distortion. Also, it provides an low-impedance
output to the next stage ie. the Amplitude Modulator.
Parts List
The parts remain same as the previous Laboratory exercise except for the
following:
• L1 = 500µH using two 1mH in parallel.
• The purpose of having RE1 is to vary it to the point where you get the
oscillator to just get into oscillation and this will ensure an oscillation
with less distortion. For PSpice you can use RE1 ≈ 1.5kΩ and you
should get it to oscillate.
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Vdd
Antenna
RL
Cant
Q1
vosc
Iaudio
vaudio
CE
Figure 2: Architecture of Amplitude Modulator
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Amplitude Modulator
Architecture
Figure-2 shows the architecture of the proposed Amplitude Modulator based
on a simple multiplier circuit. The oscillator signal (vosc ) is fed to the base
of Q1 which is configured as an common-emitter amplifier with CE chosen
such that it is very low-impedance at the oscillation frequency.
The bias is provided by the current source Iaudio which is modulated by the
input signal from the PC. Since the gain is directly proportional to the bias
current, the amplitude is modulated with the input signal (vaudio ). Finally,
the antenna, which has an inherent inductance associated with it, is made to
resonate with Cant so it carries only the RF signal and rest of the frequencies
are filtered out.
Circuit Design
Figure-3 shows the reference design for the Amplitude Modulator. vosc is the
signal from the Colpitt oscillator in Figure-1. Cc1 , R6 , R1 , R2 , Q1 and CE for
the common-emitter amplifier for the carrier signal (vosc ). Cc2 , R7 , C2 , R3 , R4 , Q2
and RE form the modulator of the bias current for the RF amplifier.
The collector current of Q1 will not only contain the carrier signal with
it’s modulated sidebands, but also the baseband signal (ie. audio signal).
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Vdd=9V
CVdd
C1
R5
R1
L1
Lant
500µH
400µΗ
10µF
vc1
Cant
vosc
CC1
R6
From the
oscillator
Q1
2N3704
vb1
R2
Vdd
ve1
R3
CC2
400Ω
R7
vb2
vin
Q2
2N3704
CE
ve2
R4
2VPK
RE
C2
Model of the PC
lineout
Figure 3: Reference Circuit for the Amplitude Modulator
L1 , C1 , R5 form the band-pass filter to filter out everything but the signal to
be transmitted.
With everything filtered out but the AM signal, Lant , Cant is made to resonate at the carrier frequency such that all the AM signal goes through Lant .
Since the AM band frequency is so low that in order to have a resonating
dipole antenna we need a very long wire (about 100m of wire), we instead
use a ferrite core inductor which has a small radiating resistance.
The audio signal is going to be provided from the line-out port of the PC
which can be modeled as an Thevnin source with max 2V peak and a source
resistance of 400Ω as shown in Figure-3.
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Guidelines for Calculating Component values
• Choose R1 and R2 such that vb1 ≈ 5V and the source impedance for
the Thevnin equivalent is ≈ 10kΩ.
• Choose R3 and R4 such that vb2 ≈ 2.8V and the source impedance for
the Thevnin equivalent is ≈ 10kΩ.
• Choose RE such that the DC emitter current of Q2 is approximately
1mA.
• Choose C2 such that it forms a low-pass filter from vin to vb2 and choose
the worst-case cut-off frequency to greater than 20KHz.
• Choose CE such that the magnitude of the AC impedance of CE is
3 − 10Ω at 500kHz.
• R6 and R7 can be chosen to be 100kΩ potentiometers.
• Cc1 forms the high-pass filter to isolate the DC from the oscillator.
Choose an appropriate value of Cc1 such that the low-frequency −3dB
point is well below 500KHz for example 100 KHz.
• Similarly choose a value for Cc2 such that the low-frequency −3dB
point less than 10Hz.
• R5 , C1, L1 forms a band-pass filter. Choose C1 for ωo = 750KHz and
R5 such that the bandwidth is approximately 300KHz. Hint: See
Lecture-21 notes for relation between Q and bandwidth of a parallel
R-L-C network.
• Choose Cant such that the series Lant , Cant resonate at ωo = 750KHz.
Add a variable capacitor in parallel when building the circuit to tune
it to the exact frequency.
Guidelines for PSpice Simulation
• After you get the oscillation, choose R6 such that the oscillation amplitude at the base of Q1 is small enough that it’s in the small-signal
domain.
• Provide a single tone of 5 kHz at the input as your voice signal.
• When you provide the input signal, you can check if the modulating
signal is too big or small by monitoring the collector current of Q1 .
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• Since the collector current will contain large portion of the input signal,
it will be hard to see the amplitude modulation in time-domain. So,
do a FFT on the IC1 and you should see two sidebands related to the
input signal.
• Monitor the voltage at collector of Q1 to make sure the signal amplitude
is small enough not to distort your signal.
• Monitor the current in Lant . It should contain a healthy amplitude
modulated signal and again you can use FFT to measure that.
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