[SI-LIST] Re: FW: DC-blocking transmission-line
- From: "George Korony" <gkorony@xxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, "Si-List" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 29 Jun 2007 14:05:39 -0400
Andy,
Thanks for your posting, It is exactly what I am talking about. Maybe my
English is not good enough and I can not explain things as clearly as you
did, thanks again!
George
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Andrew Ingraham
Sent: Friday, June 29, 2007 1:21 PM
To: Si-List
Subject: [SI-LIST] Re: FW: DC-blocking transmission-line
> By definition the impedance of a capacitor is 1/(j * omega * C). It is
> not constant. The best you can do is to adjust C to place the
> capacitance within the range you need, over the bandwidth you desire.
Scott, my read is he's not talking about that capacitance and not that
impedance. For a DC blocking application, C is large so its impedance over
the useful range of frequencies contained in the signal ought to be much
less than Zo, not matched to it.
Anyway, we all know from transmission line theory that, even though
1/(j * omega * C) is not constant (where the C is capacitance per unit
length), the Zo of the line is moderately constant.
I think he is talking about the mutual 'stray' C between the capacitors.
That, combined with their mounted inductance, results in some effective
impedance of the mounted capacitor pair to differential signals. If the
board uses a pair of 50 ohm loosely-coupled co-planar traces, then one might
want the capacitor pair to have an effective 100 ohm characteristic
impedance to differential signals, treating the capacitors as a short
differential transmission line.
That's got nothing to do with passing your lowest frequencies of interest.
Except that capacitor value influences construction and inductance.
> does anyone have a
> different approach to this problem?
Adjusting pad sizes on the board may help as you approach (but not encroach
into) the frequencies where the capacitors need to be treated as a
transmission line.
Andy
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