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Finding the Maximum Fan-Out of a distribution network

  1. Apr 11, 2013 #1
    1. The problem statement, all variables and given/known data
    A V_s=5V source, with Thevenin resistance R_s=1kΩ, drives a certain number h(fanout) gates, each of which is modeled as a C_g=100fF load. The gates are driven simultaneously (i.e., in parallel). You are given: V_OL= 1V, V_OH = 4V. Ignore the wire resistance. What is the allowed fanout (h, as a numerical value) such that signals up to 333 MHz can propagate satisfactorily?

    2. Relevant equations

    Xc = 1/(Cjw) , V=IR , C(total parallel) = C1 + C2 + C3 +...etc

    3. The attempt at a solution

    My understanding is that the capacity of the fan-out has to do with the current being driven out of the source. Given V_OH which I believe is the minimum output needed to reach state '1' on the gate, the voltage node at the Capacitors in parallel must be equal or greater than V_OH. This is as far as I got, but I am not sure my line of thinking is correct
  2. jcsd
  3. Apr 11, 2013 #2

    rude man

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    Gold Member

    That's quite right.

    The idea is the 1K source takes time to charge the parallel-wired input gates from V_ol to V_oh or the reverse.

    Assume the 333 MHz is a square wave. Then your concern is the time to get from V_ol to V_oh and from V_oh to V_ol.
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