Circuit Theory

Discussion in 'Homework Help' started by sunny1982, Apr 3, 2013.

Dec 27, 2012
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0
Hi folks its the annoying me again, I've got another problem i need some advice on, although i don't expect anyone to complete my problems i always appreciate the advice and the guidance i recieve off you intelligent folks. For this particular question I just need advice on how to start the question and which part of the tutorials notes I should look at.

A complex waveform is given by the expression

V = 100 sin(100∏t) + 50 sin(200∏t + ∏/8)

Determine:-
a) The amplitude of the fundamental

2. MrChips Moderator

Oct 2, 2009
12,624
3,451
This is a trick question.
Which one is the fundamental?
What is the amplitude of the one you select to be the fundamental?

Dec 27, 2012
41
0
A*sin(omega*t + phi)
is just a way of describing a sine wave with amplitude = A, frequency = omega/2pi, and phase angle = phi.

4. MrChips Moderator

Oct 2, 2009
12,624
3,451

Dec 27, 2012
41
0
Do you think this is correct?

the fundamental amplitude is 791.006?

6. MrChips Moderator

Oct 2, 2009
12,624
3,451
Nope. How did you arrive at that?

7. WBahn Moderator

Mar 31, 2012
18,085
4,917
I don't think it's a trick question at all. The two frequencies given are harmonically related with one of them being an integer multiple of the other.

Now, if the answer came back that the fundamental frequency was 25Hz with an amplitude of 0V, then that would not only be a trick question, but it would be an asinine trick question.

But when given a signal in which all of the components are harmonics of one of them, it is only reasonable to conclude that the latter is the fundamental frequency.

Had the two not been harmonically related, then the question of which is the fundamental would have been ambiguous (perhaps even meaningless).

sunny1982 likes this.
8. KCHARROIS Member

Jun 29, 2012
292
1
Sorry for interfering or possibly giving the answer but I'm not quite sure what the answer is either but I believe the answer is 100V peak because V = 100 sin(100∏t) + 50 sin(200∏t + ∏/8).

The first equation being the fundamental and the second equation being the information?

sunny1982 likes this.
9. WBahn Moderator

Mar 31, 2012
18,085
4,917
Yeah, you are pretty much just giving the answer outright.

As Sunny1982 stated in Post #3:

The factor multiplying the sin() function is the amplitude. But then he proceeded to completley ignore that in his next post.

The "amplitude" of a sinusoid is simply the peak excusion away from zero.

But your last statement is not correct (or, more accurately, is a case of apples and oranges).

I think you are confusing the concept of a "fundamental" frequency with that of a "carrier" frequency. The fundamental frequency is the base frequency of a periodic waveform and is just as much a part of the information content as any other component. All other components are at frequencies that are integer multiples of fundamental. In a modulation scheme, the "information" signal is offset from the carrier and has it's own fundamental frequency (assuming it is periodic) independent of the carrier.

sunny1982 likes this.