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Choice of DC blocking capacitors in Microstrip Design

Discussion in 'Electronic Design' started by Alec, Aug 11, 2008.

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  1. Alec

    Alec Guest


    I was wondering as to how to choose the correct value of a DC
    capacitor with least possible reactance with atleast 5V and 20mA.
    Center Frequency 910MHz. The value of capacitor that i need is above

    I know the thumb rule that SRF(self resonant frequency) > 2 Operating
    frequency. but i enquired both Farnell and Rscomponents distributors
    in UK as am here .., Only low value capacitors satify the above
    criteria of SRF. How do i proceed with the same...

    Thanks a lot
  2. Alec

    Alec Guest


    Is it the same for 0805 case or is there a difference.

  3. Alec

    Alec Guest

    AVX have capacitors whose SRF =950MHz at 47pF for 0805. 33pF --> srf =
    880MHz for 1210
  4. Let's look at Alec's figures (a different post in this thread) for an
    0805 47pF having self resonance at 950 MHz.

    I work that out to .6 nanohenry.

    Impedance of .6 nanohenry at a gigahertz is a little less than 4 ohms.

    Suppose that .33 uF 0603 also has .6 nanohenry of inductance. Self
    resonance would be a mere 11.3 MHz. Does that sound like what you expect?
    Impedance at a gigahertz is still a little under 4 ohms.

    Just to explore further - characteristic impedance of a microstrip .03
    inch wide, .015 inch thick, .063 inch over a ground plane.

    Page 6

    I figure 82 ohms for unity dielectric factor. That works out to 273 nH
    per meter (If I did that right for unity permeability and dielectric
    factor), .416 nanohenry for a piece of such conductor .06 inch long.
    Does this sound close to the inductance of an 0603 capacitor? If the
    capacitor's ESL is close to that of a same-length same-width trace, then
    the capacitor is just a capacitor or effectively just a piece of trace at
    frequencies even well past the self-resonance frequency figure.

    One more thing - such a capacitor will have a litle more inductance and
    a little lower self-resonant frequency in free air than it will close to a
    ground plane.

    - Don Klipstein ()
  5. Tom Bruhns

    Tom Bruhns Guest

    I'll second that, with a bit more explanation: smt caps have
    inductance that's determined by their physical size. A piece of wire
    has inductance, after all; a length even .06 inches or .08 inches of
    microstrip has inductance. What it also has, though, is capacitance
    to a ground plane. The distributed inductance and the distributed
    capacitance cause it to have a particular characteristic impedance,
    sqrt(L/C) (neglecting the reactive part of the impedance caused by
    resistance, which is generally quite small in the GHz region). If you
    model the MLCC smt part as a conductor the length, width and thickness
    of the part, you'll be really close to the way it actually performs in
    the circuit. That is, make its inductance part of the transmission
    line you mount it in. If the capacitor is slightly narrower than the
    microstrip trace, the impedance should remain very close to constant
    through the capacitor. If you want to get fancy about it, use the
    freeware ATLC program to calculate impedance for that cross-section;
    or use Agilent's ADS to accurately predict performance to well beyond
    1GHz, if you put in an accurate model of your system. -- The reason
    I say that ideally the capacitor will have a width slightly less than
    the microstrip trace is that the added height of the part adds
    capacitance to the ground plane and slightly decreases the series
    inductance compared with a thin trace the same width. But...the
    propagation velocity of the line is such that .06 inches of line
    represents only about 3 electrical degrees at 1GHz using an FR4
    substrate, less for lower dielectric constant substrates. One way to
    think about what that means is that a reflection off the "leading"
    edge is very nearly canceled by the corresponding reflection off the
    "trailing" edge, where you have an impedance discontinuity only that
    long. If you try to think about this stuff in terms of self-resonant
    frequencies, you'll get yourself all needlessly worried. The model I
    suggest above is far closer to what's really going on. (This assumes
    the series reactance of your capacitor is very small compared with the
    transmission line impedance, of course; otherwise, just include that
    as a series impedance at that point along the line.)

  6. Alec

    Alec Guest

    I am designing a phase shifter hybrids(branchline) reflection type
    using pin diodes. I presumed according to the datasheet in
    having a graph with series resonance and freq plot . for my center
    freq 910MHz this capacitor has SRF of 940Mhz... But theoretically, xc
    = 1/cw ... will give a reactance of 3 ohm but this is where where the
    parasitic inductance would be best compensated is it not...
  7. Alec

    Alec Guest

    wat is the minimum length of a transmission line that would not hinder
    soldering of capacitors(0805 case).., less than 0805 would be
    difficult to solder for me.. :)

    for the input port to the hybrid is an extra section of line of small
    length necessary to feed in thru coaxial.... cause even a small
    addition of a section line will result in phase change...?

  8. Tom Bruhns

    Tom Bruhns Guest

    Yes, since 50 ohm microstrip on 1.6mm (1/16 inch) FR4 with no internal
    planes is over 2.5mm wide (over 0.1 inch wide), you'd want to do
    something like what John suggests for that, if you really want to keep
    things constant RF impedance. I tend to deal with microstrip on
    multilayer boards where the trace width is very similar to the width
    of an 0603 part. It becomes one of those things you think about in
    system design: the board stackup is not independent of the passive
    part selection. But again, until you get to pretty high frequencies
    (and especially if you are trying to keep things very wideband) it
    tends to not be a problem. If you are dealing with narrow-band, like
    I suspect the OP was ("center frequency 910MHz..."), you can pretty
    easily tune out the small effect of an 0.08" long capacitor which
    doesn't exactly maintain the microstrip's impedance. If I model an
    0.08" long 80 ohm section (typical microstrip velocity factor)
    inserted into a 50 ohm line, at 1GHz, I see a return loss degradation
    to -28dB, and a whopping 6 millidegree insertion loss due to
    mismatch: both probably not worth worrying about outside of precision
    measurement systems. I expect that a conductor of the same cross-
    section as an 0805 cap, as a microstrip trace on a 1/16" board with
    ground on the other side, would be a lower impedance than 80 ohms,
    probably about 70-75 ohms.

  9. I use it all the time. In fact, I do layouts where the mask is
    already cleared off the ground where I know I'll probably want to tap
    in. (And making sure there are plenty of gnd vias there.)

    I've been using 47 mil od with sma, up to 8G, for now.

    Regarding the DC blocking, I have to echo what JL sez. I've tried to
    measure 0402 L, and estimate it to be around 0.2 nH. If the
    application is wideband, then just put the biggest cap possible, as
    0.2 nH is ignorable to fairly high frequencies. If narrowband, then
    just pick for the SRF, as it will be very slightly less series Z.

    Also, in some cases and at higher frequencies the dissipation R can
    become as significant as the inductance X. So a jelly-bean X7R might
    not be so good in that respect. I think ATC has some better Q
    wideband caps.
  10. Whether it is 50 or 75 doesn't really matter. If you are at a high
    frequency that it matters, then the launch onto the board will
    probably be an edge mount 3.5 (or similar) to either microstrip or
    grounded co-planer waveguide. Then transitioning to stripline is yet
    another discontinuity.

    There should be no annular rings on the inner layer pads for the "via"
    down to the stripline layer. (That layer has the annular ring, of
    course.) Also, back-drilling the via is not too expensive from some
    manufacturers. (Cheaper than blind vias.) Instead of, say 30 mil of
    "hanging via," you can get it down to maybe 2-7 mil with the back-
    drill tolerance. To state the obvious, what you do depends upon
    you're requirements.

    To really get an idea of the transition's performance before you build
    it, you'd need a full 3D simulator. HFSS or CST, that is.

    THis was amusing at a quick scan:

    from googling:
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