There is a reason for the 50ohm impedance, meaning you want to match it as
close as possible to reduce the effects of a mismatch and thus undesirable
effects of it.  It is the characteristic impeadance of the output driver
that you want to try to match as close as possible to reduce losses, etc.

I am not sure what you are using to layout the board, but if you have
concerns (and you should) then perhaps using some SI software to analyze
this might be usefull.  Cadence has SpectraQuest (hard to use), Mentor has
Hyperlynx (easier to use).  You might be able to download a demo version to
analyze just this part of the circuit, but you need to request the correct
interface from the board file.  PADs is the Mentor product, so by default it
interfaces.  It will read in the stackup and the layout and give you the
data you need.  Change the impedance and see how it affects the signal
integrity.


>From: Damien Cahill <damiencahill@AUSTARNET.COM.AU>
>Reply-To: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [EE]: RF Transmission Line PCB Routing
>Date: Fri, 20 Feb 2004 10:55:04 +1000
>
>Hi List,
>
>I have a reasonably simple question.  I am currently routing an RF PCB
>which
>uses a 1GHz DDS synthesiser.  The RF signals on the board are up to 400MHz
>in frequency and obviously I must consider the impedance of any copper
>tracks I lay.  What I am unsure of though is what impedance should I be
>aiming for with these tracks.  Should it match the DDS outputs which are
>all
>50ohm or should it be as low impedance as possible to reduce resistive
>losses.  I eagerly await the lists omnipotent response.
>
>Cheers,
>Damien Cahill
>
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