[SI-LIST] Re: Matching impedance
- From: Jim G Roberts <jgroberts@xxxxxxxxxx>
- To: whizplayer@xxxxxxxxx
- Date: Fri, 31 Jan 2003 19:12:50 +0100
Hi Bob,
I assume by equlisation you are referring to compensation for the
frequency dependent losses of a transmission line.
This equalisation does not compensate for mismatches.
As far as the mentioned capacitive parasitics e.g pads, vias, etc. if these are
distributed relative to the wavelength you are considering (risetime dependent)
then you can consider them as reducing the line impedance. However if not they
are lumped elements that will require compensation with a series inductance.
I assume your matching network is resistive and that you have allowed for
attenuation and matching from both directions.
When measuring on board impedance you must take special care that cable outer is
well grounded and the inner
is relatively short. If their is an appropriate place to disconnect the line and
mount a termintion resistor you can callobrate the cable and board connection on
your s-parameter testset. If using TDR the mismatch will reduce the risetime and
accuracy of the pulse.
--
Regards, __________ James G Roberts
/___ ____ | jrobert@xxxxxxxxxxxxxxxxxxxxx
Jim __ / /___/ / jgroberts@xxxxxxxxxx
/ /_/ /---| | Room: BE436, Hilversum
\____/ /_/ Tel: +31 35 687 4308 Fax: 5976
Bob Patel wrote:
> Hi! For most applications the impedance requirement is
> say 50ohm single ended or 100ohm differential and so
> on a PCB the traces are etched to meet this impedance.
> Now, for some signals it is not critical to maintain
> absolute 50/100 and for higher speed devices, chips
> have equalisation circuitry that takes care of some
> amount of noise to a certain degree. But the problem
> comes for chips that don't have equalisation.
> I have an interconnect that has 75ohm output impedance
> driver & 50 ohm input receiver. Hence I need to put
> matching network. So, the design of the interconnect
> is Driver-->75ohm trace-->matching network-->50ohm
> trace-->receiver. Now, I keep the 75ohm trace as short
> as possible. But the problem I have is that if I look
> at the impedance from the driver upto the matching
> network it will not be 75ohm but lower because of
> added parasitic capacitance of pin, via, pad of
> matching network. Hence, I was thinking of removing
> the non-functional pads on the driver pin(thru-hole),
> keep the trace impedance slightly above 75ohm say
> 80ohm, that way the effective impedance seen by the
> driver is around 75ohm and then I can plug in the
> values of resistors to give 75ohm termination at that
> point.On the other side since the trace is 50 ohm it
> will terminate into the 50ohm termiantion at the
> receiver.
> Any thoughts or am I on the wrong track?
> Also, what is the best way to measure impedance of
> assembled board. I solder an SMA cable to the pin(pt.
> of interest) and look at the impedance.
> Thanks in advance
> Bob
>
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