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Filter problem

  1. Apr 17, 2005 #1
    I have this filter problem,
    I am given a magnitude graph (magnitude vs frequency, with a point on it) of voltage across input current source and a sketch of a circuit: black box connected with a current source (cos(wt) and voltage across it is Vmcos(wt + theta)) and the question is to figure out what's in the box and values. Looking at the graph, it is low-pass filter

    So far we covered steady-state sinusoidal response and phasors. But all the examples in the book are with voltage sources so I am thrown off, I am not sure if I can do source transformation on it!

    So, I tried getting ratio of v/i and using phasor e-notation, I got Vm*e^theta, so does that mean that since impedance has positive imaginary part, it's inductor and resistor in series? What would a point on the magnitude voltage graph give me?

    Any hint on this would be GREATLY appreciated...
    Last edited: Apr 17, 2005
  2. jcsd
  3. Apr 18, 2005 #2
    Missing ???

    The problem here is that a multitude of circuits may do the same thing
    ( you left off the sketch ) without which a solution is impossible .
    However this sounds like Homework so I am not inclined to repy.
  4. Apr 18, 2005 #3
    I do not know how to use Latex, so I described the graph as a graph of low-freq. filter: as w->infinity, |v| -> 0, since this is the only information that was provided for us.
    I resolved it with a TA, but thanks for making me feel welcome. I did not hide it was a homework, and it wasn't like I did not make an effort in trying to understand this. I guess I had a wrong idea about the point of this forum: help people; but evidently students are not considered to be "people" by your standards.
    No hard feelings.
    Thanks for your time [edit] and that other time you answered my question[/edit].
    Last edited: Apr 18, 2005
  5. Apr 18, 2005 #4
    Oh Ray is always like that... he's just kidding.

    We know you tried it out --- but he has a point, I think I oculd help you out if you attach the pdf of which the plot is drawn. Or a link to your course webpage could be helpful!
    Last edited: Apr 18, 2005
  6. Apr 20, 2005 #5
    To Elver -- sorry I think I was a little tired. The point here is that exact info is required
    i.e the graph shape matters. You say that you need to get what is in the box AND values . You can only do this in very simple cases which one may be able to judge by the shape of the graph. I assume (but do not know ) that the point is chosen to reveal
    a value of loss at a specific frequency leading to values.
    In electronics the circuit and graph info is vital --- I do not use Latex either --- I found that any drawing package which can produce a small Jpeg file can be used here
    I happen to use Corel Draw Here is a sample of what it can do in a couple of minutes ( the Jpeg is at 55 dpi to keep it small ) .everything here is drawn or typed and placed . it's not a schematic package ).
    I do not use BMP's because unless they are zipped they are too large -- and if they are zipped they cannot be viewed on the forum.

    Attached Files:

    Last edited: Apr 20, 2005
  7. Apr 20, 2005 #6
    Oh, I did not expect you to go at such length :smile:
    Actually that is exactly what the circuit is supposed to look like. Next time I will be more considerate when it comes to details and precision of the problem posting. The tip on drawing is very helpful.
  8. Apr 21, 2005 #7
    To Theelectrichild --- Hey I am not ALWAYS like that ---- but the solution to a problem IS a GOOD Question . Ray.
    But you are quite right about the kidding !!!!!!!!!!!!!!!
    and thank you for keeping the discussion light hearted.
    Last edited: Apr 21, 2005
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