PW 2 : Amplitude Modulation

Devices and Components:
1 electronic board
2 multiplier functions AD633
1 two ports π electronic circuit
1 50 kHz passband filter
1 wire Jack-M cable > bananas
1 loudspeaker
LPRO – Wireless Network and Security
PW 2 : Amplitude Modulation
1 Measurement Objectives
The measurements realized and studied are:
• Double Side Band Amplitude Modulation (DSB-AM)
• Double Side Band Suppressed Carrier (DSBSC)
• The envelope detector demodulation method
• The product detector demodulation method
2 Theoretical part
1 diode : Ge 1N541 (Ge)
Resistor : 15 kΩ
Capacitor : 10 nF
1 second function generator
1 wire Bananas > Jack-F
2007-08
2.1 Choice of the modulation index or modulation depth
The AM modulator is a multiplier: VS = KVX.VY with a constant parameter K = 0,1 V -1 , VX is a
sinusoidal signal with a carrier frequency fP = 100 kHz and a peak amplitude VP = 5V. The base band
signal is characterized by a sinusoidal signal VY with a frequency fM = 1 kHz , a peak amplitude VM = 2V
Calculate the average voltage V0 added to VY to obtain the two following cases:
• Double Side Band Amplitude Modulation (DSB-AM)
• Double Side Band Suppressed Carrier (DSBSC) with a modulation index m = 50%
2.2 Product detector demodulation of a DSBSC modulation
A signal VMA is a DSBSC modulation and is characterized by:
The carrier is a sinusoidal signal with a peak amplitude VP and a frequency FP
The low frequency information signal is a sinusoidal signal with a peak amplitude VM and a frequency fM
Trace its spectral response:
The product detector demodulation is composed of a multiplier in order to obtain the product between the
signal VMA and the signal VP. Trace on the next figure, the spectral result of this product.
IUT of Grenoble - WINS PW2 AM - 1 10/10/07

At the following of the multiplier, band-pass filter is added. Trace on the previous figure the filter design
intended in order to obtain the demodulation.
Trace on the next figure the spectral response after demodulation.
3 Measurement part
3.1 Amplitude Modulation using a multiplier function
Insert the multiplier function circuit on the holes
board. The circuit inputs are characterized by the
following letters X, Y et Z. The output S gives the
next voltage signal:
S = K . (X . Y) + Z
K is a constant parameter.
3.1.1 The multiplier.
Measure the constant parameter K with the following input signals:
- At the two inputs X and Y, a dc voltage signal with an amplitude of 1 V
- At the input Z, a null voltage signal.
Measure at the output S the voltage signal. Deducing the constant parameter K.
Now, only the signal relied at the two inputs is replaced by a sinusoidal signal with an effective amplitude
V = 2 V and a frequency F = 1 kHz. Its average value is null.
Acquire the input and output voltage signal.
Justify the frequency, the average and the peak amplitude values obtained at the output signal S.
3.1.2 The DSBSC modulation
• Apply at the input X a sinusoidal carrier signal (⇒
HAMEG 8030 source generator) with a peak
amplitude of 5 V and a frequency of 100 kHz. Its
average value is null.
• Apply at the input Y a sinusoidal signal with a lower
frequency of 1 kHz and a peak amplitude of 2V (⇒ used the second source generator) Record the
DSBSC modulation and the low frequency information signal together.
Important remark: to observe the AM signal, it is necessary to trig the oscilloscope with the lower
frequency signal. The trace is perfect with analog mode which is not the case with numerical mode.
To get the screen copy of the oscilloscope, select the mode acquire/Envelope ON. Then, come back
to the normal mode. Give the characteristics of the modulated signal.
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3.1.3 The DSB-AM
Use the offset voltage of the second generator to modify the
modulation index. To measure the offset voltage, use a
multimeter at the position “V=” .
• Control and record this value in order to have a
modulation index m = 100%
(Measurement method is described at the following address http://www.t14 at the heading
“Technical help/Measurement methods”)
Acquire this AM and the low frequency information signal together. Give the characteristic
parameters of the lower frequency signal
• Acquire the trapezoidal figure with the X-Y mode selecting the DISPLAY / X-DEFL / ON function.
In this case the source chosen is AM signal.
• Repeat the same measurements with a modulation index m = 50%.
3.1.4 Amplitude Modulation spectral response
Due to a weak resolution of the FFT module of the oscilloscope, the lower frequency will be multiplied by
10 in order to reach a frequency of 10 kHz. In this case, the different frequency peaks in the spectral
response is sufficiently different to be observed.
To acquire the spectral response read at the next address http://www.t14 at the heading “Technical
help/Measurement methods”.
- Reduce the time base to observe only about ten periods of the signal
- Select the button MATH
- Select FFT on CH2 if the AM signal is connected to this channel (CH1 if AM signal is connected to
the other channel)
- Activate ON
- Do not observe the source, Param = ABS
- Increase the time base until to see two or three peaks.
- Display the frequency activating the cursors function
• Acquire the DSBSC-modulation spectral response
• Acquire the DSB-AM spectral response with a modulation depth m = 50%.
• To conclude, which is the frequency band of this amplitude modulation ? And which is the
parameter associated to this frequency band ?
3.2 Envelope detector
Band-Pass High-Pass
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Be careful : do not forget to replace the old frequency of the low frequency
information signal by the previous value 1 kHz !
Set the V0 in order to have a DSB-AM with a modulation depth m = 50%.
Using the circuit shown in the following figure realize the envelope detector
defined in the previous theoretical part with a circuit composed of a diode
1N451 (Germanium => low threshold).
• Check that this detector demodulates the amplitude modulation signal
acquiring simultaneously the low frequency information signal and the
modulated signal.
• Find the maximum frequency fMAX of the low frequency information
signal which gives a correct envelope detection the defect appears clearly using the X-Y mode
(modulated and low frequency signals). Explain that comparing the envelope slow with the exponential
evolution of the capacitance discharge.
• Which do appear if a DSBSC modulation is applied at the input of the envelope detector?
3.3 Product demodulation
Insert a second multiplier circuit on the holes board.
• Apply at the input X the carrier signal (⇒ HAMEG
8030 source generator) with a frequency of 100 kHz
and peak amplitude of 5V.
• Apply at the input Y, the previous DSBSC modulated
signal.
Using the filter function, realize at the output of the
multiplier the pass-band filter defined in the theoretical
part.
3.4 Low-Pass filter
Low-Pass filter
• Acquire the modulated signal and the low frequency information signal together.
Measure the cut-off frequency at -3 dB of the transfer function associated to the base band signal used for
the modulation and the signal obtained with the product demodulation module. For that, decrease the
frequency value of the low frequency signal to 500 Hz and apply the demodulated signal at the input of the
voltmeter Ferisol. Read the amplitude with dB scale and then increase the frequency value until a decrease
of 3 dB of the level.
• Which is this cut-off frequency?
3.5 Audio application
Replace the low frequency signal from the second generator by the signal coming from the audio card of
the PC with the cable (bananas and Jack-M connector).
Chose a music file, as an example AmeliePoulain.wav (Intranet , Heading : TP / En GTR 1A / Cycles1 /
AM).
Apply the demodulated signal on the loudspeaker with the cable (bananas and Jack-F connector).
The demodulated signal could restore the music chosen previously.
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