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Amplify clock signal?

Discussion in 'Electronic Basics' started by Amanda Robin, Jul 21, 2006.

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  1. Amanda Robin

    Amanda Robin Guest

    I bought some ECS -100 i MHz clock oscillators to use as an external
    clock to set a cutoff frequency in a Maxim lowpass filter (MAX281).

    If I am reading the Maxim specs right, the oscillator's output voltage
    is too low. The notes say that when using an external oscillator, "the
    input on the COSC pin must swing close to the power rails (V+, V-)." I
    will probably power it with +1 5 V.

    The notes go on to say: "Although standard 74HC00 series CMOS gates do
    not guarantee CMOS levels with the source and sink currents of the COSC
    pin, they will in reality drive the COSC pin. CMOS gates conforming to
    standard B series output drive have the appropriate voltage levels and
    current to simultaneously drive several chips."

    I looked up the 74HC00 series CMOS gates and found that they are NAND
    gates. Will they really operate to amplify my clock signal? What am I
    missing, here? I also read some previous threads in the newsgroup that
    mention using a comparator for such, which I understand a little better.

    If anyone would like to try to explain this to idiot child here, thanks
    in advance.

    Amanda
     
  2. Don Bruder

    Don Bruder Guest

    It boils down to this:
    Your oscillators don't have enough "oomph" to directly drive the pin on
    the 281. So you need to feed the oscillator output into a NAND - your
    oscillator into input A, input B grounded (Or whatever other combination
    works - a pair of NOTs in line with each other would do just as well) to
    give you the signal at the "Y" pin at a level that *DOES* have enough
    "oomph" to drive the 281's pin.

    In a way, the gate is acting like an amp, but that's just a side-effect
    of it doing its primary function - the logic operation - It sees your
    weak-but-present 1 from the osc on A, and the grounded (0) B input, says
    "1 NAND 0 equals 1", and outputs a "full strength" 1 signal, since for
    gates like the 7400, a weak "on" is no different than a super-strong
    "on".

    And to clarify something before you confuse yourself:
    The "7400 series" isn't just one chip - The 7400 is indeed a NAND gate.
    But there's the 7401, and the 74243, and the 7411, and... (A whole bunch
    of them with numbers that start out "74", some with letters, like
    74LS127 or 74HCT04) each one doing its own thing. Don't confuse one chip
    of the series with the whole series! By referring you to "the 7400
    series", they're telling you that chips in that series are probably
    going to do the job of boosting your clock so that your signal can be
    used by the 281, even though the 281 docs may not think so.
     
  3. Amanda Robin

    Amanda Robin Guest

    (a really nice explanation as to why a NAND gate in the 7400 series
    would effectively "amplify" a clock signal)

    Thanks very much. Off to find one that will work for me.

    amanda
     
  4. GPG

    GPG Guest

    You want 74Cxx or 40xx at 15V
     
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