LAB 1 F.A.Q.

Updated on 12-Jan-2000

1. (2.1) My answers (for amplitude and phase) from the plots don’t match the ones calculated. They are off by 5-10%. I think it is because of the rounded figures that I used while dealing with answers involving pi.

ANSWER: There will be, of course, some errors in reading values from the plots. 10% seems high. You can “blow up” the plots. Here is a trick: type zoom on in the MATLAB command window. Then select a region of the plot with the mouse. MATLAB will zoom in on that region. In MATLAB v5.3 the zoom tool is part of the plot window. Help zoom should provide more details.

2. (2.1 (a)) I know how zoom works and I have zoomed in on the first positive peak after 0 and calculated the time delay. Then using -t1*omega = (time shift), I calculated time shift but it isn’t right. I started with 150 degrees (or 0.83333*pi) and I end up with 1.16*pi. I calculated the period using the graph and it is correct. The amplitude, of course, is also correct. What could be wrong? Here is the line I used for one of my functions:

xx1 = A1*cos(2*pi*400*tt+P1); %Sinusoid function 1

ADDENDUM 1: I have figured it out for the first one, but the second one never works. We should use the first three digits of our SSN in degrees for the second time shift. Mine is 595, which, if I am correct, is 3.30555*pi, but when I find the closest peak to zero and calculate the time delay and then calculate the time shift it is always wrong.

ANSWER: Are you taking the “phase ambiguity” into account? Remember: you can always add/subtract integer multiples of 2*pi. 3.30555*pi is the same as 1.30555*pi


3. (2.1 (a)) It says to make measurements of the time-delay and amplitude from the plots. Is there a command to do that or do we just estimate by hand?

ANSWER: Do it by HAND. In fact, you should avoid “naked plots”. For the lab, we prefer plots that are dressed up with some hand-written annotation to show that you can pick out the important information on the plot. Also, it is hard to draw arrows in MATLAB that point to the interesting stuff.

UPDATE: Graph annotation (including arrow drawing) is much easier with MATLAB v5.3 because Mathworks added a new annotation toolbar to the figure. Just click on the arrow button and drag the arrow where you want to place it on the plot! You can change plot properties and add text as well.

4. (2.1(c)) I am unsure what exactly is expected in this part; I have shown how x3 can be obtained from x1 and x2 using the phasor addition theorem. Is that all I have to do, or is there something else I am missing?

ANSWER: If you have computed x3 by phasor addition, all you need to do is verify from the plot (of x3 vs. time) that the amplitude and phase of x3 are as calculated to within numerical precision; use the zoom command to get accurate values from the plot.