# NCEES 538, 539, and 540



## schmidty99 (Mar 9, 2011)

Just wondering if anyone would have any information to offer on these problems. They are communication based problems, and I am having a difficult time finding information on them, both in my books and on the web. I can understand what they are telling me from the answers given, but I'm trying to research the problems a little more to learn about the concepts. Just not finding much. Any info or resources would be appreciated. Thanks!


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## CincinnatiControlsGuy (Mar 10, 2011)

This is certainly one of the major problems with the EC&amp;C module of the electrical PE exam: nobody takes it so there aren't too many resources available. Honestly, the EERM manual is the only thing that I used, though technically I had information in my microwave theory textbook from school. The key here was understanding how to work with decibels and dealing with units. I can help you through some sticking points, but I don't know of any other sources besides maybe the Kaplan book.


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## schmidty99 (Mar 10, 2011)

Thanks GB, I'll take any help I can get. I looked at the EERM, a communications book by Taub, a electromagnetics book, then the web, and really didn't find an good explanation. For example, I think 538 states that to find the SNR, you take the difference between the noise floor and the noise power. Ok, fine, but I can't find anywhere that AM modulation causes the power to decrease by half or 3 dB (AND if that's only true for 100% modulation index). Which is then subtracted from the difference to get the answer. ???


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## CincinnatiControlsGuy (Mar 10, 2011)

schmidty99 said:


> Thanks GB, I'll take any help I can get. I looked at the EERM, a communications book by Taub, a electromagnetics book, then the web, and really didn't find an good explanation. For example, I think 538 states that to find the SNR, you take the difference between the noise floor and the noise power. Ok, fine, but I can't find anywhere that AM modulation causes the power to decrease by half or 3 dB (AND if that's only true for 100% modulation index). Which is then subtracted from the difference to get the answer. ???


Look through the paragraph under equation 60.10 in the EERM; they do mention that half the power is carried in each sideband. However, these were easily some of the more difficult problems in the NCEES practice exam book and I don't think I ever fully understood them. Do you have the practice exam book that came with the EERM? Those problems were much simpler, but helped enlighten me on communication theory basics. Anyway, one book that you should DEFINITELY have as a reference is a copy of the ARRL handbook. For practice problems try "Electrical Engineering 360 Problems &amp; Solutions" and "Electrical Engineering PE problems and solutions". Some of the problems are ridiculously difficult, but some of the easier ones may help you out (when I say easier, I mean possibly representative of an exam question). I'm sorry if I'm not much help but communications theory and RF are my weakest areas by a country mile. Luckily, the exam won't contain many of these problems as per the published exam breakdown.


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## schmidty99 (Mar 10, 2011)

GroesbeckEE said:


> schmidty99 said:
> 
> 
> > Thanks GB, I'll take any help I can get. I looked at the EERM, a communications book by Taub, a electromagnetics book, then the web, and really didn't find an good explanation. For example, I think 538 states that to find the SNR, you take the difference between the noise floor and the noise power. Ok, fine, but I can't find anywhere that AM modulation causes the power to decrease by half or 3 dB (AND if that's only true for 100% modulation index). Which is then subtracted from the difference to get the answer. ???
> ...


Thanks!! I did find some information on 540 tonight, I should have been searching for "relative power amplitudes of multipath components." Here's the links I found so far if you want to see them.

http://www.mike-willis.com/Tutorial/PF15.htm

http://alexandria.tue.nl/extra2/afstversl/E/625088.pdf

I did purchase a copy of the ARRL handbook (nothing on this in there), and have the EERM sample exam. I'll look at the other PE problems you mentioned, but frankly its getting down to crunch time, so the problems in the EERM sample exam and EERM practice problem will keep me plenty busy.

I appreciate your help with this and all the previous posts. I know it takes time out of your schedule/life. But I know for a fact that without this Board and you especially, I will not pass this exam. With your help, I at least have a fighting chance! Stayed tuned, I'll probably be knocking(begging) for help again in the coming days and weeks.


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## benbo (Mar 11, 2011)

> Ok, fine, but I can't find anywhere that AM modulation causes the power to decrease by half or 3 dB (AND if that's only true for 100% modulation index). Which is then subtracted from the difference to get the answer. ???


I am a little bit in the dark becuse I don't have this book of problems, so what I say may be completely irrelevant. I did okay confusing you last time when I knew what the problem was.

If this is single sideband operation that would decrease the amount of power used by 3dB. Each sidenband has half the amplitude of the carrier, and they ususally suppress the carrier since it carries no information.

Anyway, ignore this if it has nothing to do with the problem. It just came to mind when you mentioned "half power."


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## schmidty99 (Mar 12, 2011)

I think you're on the right track with it. Thanks!


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## benbo (Mar 12, 2011)

schmidty99 said:


> I think you're on the right track with it. Thanks!


Probably not, but thanks for making me feel better.

Please keep posting things here. It's been a while since I took the test and I like to see people discussing the problems to refresh my memory. So if I can't help maybe I can review by reading what you and GB post here.

BTW - I was told by an instructor that the communications part of the test is the hardest to study for, because there are so many things they could potentially ask. There's a lot of luck involved in that part of the exam - on which questions you get. So don't expect to be able to answer every comm question. You'll probably pick up more points in electronics and control.

Good luck!


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## schmidty99 (Mar 12, 2011)

Ok will do. Thanks for the pointers! I was planning to take the last couple weeks and brush up on my circuit analysis and general electrical engingeering to try to make sure I have those down cold. But, like I've said before, there's room for improvment everywhere!


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## lomenop (Mar 31, 2011)

For what its worth, here is my understanding.

SNR = 10log(Ps/Pn)=10log(Ps)-10log(Pn)=10log(0.5*m*Pactual)-10log(Pn)

They problem states the modulation index, m, is 100% =&gt; m=1

=10log(0.5*1*Pactual)-10log(Pn)

=10log(0.5)+10Log(Pactual)-10log(Pn)

= -3dB + -70dBm - (-102dBm)

= -3dB + 32dB

= 29 dB

to see where I got the 0.5*m, expand equation 48.5(on the latest EERM book with is now ELRM) using trig

Smod(t) = A (m*cos wmod*t + 1 )*cos wc*t

= 0.5*A*m*cos (wc*t + wmod*t ) + 0.5*A*m*cos (wc*t - wmod*t ) + Acos wc*t

This expanded equation also explaing problem 537


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## schmidty99 (Apr 1, 2011)

Thanks! It makes more sense from that, than just the NCEES answer which is basically "magic."


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