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Bypass capacitor values at RF

Discussion in 'Electronic Design' started by Paul Burridge, Aug 9, 2003.

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  1. Hi chaps,

    I'm just starting to build Frank's circuit for the VCO he kindly
    designed for me for 40Mhz. Frank's chosen values for bypassing are all
    22nF caps; but I'd like to use 10nF ones if possible, since I've got
    200 of them and no 22s whatsoever. I can't see a problem with this but
    thought I'd best check with the Panel first. Can any far-sighted
    individuals see a difficulty in this expedient?

    Thanks awfully,

    p.
     
  2. Active8

    Active8 Guest

    they'll be half as effective. parallel two of your 10nF caps.

    mike
     
  3. I wasn't following the thread at all, but it seems like something smaller
    has been traditionally common at 40MHz. More like .001uf aka 1nF.

    Maybe there's something special about this design, but even if you stick
    with the larger values I can't see a lot of different between 22nF
    and 10nF. Likely more important is making sure the lead lengths are
    small, and picking capacitors which don't have a lot of built in inductance.

    Michael
     
  4. That's the kind of value I'd have expected to see, but no doubt Frank
    had good reason for specifying the larger ones.
    The ones I've got are all Mylars. I've no idea how much paracitic
    inductance they have and without access to a network analyser I'm
    unable to tell what their SRF is. But I should be okay with Mylars,
    surely?
     
  5. What often happens to RF byp caps is the inductance of the leads
    resonates with the cap to form a series resonant circuit. Works great
    at the Fr, but above or below it becomes less of a bypass. So it
    depends on the capacitor, lead length, etc. Ceramic disks make good
    bypass caps. You can find out by dipping it with the leads shorted.

    Sso post the URL of the circuit so we can check it out. Sorry, no, I
    haven't read other followups.

    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  6. Not necessarily. See my other followup.
    --
    @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@
    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
    @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@
     
  7. Paul Burridge wrote:
    (snip)
    Not so surely. The cheap (usually green) mylars from China have their
    wire leads attached to the ends of two metalized mylar strips that are
    then wound around the pair of lead ends. They are more of a wound
    transmission line than a lumped capacitor (Arrrggg!). Good quality
    mylars have the metalization or foil extended from both ends of the
    roll, and the leads attached such that there is a very short
    conductance distance and low resistance from the lead to any part of
    the capacitor. Crush one of yours, and see how it is made.
     
  8. That's very good of you, Frank, but I'm determined to press ahead and
    get this thing built over the next day or so as time is getting very
    short now. Posting it would take too long, I'm afraid and I've done
    all the layout stuff now anyway so am most of the way there.
    I'm sure they're the best solution from the performance side of
    things, but I have to admit I'm extremely reluctant to get involved
    with SMDs unless it's totally unavoidable!
     
  9. Reg Edwards

    Reg Edwards Guest

    "I believe history will be kind to me, since I intend
    ---------------------------------------------

    Winston admitted to being one of the most proficient warmongers of the
    century.

    It's time this sort of signature was abolished from the network.
     
  10. Are you volunteering to be the standard maker and censor?
     
  11. Speaking of standard makers, does somebody know what has happened to
    John Woodgate? I miss him.
     
  12. Paul Burke

    Paul Burke Guest

    He seems to have taken up archaeology in a big way: his posts constitute
    about a quarter of the Britarch mailing list. Cantankerous and sensible
    as always.

    Paul Burke
     
  13. The frequency response of a bypass capacitor drops to near zero where it
    resonates with its lead inductance, and then RISES with incresing
    frequency after that.

    You have to plot or RX bridge the capacitor to see what it does at the
    frequencies of interest.

    "Larger" capacitors are usually "worse".

    --
    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email: fax 847-574-1462

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  14. Signs of a really alert brain, can change field of interest.

    I'm thinking of taking up oil painting myself, after seeing some
    painters course (by Bob Ross) on tv. It made me look at the world in a
    new way, I see things I never saw before and I often think about how I
    could paint things I see.

    Bob Ross has hi-jacked my brain. And I like it.
     
  15. People of your age should know better. You're talking like a Taffy,
    which I assume you're not by your callsign. So you've no mitigation.
     
  16. And that's why in the old days, it was common in hobby circuits to
    show .o1uFs up to a certain frequency, and then the switch to .001uFs
    for the next range. Theoretically the .01uFs were still fine, indeed
    they'd be even better the higher you went in frequency, with the
    reactance getting ever smaller. But since they tended to be physically
    larger, a switch to a lower value bypass was the norm, because the
    inductance from it's leads would be (hopefully) lower.

    Michael
     
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