DE LA SALLE UNIVERSITY-DASMARIÑAS

College of Engineering, Architecture and Technology

ECET412L- PRINCIPLE OF COMMUNICATIONS LABORATORY

Expt. No. 4

Double Sideband AM with Full Carrier (DSBFC)

Submitted By:

Maria Rosalina C. Reyes

ECE41

August 19, 2014

Engr. Conrado D. Monzon

Instructor

OBJECTIVES

1. To investigate the dynamic characteristics of Double Sideband Full Carrier AM

signals.

2. To observe the behaviour of DSBFC AM signals with different modulating

signals.

3. To calculate the depth of modulation in AM using different methods.

4. To investigate the effects of overmodulation in AM signals.

5. To investigate the spectral characteristic of DSBFC.

THEORY/DISCUSSION

Double sideband (DSB) is one of the easiest modulation techniques to

understand, so it is a good starting point for the study of modulation. A type of DSB,

called binary phase-shift keying, is used for digital telemetry. Amplitude modulation

(AM) is similar to DSB but has the advantage of permitting a simpler demodulator, the

envelope detector. AM is used for broadcast radio, aviation radio, citizens’ band (CB)

radio, and short-wave broadcasting.

In radio communications, a sideband is a band of frequencies higher than or

lower than the carrier frequency, containing power as a result of the modulation

process. The sidebands consist of all the Fourier components of the modulated signal

except the carrier. All forms of modulation produce sidebands.

Amplitude modulation of a carrier wave normally results in two mirror-image

sidebands. The signal components above the carrier frequency constitute the upper

sideband (USB), and those below the carrier frequency constitute the lower sideband

(LSB). In conventional AM transmission, the carrier and both sidebands are present,

sometimes called double sideband amplitude modulation (DSB-AM).

In communication systems information is transmitted from one place to another

using electrical signals. Usually the information bearing signals are not suitable for

transmission due to its propagation qualities . Also, since these signals generally exist in

the same frequency range it is necessary to transmit them using different frequency

allocations to avoid interference. One of the methods used to solve these problems is

linear modulation, which is merely the frequency translation of the spectrum of the

information signal to a usually much higher frequency. The translated spectrum can be

modified before transmission in different forms resulting in different linear modulation

schemes. Specifically, there are four linear modulation methods: double-sideband

(DSB) (also known as double-sideband with suppressed carrier DSB-SC), amplitude

modulation (AM) or DSB-LC (large carrier), single-sideband (SSB) and vestigial-

sideband (VSB). This experiment examines the characteristics of the DSB modulation.

DATA AND RESULTS

SINUSOIDAL WAVE

Diagram 1. Modulating and Carrier Signals

Vp-p: 4.4 V

Diagram 2. Modulating and Modulated Signal

Vary the frequency and amplitude of the modulating signal. What do you observe?

Overmodulation was observed.

Measure the peak-to-peak values of the maximums (A) and the minimums (B) of the

AM signal.

A= 6.48 V B= 2.4 V

Calculate the depth of modulation, m, using the formula:

m= (A-B)/(A+B)

m= .44 %m= 44%

m= Vm/Vc

m= .45 %m= 45%

Compare the results.

The results gave off almost the same answers.

Diagram 3. Trapezoidal Pattern of AM Wave

Measure the longer vertical side of the trapezoid.

A= 6.5 V

Measure the shorter vertical side of the trapezoid.

B= 2.4 V

Calculate the depth of modulation, m, using the formula above as above with the

measurements obtained in the trapezoidal pattern.

m= .46 %m= 46%

What do you observe?

Based from the results obtained. The values of m, from the previous results is almost

the same with the trapezoidal pattern result.

Diagram 4. Dynamic Characteristic of Overmodulated AM

Table 1. Spectrum of DSBFC

MEASURED THEORY

f

(kHz)Vcm (volt)

f

(kHz)Vcm (volt)

18 -12.5 dB 18 2.4

20 3.45 dB 20 4.4

22 -12.9 dB 22 6.4

Diagram 5. Spectral Characteristic of DSBFC

Table 2. Spectrum of DSBFC

MEASURED THEORY

f

(kHz)Vcm (volt)

f

(kHz)Vcm (volt)

16.9 -5.75 dB 17 3.4

20 3.45 dB 20 4.4

23.1 -5.79 dB 23 5.4

Diagram 6. Spectral Characteristic of DSBFC

SQUARE WAVE

Diagram 1. Modulating and Carrier Signals

Diagram 2. Modulating and Modulated Signal

Overmodulation was observed.

Measure the peak-to-peak values of the maximums (A) and the minimums (B) of the

AM signal.

A= 6.6 V B= 2.4 V

Calculate the depth of modulation, m, using the formula:

m= (A-B)/(A+B)

m= .47 %m= 47%

m= Vm/Vc

m= .45 %m= 45%

Compare the results.

Both of them yield the same result.

Diagram 3. Trapezoidal Pattern of AM Wave

Measure the longer vertical side of the trapezoid.

A= 6.6 V

Measure the shorter vertical side of the trapezoid.

B= 2.4 V

Calculate the depth of modulation, m, using the formula above as above with the

measurements obtained in the trapezoidal pattern.

m= .47 %m= 47%

What do you observe?

The trapezoidal pattern of the AM wave is the same with the previous results of m.

Diagram 4. Dynamic Characteristic of Overmodulated AM

OBSERVATION/CONCLUSION

Based form the experiment conducted, the Double Sideband Full Carrier is one

type of amplitude modulation. The carrier is then enveloped by the modulating signal

where it is being transmitted with a variation in its amplitude. This is one characteristic

of an AM signal. In a DSFBFC signal, the modulating signal, the upper sideband and

the lower sideband does not intersect each other. One characteristic an AM signal that

is also being exhibited by a DSBFC signal is the depth modulation. Depth modulation

can be calculated in many ways. First, by getting its peak-to-peak values of the

minimums and maximums of the signal and by simply getting the ratio of its sum and

difference a value of m can be obtained. Second, by getting the Vm and Vc of the signal

and dividing them. Lastly, the trapezoidal pattern method by determining the shorter and

longer side vertical side of the trapezoid, m can be determined. Any of these three

methods will yield to the same result. Overmodulated AM gives of more than 100%

modulation of the carrier, wherein the modulating signal creates smaller enveloped

signals. This is commonly a fault in transmission. It is referred to as “off the scale”

signal. The spectral characteristic of the AM signal gives the view wherein the signal is

plotted in frequency vs. time. The DSBFC spectral characteristics shows three

frequencies. The frequency itself, the modulating frequency and the carrier frequency.

Based on observation, these three were seen. The amplitude of the frequency is longer

and the amplitude of the fm and fc were the same, showing the sidebands.

REFERENCES

http://nptel.ac.in/courses/IIT-MADRAS/Principles_Of_Communication/pdf/Lecture19-

20_AM_DSB-SC.pdf

http://electronics.stackexchange.com/questions/107128/what-is-overmodulation

http://en.wikipedia.org/wiki/Amplitude_modulation

http://en.wikipedia.org/wiki/Double_sideband

http://en.wikipedia.org/wiki/Overmodulation