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Plotting the upper bound of harmonic content - yes it's assignment time again.

**Simon**:

The fact that it is nominally a square wave with given waveform makes it trapezoidal. I's not sure what difference the shape makes on upper frequency. They teach us 1/pi*Tr which seems to make sense as to represent a faster changing signal you need higher frequency harmonics.

**MrAl**:

--- Quote from: Simon on February 25, 2019, 11:05:39 pm ---The fact that it is nominally a square wave with given waveform makes it trapezoidal. I's not sure what difference the shape makes on upper frequency. They teach us 1/pi*Tr which seems to make sense as to represent a faster changing signal you need higher frequency harmonics.

--- End quote ---

Hi,

The shape makes a big difference not really on the upper frequency but on the upper frequency chosen as the upper limit.

Recall that for a square wave we have harmonics that have amplitudes that decrease as the inverse of the harmonic number:

An=K*1/n

but for a sawtooth we have harmonic amplitudes that decrease as the inverse sqaure:

An=K*1/n^2

so you can quickly see that the shape makes a big difference because the amplitude decreases much faster for a ramp.

Note that for the 10th harmonic of the square wave we have amplitude:

A10=K/10

while for the sawtooth we have:

A10=K/100

where K is the same for both.

You see the difference now.

In the practical sense if we chose 10 as the highest harmonic number, that would mean for the square wave we would quickly have amplitude that is 10 percent of the fundamental while with the sawtooth we'd only have 1 percent of the fundamental. 1 percent is often acceptable while 10 percent often is not acceptable.

**GrimaWormT**:

Hello all hope you are all keeping well.

For part (b) of the question I am stumped. It should be fourier analysis but for the life of me I can't seem to get it to click.

I was going to just apply the formula x(t)=An * Sin (n*2*pi*Fo*t) and punch in 3 & 101 for the n figures, but can't tell how I'm meant to have a "t" figure?

Any help from one of you big brained folk would be much appreciated.

**Simon**:

Hi, are you doing this module? condolences. I recently got a small 12/5V 1A SMPS to pass military grade EMC with little sheilding and my study of this module did nothing to help in that :-DD.

So to question section b specifically, my answer was:

--- Quote --- The third harmonic calculated using the formula on page 3 of EC-3-2 is xxxxxxxx. The 101st harmonic is xxxxxx.

--- End quote ---

I got into the habit of reminding them of their own text that had driven the answer especially when the explanation made no sense and it seemed to be a case of monkey see monkey do like it is here as they have over condensed and over simplified the theory.

Who is the tutor on this now? if it is Andy, he's a lovely chap and is not hard on his students in marking if they demonstrate effort and grasp of the subject. Don't be afraid to answer the way you want to after giving them what they want, showing that you understand beyond the scope of the question may get you some forgiveness for minor errors.

Always remember when answering these questions, forget what you may already know, work out which bit of their theory they are talking about as that is all they are interested in and given the strange way they explain stuff questions can only be correctly answered using their explanations. The stranger the question the more it was targeted at their strange theory. The sad thing about the EMC stuff is that you can't just pick up an alternative book about it as they are several hundred pages long, expensive and very in depth.

Don't bother with the distributed control systems module, it's not worth the electrons used to store it in memory.

**Simon**:

Oh as for "t" ignore it unless you are doing calculations at a specific angle in the cycle. For your fundamental it is "1" when you calculate on harmonics, it's the harmonic number.

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