[SI-LIST] Re: Matching impedance
- From: Betty Luk <bl87@xxxxxxxxxxx>
- To: scott@xxxxxxxxxxxxx, whizplayer@xxxxxxxxx
- Date: Mon, 03 Feb 2003 15:07:43 -0500
Hello,
I am a bit confused as to when certain impedance matching schemes should be
used. (This is related to the question I posted last week regarding
impedance matching for high freq PCB). There seems to be two major categories
1) resistive termination (this seems to be the accepted method for most PCBs)
2) as referenced below, Microwave techniques (such as stub matching,
tapered lines, etc)
Which method should be used when?
Thanks in advance,
Betty
At 01:24 PM 1/31/2003 -0800, Scott McMorrow wrote:
>Bob,
>
>Chapter 5 of Pozar's Microwave Engineering discusses all of the
>different methods for impedance matching and the limits of each. If you
>have control over the package of the device, or if the package of the
>driver is a 75 ohm impedance, then you could use a tapered transmission
>line to provide a match out to the 50 ohm system impedance.
>
>regards,
>
>scott
>
>--
>Scott McMorrow
>Teraspeed Consulting Group LLC
>2926 SE Yamhill St.
>Portland, OR 97214
>(503) 239-5536
>http://www.teraspeed.com
>
>
>
>
>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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