Re: Choice of DC blocking capacitors in Microstrip Design

On Mon, 11 Aug 2008 10:05:38 -0700, "BobW"
<nimby_GIMME_SOME_SPAM@xxxxxxxxxxxxxx> wrote:





"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:g0o0a4t3lrj5v5vt2pgs11av2kh7bqtnlm@xxxxxxxxxx
On Mon, 11 Aug 2008 08:48:46 -0700 (PDT), Alec
<yttoh.etis@xxxxxxxxxxxxxx> wrote:

Hi,

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

[snip]


There's no reason for that rule. Most of the time a reasonable, like
0.33 uF, 0603 maybe, surface-mount cap has a very low RF impedance,
essentially the same ESL as most any other 0603 cap. SRF doesn't
matter, impedance does.

A 0.33 uF 0603 cap is a very low z well into the GHz. Ideally, match
the cap body width and the trace width.

John


Whose 0.33uF do you use that is not well above its resonance at 1GHz?

Bob

Consider a 1 uF 0603 cap. Its ESL may be ballpark 600 pH, so its SRF
is around 6.5 MHz. So what? At UHF frequencies, it's still 600 pH. If
you bought a 10 uF 0603, or a 0.1 uF, it would still look like 600 pH
at high frequencies.

The SRF *doesn't matter*. The working impedance does.

If you want a really low impedance DC block, make the microstrip trace
wide, break it with a tiny slit, and pave it over with a row of 1 uF
0402 caps as wide as the trace. They're short, so have even lower ESL,
and in a 50 ohm trace, depending on width, you may be able to get 2 to
5 in parallel.

John

.

Relevant Pages

  • Re: Choice of DC blocking capacitors in Microstrip Design
    ... uF, 0603 maybe, surface-mount cap has a very low RF impedance, ... essentially the same ESL as most any other 0603 cap. ... Its ESL may be ballpark 600 pH, so its SRF ... If you want a really low impedance DC block, make the microstrip trace ...
    (sci.electronics.design)
  • Re: Choice of DC blocking capacitors in Microstrip Design
    ... capacitor with least possible reactance with atleast 5V and 20mA. ... uF, 0603 maybe, surface-mount cap has a very low RF impedance, ... essentially the same ESL as most any other 0603 cap. ... the cap body width and the trace width. ...
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