University of Pittsburgh

Experiment #10 Lab Report

Submission Date:
11/27/2017

Instructors:
Dr. Minhee Yun
John Erickson
Yanhao Du

Submitted By:
Nick Haver & Alex Williams
Station #16

ECE 1201: Electronic Measurements and Circuits Laboratory

I. Introduction
The purpose of this experiment was to generate and examine amplitude modulation (AM) signals. AM signals can be used
to transmit information through a variety of mediums; in the case of Experiment #10, co-axial cable. Most widely
recognized for public radio applications, amplitude modulation works by transforming a high-frequency carrier signal
based on a second signal, commonly referred to as the message signal. Whereas frequency modulation (FM) transforms
the frequency of a carrier signal, amplitude modulation, as the name suggests, transforms the carrier signal’s amplitude.

II. Procedure
All signals used in Experiment 10 were created using the function generator. Modulation of a carrier signal was carried
out using two methods, first using the function generator’s internal modulation function to modulate the carrier frequency
with a defined message frequency. Channel 1 was set on the function generator to produce a sine wave with amplitude of
1 V (Vpp = 2 V). To illustrate real-world amplitude modulation characteristics, carrier and message frequencies were
selected comparable to those used for AM radio. In Pittsburgh, 1020 KDKA, an AM radio station, broadcasts at 1020 kHz,
or 1.020 MHz, which we chose to use as a carrier signal.

Using the function generator’s internal modulation function, the carrier signal was modulated using a message frequency
of 1.0 kHz. In the case of AM radio, message signals are often those from a radio host’s microphone or a music system.
The human ear can detect audio at frequencies in the range of 20 – 20,000 Hz, while most music is in the range of 20 –
5000 Hz, so modulation with a message frequency of 1.0 kHz is a relatively real-world scenario. This amplitude modulation
is shown in Fig. 1.

The function generator was then used to change modulation depth. As its name suggests, modulation depth dictates the
degree to which the carrier signal is modulated. Under normal circumstances, modulation depth is 100%. Depths of less
than 100% correspond to less modulation while depths greater than 100% correspond to more modulation. AM signals
with modulation depths of 0%, 50%, and 120% are shown in Figs. 2, 3, and 4.

Next, modulation was carried out using a second method, external modulation. In this case, a carrier signal produced by
the function generator is modulated with an external message signal input. In this experiment, this message signal input
was generated by Channel 2 of the function generator, while the carrier frequency of 1.02 MHz remained on Channel 1.
To do this, a co-axial cable was connected from the Channel 2 output to the external modulation input, and the function
generator was configured to trigger on the modulation signal. For the case of external modulation, modulation depth could
not simply be adjusted using the function generator’s internal settings. Instead, modulation depth was modified by
adjusting the amplitude of the message signal. Modulation depth varies proportionally with message signal amplitude, as
shown in Table 1.
Table 1: Modulation Depths and Corresponding Message Signal Amplitudes
Modulation Depth Message Signal Amplitude (V)
10% 1.0
50% 5.0
60% 6.0

External modulations of the 1.02 MHz carrier signal with 1 V amplitude, using a 1.0 kHz message signal with the various
amplitudes listed in Table 1 are shown in Figs. 5, 6, and 7.

Next, message signals other than the sinusoidal waveform were used for modulation, and their outputs were evaluated.
While waveforms such as triangle and square waves would not normally be seen in audio transmission, their use as
modulation signals produced interesting results. Shown in Fig. 8 is the amplitude modulation using a 1kHz square wave,
rather than a sinusoid. In Fig. 9, the sinusoid was again replaced, this time with a triangle wave.

AM signals are often characterized by the function shown in Eq. 1. In this equation, ωc is determined by the carrier
frequency, while carrier signal amplitude determines constant A.

𝑠(𝑡) = 𝐴[1 + 𝐵𝑚(𝑡)] cos(ω𝑐 𝑡) (1)

III. Summary of Results

Figure 1: Internal Amplitude Modulation of 1.02 MHz Figure 2: Internal Amplitude Modulation at 0%
Sinusoid with 1.0 kHz Message Signal Modulation Depth

Figure 3: Internal Amplitude Modulation at 50% Figure 4: Internal Amplitude Modulation at 120%
Modulation Depth Modulation Depth

Figure 5: External Amplitude Modulation with Message Figure 6: External Amplitude Modulation with Message
Signal Amplitude of 1.0 V Signal Amplitude of 5.0 V
 Figure 7: External Amplitude Modulation with Message Figure 8: External Amplitude Modulation with 1.0 kHz
Signal Amplitude of 6.0 V Square Wave

Figure 9: External Amplitude Modulation with 1.0 kHz

IV. Conclusion
Signal modulation can be done at a variety of depths, but from our experiments it appears that using 100% or less
modulation is ideal when working with signals. Anything above that results in dead zones like those seen in Fig. 4. At
100% modulation, the changes to the signal are clearest, but even when looking at modulation depths of 50% the changes
are still relatively clear. As expected, when there is no message signal (in other words, a modulation depth of 0%) the
reading is a rather uniform output.

Other types of waves can also be used to modulate the signal, and will result in various shapes. Square waves produce an
output with two parts, a high output and a low output that it jumps in between, which is to be expected as a square wave is
very similar. The triangle wave resulted in a modulated signal whose amplitude steadily increased then decreased in both
directions, which also follows logically from a triangle wave. Modulating the amplitude of these two difference functions
affected the output the same way that modifying the sine wave message signal affected the output.

Procedure B: Envelope Detector

I. Introduction
The purpose of an envelope detector is to separate the message signal from the carrier signal in an AM signal. Its name
comes from the fact that an AM signal can be thought of as a carrier signal contained within the envelope of the message
signal. In this sense, to extract information from an AM signal, then envelope must be detected while the contents of the
envelope may be discarded. When it is considered that the envelope and its contents are two signals with different
frequencies, it becomes apparent that a filter-type circuit will be necessary to construct the envelope detector.
II. Procedure
First, the envelope detector circuit shown in Fig. 10 was constructed in PSPICE. In choosing values for R and C, it was
assumed that message signals would remain less than 50kHz, and signal of greater frequency would be filtered out. A
frequency of 50kHz corresponds to a period of 20 µs, which was chosen as our R-C time constant. Therefore, values of
2.0 kΩ and 0.01 µF were chosen for R and C. The circuit was simulated with the AM signal used in Procedure A. The
envelope detector was first simulated with only the message signal, shown in Fig. 11. Considering that the diode will only
allow the positive portion of the signal to be seen at the output, the envelope detector input and output shown in Fig. 11
appear as expected. The envelope detector was then simulated with the AM signal, first with an amplitude of 2.5 V, then
with an amplitude of 5.0 V, shown in Fig. 12 and Fig. 13.

The envelope detector circuit was then constructed on the protoboard using the same R and C values used in the PSPICE
simulation. First, only the message signal was connected to the input of the envelope detector, and the input and output
were observed, as shown in Fig. 14. The output (shown in yellow) appears as expected, essentially a rectified version of
the input message signal, due to the diode in the envelope detector. The voltage decrease seen between the input and output
is also due to the built-in voltage of the diode. Next, only the carrier signal was connected to the input of the envelope
detector, and the input and output were observed, as shown in Fig. 15. The output (shown in yellow) is a DC signal, or
essentially zero with some DC bias due to the capacitor in the envelope detector.

The AM signal used in Procedure A was then connected to the input of the envelope detector. For a 1.02 MHz carrier
signal with 1.0 kHz message signal, the input and output of the envelope detector are shown in Fig. 16. While the general
function of the envelope detector is noticeable, and the circuit appears to be functioning properly, the performance may
not be ideal. For example, the amplitude of the output signal appears to only be approximately half of that of the input
signal. This loss in amplitude could lead to problems with properly receiving information from the message signal in real-
world use. Given that the envelope detector used in the experiment is quite primitive, containing only two passive elements,
it can be concluded that a more optimized circuit could provide higher quality envelope detection.

Next, the modulation percentage of the AM signal was modified, and the envelope detector output was observed. Envelope
detector input and output or modulation percentages of 0%, 50%, and 75% are shown in Figs. 17-19. As expected, at 0%
modulation, no modulation occurs at the input signal, and no output waveform is observed. For 50 % and 75% modulation,
output waveforms with amplitudes lower than that in Fig. 16 are observed. Modulation percentage was then decreased to
40%, which was said to be the lowest modulation percentage at which the envelope detector could accurately reproduce
the message signal, as shown in Fig. 20. In addition to modulation percentage, amplitude of the AM signal also had an
effect on the fidelity of reproduction. Increasing the amplitude from 1 V to 2 V, for example, while decreasing modulation
percentage to only 20%, resulted in a higher quality output that that in Fig. 20, as shown in Fig. 21.

Function generator settings were reset to a 1 V amplitude and 100% modulation. Next, a 10 kΩ potentiometer was used to
vary the value of R in the envelope detector. It was observed that increasing the resistance increased the amplitude of the
output waveform while decreasing the resistance decreased the amplitude of the output. R was first increased from 2.0 kΩ
to 9.3 kΩ, as shown in Fig. 22. Then R was decreased to 740 Ω, as shown in Fig. 23. The direct correlation between R and
output amplitude can be attributed to the fact that the function generator contains some internal resistance, meaning that
the function generator and envelope detector act as a voltage divider. When envelope detector resistance (R) is increased,
envelope detector voltage, or output amplitude, is also increased, and vice-versa.

In most real-world AM applications, the incoming AM signal is extremely weak, and must be amplified before the message
is recovered from the signal. To simulate this, AM signal amplitude was decreased to only 0.50 V, and the envelope
detector input and output were observed, as shown in Fig. 24. In this case, the amplitude of the recovered message is
extremely low, and it is likely that there would be errors in receiving information from the signal if it were to be directly
recovered without amplification.

Lastly, the frequency of the message signal was increased from 1.0 kHz to 50 kHz. While this high of a message frequency
is not likely to be seen in real-word AM applications, the resulting waveforms are worth noting. First, given that our
envelope detector was originally designed to work for message signals below 50kHz, the circuit was modified with a
resistance of 2.0 kΩ and a 0.001 µF capacitor, giving a time constant of 2.0 µs, or a frequency of approximately 500 kHz.
For an input with carrier frequency 1.02 MHz and message frequency 50 kHz, the output of the envelope detector is shown
in Fig. 25.
III. Summary of Results

Figure 10: Envelope Detector Circuit Constructed in Procedure B

Figure 11: PSPICE Simulation of Envelope Detector with Message Signal

Figure 12: PSPICE Simulation of Envelope Detector with AM Signal, Amplitude 2.5 V
 Figure 13: PSPICE Simulation of Envelope Detector with AM Signal, Amplitude 5.0 V

Figure 14: Envelope Detector I/O, Message Signal Only Figure 15: Envelope Detector I/O, Carrier Signal Only

Figure 16: Envelope Detector I/O for 1.0 kHz Message Figure 17: Envelope Detector I/O for AM Signal with 0%
Signal and 1.02 MHz Carrier Signal Modulation
 Figure 18: Envelope Detector I/O for AM Signal with 50% Figure 19: Envelope Detector I/O for AM Signal with 75%
Modulation Modulation

Figure 20: Envelope Detector I/O for AM Signal with 40% Figure 21: Envelope Detector I/O for AM Signal with 20%
Modulation Modulation and 2.0 V Amplitude

Figure 22: Envelope Detector I/O for AM Signal with R = 9.3 Figure 23: Envelope Detector I/O for AM Signal with R =
kΩ 740 Ω
 Figure 24: Envelope Detector I/O for AM Signal with 0.5 V Figure 25: Envelope Detector I/O with R = 2.0 kΩ, C = 0.001
Amplitude µF and 50 kHz Message Signal

IV. Conclusion
For Procedure B, the analysis of the detector using the PSPICE simulation yielded a decent capture of the data for our
simulation. There was a voltage drop across the diode, which caused the output of the detector to be lower, however it still
accurately reflected the changes in the input signal. When the input amplitude is increased to 5 V, the voltage drop is less
apparent and so the capture is even better under those circumstances.

When using the oscilloscope to capture information from the function generator, a similar effect was observed, where the
output from the envelope detector was at a lower voltage. There was also a problem with the beginning and the end of the
sinusoidal signal where, due to the voltage drop, it would miss that part of the input and only reflect the middle of the
input. However, the center area of the sine wave was effectively captured by the envelope detector.

When changing the modulation percentage, the performance of the detector varied. For smaller modulation depths, it had
greater trouble accurately representing the data. This is most likely because when the modulation depth is less, the changes
occur at lower voltages and so are more likely to be too small for them to be above the activation voltage of the diode.
However, at an amplitude of 2 volts with a modulation depth of 20%, as in Fig. 21, it can be seen that the detector does
still succeed in capturing part of the change from the message signal. Overall, the effectiveness of the detector is changed
both by the modulation depth and the amplitude of s(t).

The envelope detector is a low pass filter, and so changing the resistance will modify the frequencies at which it begins to
cut off the signal. Using the decade box to modify the resistance up to 9.3 kΩ, the envelope detector still functioned
effectively up to about 13 kHz before the output signal was no longer discernable.

When decreasing the amplitude of the signal to 0.5 volts, it can be seen from Fig, 24 that the recovered signal is barely
discernable, and is subject to far more error. This supports the statement from the lab report that it is important to boost
the amplitude of the signal before using the detector.

When increasing the message signal to 50 KHz, the envelope detector has greater difficulty reading it due to the fact that
the detector is a low pass filter. As the frequency increases, more information is filtered out. Looking at Fig. 25, the filter
is seen losing more information than the lower frequency signals. By taking the specs provided from Fig. 25 and
substituting the 2 kΩ resistor with a 1 kΩ resistor, the time constant of the filter is changed from 50 kHz to 100 kHz as it’s
-3dB point. This allows for a more effective capture of this higher frequency signal.
Experiment Conclusion
The purpose of this experiment was to generate and examine amplitude modulation (AM) signals. Several characteristics of amplitude
modulation were examined, including modulation percentage, as well as the effects of varying the frequency and amplitude of both the
carrier and message signals. A basic envelope detector circuit was constructed to separate the message signal from the carrier signal in
an AM signal. The functionality of the envelope detector was observed, and R and C parameters of the circuit were varied for different
uses.

References
ECE 1201 Website: http://engrclasses.pitt.edu/electrical/faculty-staff/gli/1201/