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Caps and Resistors for RF applications

Discussion in 'Electronic Components' started by O. P. E., Apr 11, 2007.

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  1. O. P. E.

    O. P. E. Guest

    I am trying to make the ns pulse generator in Linear's App Note 98.
    The basic idea of the circuit is to use comparators to produce a
    delayed pulse and an inverted pulse, and then use an AND gate to
    produce a pulse. It uses a resistor and capacitor to form the delay
    lines. It specs 8pF caps and the circuit also has a variable resistor
    in it.
    be in the RF "side" of the circuit. Still, I am planning to use
    surface mount components to reduce RF problems, and I was thinking of
    purchasing some "High frequency" ceramic caps, as Mouser has some from
    AVX and Murata. I am wondering whether these parts RF properties will
    be so different from that of "standard" ceramic caps as to cause
    problems? (The app note and associated articles do not specify the
    caps beyond 8pF).

    Also, I was planning to use a surface mount pot to reduce RF problems
    with the leads. Since pots are relatively large compared to other
    components, I was wondering if it is likely to cause problems. Would
    wrapping it in AL foil help?

    Thanks
     
  2. I think that would just be a new source of stray capacitance. Some SMT pots
    are very small, so it's worth looking for one, if it's a preset you want.
     
  3. LVMarc

    LVMarc Guest

    Depending on the RF freq uencies you use, the smt pots may not be a good
    idea! The size and convulted leads add inductance, at some point the jwL
    impedance becomes significant..thats your personal cutooff frequency. if
    you below that go for it, if not use RF resitors and caps.

    what makes the rf labeled r and c different? these parts all have
    parasitic elements. caps have hidden resitors and inductnce,, inductors
    have hidden lead to lead capcitors, so these also have a

    self resonant frequency. thats the indivduals part's limit! so your "nS"
    pulse sytem needs a clear DC - > 1GC frequency......

    Pulsed applications also need "low esr" to be able to respond to the
    transtional state where the impulse from the leading edge occrus.

    Best Regards and good luck

    Marc
     
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