oops, I forgot to reply to everyone, sorry...
Thanks for all the great responses and for the references, too!
For the record,
even though my traces were less than 2 cm long,
I DID route them as impedance controlled differential pairs,
but I couldn't help wondering how much difference it made
(pun intended)
Since there are only two instances of these lines,
and I did a good job on component placement to keep the traces short,
I'm tempted to not even declare the board as having impedance requirements
(why waste the extra money if I'm confident that I did my job properly?)
just sayin'
Jack
On Mon, Sep 23, 2013 at 10:16 PM, Barry Olney <[log in to unmask]> wrote:
> Hi Guys,
>
> Bill is correct in that it is the rise time rather than the frequency that
> is of importance. However, the frequency does determine the maximum
> bandwidth.
>
> Rule of Thumb: All drivers whose trace length (in inches) is equal to or
> greater than the rise time (in ns) must have provision for termination. So
> for a 1ns rise time the max length is approx one inch.
>
> Also, the bandwidth of a 40MHz signal needs to be considered. I generally
> take the 5th harmonic (or 7th if it is extremely fast rise time). So 40MHz
> means you are really working with 200MHz.
>
> For differential pairs - they are best routed with a clearance of twice the
> trace width. So, for a 4 mil trace the clearance should be 8 mils.
>
> There are quite a few articles, on these subjects, that were originally
> published in the PCB Design Magazine on my web site if you go to
> www.icd.com.au and then "News and App Notes" from the main menu.
>
> Practical Signal Integrity
> Differential Pair Routing
> Etc
>
> Cheers,
> Barry
>
>
> -----Original Message-----
> From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
> Brooks,
> William
> Sent: Tuesday, 24 September 2013 3:25 AM
> To: [log in to unmask]
> Subject: Re: [DC] Impedance Control vs. Line Length
>
> Hey Jack,
>
>
> Is this RS232? Ethernet? Some other mode?
>
> Impedance matching is important... especially as you get into higher
> frequencies...
> At Microwave frequencies, the reflections that happen at the transition
> from
> one impedance value to another can destroy the integrity of the signal...
>
> 40Mhz sinusodal analog signal has a wavelength of about 24 1/2 feet (7.5
> meters), (1/4 wave length = 6.125 ft).
>
> A 40Mhz digital signal has frequency components that are in the Gigahertz
> to
> make the square wave signal... (that's a little different animal) ....
> The rise/fall time of the digital signal is key... A 1 GHz has a
> wavelength
> just under a foot... (0.98357 Ft) 1/4wave would be about 3 inches...
> That means that a 3 inch line on a pcb will make a great 1/4 wave antenna
> for 1Ghz signals... a 1.5 inch line works well for 2 Ghz... etc...
> receiving
> or transmitting...
>
> So check your rise times... they are the component that sets the high freq
> for your signal.... not the clock rate. The mutual close coupling of
> differential traces can help reduce x-talk due to the nature of
> differentially transmitted signals... there are tradoffs to know about in
> routing differential traces... The more tightly coupled the more sensitive
> to changes in the trace geometries... the looser they are coupled the less
> sensitive to routing influences.. Lee Ritchey talks about all of the
> aspects
> of high speed signal integrity in his 'Right the first time' book...
> http://www.thehighspeeddesignbook.com/ He did a presentation for the
> Designers Council local chapter here in San Diego awhile back and it was
> very informative...
>
> My reccomendation would be to run the lines differentially matched to the
> required impedance for the signal you are trying to route regardless of how
> short they are. Better safe than sorry... ;)
>
> Best regards.
>
>
> William Brooks, CID+
> Senior MTS (Contract)
> 2747 Loker Ave West
> Carlsbad, CA 92010-6603
> 760-930-7212
> Fax: 760.918.8332
> Mobile: 760.216.0170
> E-mail: [log in to unmask]
>
>
>
> -----Original Message-----
> From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
> Olson
> Sent: Monday, September 23, 2013 8:26 AM
> To: [log in to unmask]
> Subject: [DC] Impedance Control vs. Line Length
>
> I haven't done much impedance control lately, and I can't remember at what
> length the impedance control becomes significant.
> Is it 1/4 wavelength?
> 1/10 wavelength?
> 1/20 wavelength?
> In other words, if I have very short traces compared to the frequency,
> should I even worry about it?
> (I have some Tx/Rx pairs that theoretically should be routed as
> differential
> pairs, but they only need to be routed about an inch away. The clock is
> 40Mhz)
>
>
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