ECL question for the 11C90 prescaler

Discussion in 'The Projects Forum' started by ke0ff, Sep 13, 2011.

  1. ke0ff

    Thread Starter New Member

    Sep 13, 2011
    I have an early 80's vintage freq counter with a 11C90 prescaler on the 500MHz input. The circuit ties the /Q output of the 11C90 back to one of its modulus select inputs, with the other modulus select tied low. In looking through the data sheet for the 11C90, it appears that this connection will result in a divide by 10 output to the rest of the counter chain, which is what is desired.

    The question is, why tie the /Q back to the modulus select, when one or both of the modulus selects could simply be tied high to select the divide by 10 operation? I've seen both methods in other circuits, but the apparent majority connect /Q to one of the mod inputs. I've seen no explanations as to why one or the other of these methods has an advantage.

    FYI, the /Q output and the modulus selects are ECL



    "I know what I don't know, which means I know quite a lot"
  2. Hi-Z


    Jul 31, 2011
    I would imagine it's a matter of convenience. If you use the /Q ecl output you have a nice, local ecl signal which is high at the appropriate time. Otherwise you may have to manufacture the ecl high (you can't just tie it to Vcc).