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Finding Voltages and Power?

Discussion in 'Electronic Basics' started by Newbie 2 Electronics, Nov 20, 2003.

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  1. Hi All

    I have been given the following question from University and I am abit stuck
    on it, if anyone could point me in the right direction, please shout....

    Q) The input power of a line is Pi = 18mW and the loss Ndb = -16dB. If the
    resistance of the line is 50?, calculate:

    1) The input voltage Vi=?
    2) The output voltage Vo=?
    3) The output power Po=?

    I have found the equation of how to calculate the loss: -

    Loss Ndb= = 10log Po/Pi = dB.

    I have tried looking through a number of books, but I cant seem to get what
    is going on. The lecturer has said that he will go through it next week, but
    I would like to be able to understand it before this.

    Any ideas or hints would be good.

    Thanks. Geoff
  2. Hi,
    You haven't stated the line's load impedance which in the real world
    affects all of these things, so I will assume that it is also 50 ohms.

    1) This is just Ohm's Law as the line is acting like a 50 ohm load resistor
    as far as the input power is concerned. So, if you know the relationship
    between power, voltage and resistance, it is straightforward.

    2) Transform the formula for voltage loss/gain in a matched system, which
    is 20log(Vo/Vi) to put Vo on the left. It will come out as negative for a loss
    but that need not concern you here.

    3) Knowing Vo, it is Ohm's Law once again to find Po.

    If you have any problems with this - just email.

    Cheers - Joe
  3. Thank you for your help, I will sit down with this question tomorrow and try
    and see if I can make any more sense of it!

    Thanks again

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