[SI-LIST] Re: Matching impedance
- From: "Larry Barnes" <larry.barnes@xxxxxxxxxx>
- To: "silist" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 31 Jan 2003 14:03:35 -0800
Scott,
If you are looking for a broadband match, you have to keep in mind the =
lower end of the spectrum of the signal. The length of the taper is =
proportional to the wavelength of this frequency. An example would be =
an 8B-10B data stream, the lower end would be a series of nulls while =
the upper end would be alternating data. I seem to recollect looking at =
this technique for matching Fibre Channel signals and came up with a =
taper that needed to be on the order of 18 inches.
Larry
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Lawrence C. Barnes
QLogic Corporation
2660 Laguna Hills Drive
Aliso Viejo, CA 92656
-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Friday, January 31, 2003 2:24 PM
To: whizplayer@xxxxxxxxx
Cc: silist
Subject: [SI-LIST] Re: Matching impedance
Bob,
Chapter 5 of Pozar's Microwave Engineering discusses all of the=20
different methods for impedance matching and the limits of each. If you=20
have control over the package of the device, or if the package of the=20
driver is a 75 ohm impedance, then you could use a tapered transmission=20
line to provide a match out to the 50 ohm system impedance. =20
regards,
scott
--=20
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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