Part 1
- Download the jc10-29-square-wave-from-sine-waves-PART1.ms10
Multisim file, and run it.
- Open the spectrum analyzer if it isn't already opened.
- Enlarge the spectrum analyzer window until it fills the
screen.
- Use the mouse to drag the vertical cursor to where it lines
up with the fundamental frequency (1st harmonic), which is 1 kHz in our
case.
- Just below the oscilloscope-like screen of the spectrum
analyzer, you'll see the frequency and voltage displayed for the part
of the graph selected by the vertical cursor.
- You probably won't be able to position the cursor at
exactly 1 kHz. Just get as close as you can.
- The left and right arrows allow you to move the cursor in
small increments.
- Write down the frequency and voltage for the 1 kHz peak, as
well as for the next four peaks.
Part 2
- Download the jc10-29-square-wave-from-sine-waves-PART2.ms10
Multisim file, and run it.
- Set the five sine wave generators to correspond to the
frequencies you wrote down in Part 1.
- Adjust the five potentiometers to give you the voltages you
wrote down in Part 1.
- Because the sine-wave generators are set at 10 volts, and
the pots go from 0 to 100%, a pot setting of 75% will give you a
voltage of 7.5 volts on the pot's slider.
- In other words, if you take the percentage figure for a
pot and move the decimal point one place to the left, you'll have the
voltage being put out by the pot.
- Open the oscilloscope if it isn't already opened.
- If all the switches aren't closed, close them.
- You should get something resembling a square wave with some
small bumps on it. If we added more sine wave generators to produce
more harmonics, we could get a square wave with smaller bumps.
Fourier's mathematics says that you can get as close as you want to the
waveform you want by adding more and more harmonics.
- One at a time try opening up the switches and see what
happens to the waveform.
We rarely use sine waves to create other waveforms, with one exception
being the first Moog music synthesizers. However, we take into account
the harmonics present in non-sinusoidal waveforms and sometimes use
filters to remove certain harmonics.
An example of harmonic filtering are the ferrite beads found on almost
all computer cables. Digital electronics produces square waves, and
square waves have harmonic frequencies much higher than the frequency
of the square waves. These high frequencies radiate well, and interfere
with televisions and other electronic devices. A ferrite bead slipped
over a wire turns a portion of the wire into an inductor, and inductors
don't let high frequencies through easily. Thus, radiation from the
high frequencies present in the square waves traveling down the cables
is greatly reduced.
Inductors with multiple turns of wire are not as good for eliminating
high frequencies as are ferrite beads sliped over a wire. Whenever you
have two conductors separated by an insulator, you have a capacitor.
Capacitors allow high frequencies to pass through them easily.
Inductors consisting of coils of wire are not good at stopping very
high frequencies because of the capacitance between each turn of the
coil. A ferrite bead slipped over a wire has very low capacitance, and
is thus good at stopping very high frequencies.
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