[SI-LIST] Re: PCB tracks
- From: ray_waugh@xxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 9 Aug 2002 07:52:48 -0600
Changes in characteristic impedance do not matter as long as the mismatched
section is shorter than a tenth wavelength at the highest operating frequency,
and as long as it is offset by a second mismatch. For example, a short section
of narrow line (high impedance) can be nicely offset by a short section of wide
line (low impedance), forming a low pass filter with a cutoff frequency higher
than your highest operating frequency.
Radiation in TEM mode microstrip lines is a function of frequency, board
thickness and board dielectric constant. Thin boards and boards with a high
dielectric constant restrict the E-fields of the line (keep them close to the
ground plane) and inhibit radiation. 32 mil FR4 works very nicely to 6 or 8
GHz, and 16 mil FR4 would not radiate anything you can measure at 15 GHz.
Ray
Raymond Waugh
Agilent Technologies
ray_waugh@xxxxxxxxxxx
-----Original Message-----
From: Ismail B - CTD, Chennai. [mailto:ismailb@xxxxxxxxxxxxxxx]
Sent: Friday, August 09, 2002 3:09 AM
To: gurunath.kulkarni@xxxxxxxxx; cadpro2k@xxxxxxxxxx; SI-LIST
Subject: [SI-LIST] Re: PCB tracks
Hi GVK,
I think there will be a sudden change in charateristic impedance at 90 deg
bends due to increase in trace width at those points. This change in
impedance will set up reflections on the trace. As far as EMI is concerned I
have a feeling that pointed edges do radiate more that curved edges.
However, Im not sure about other members comments (Perhaps, I have not
experimented on this ). Generally, We prefer 45 deg bends on our boards.
Regards,
Ismail
-----Original Message-----
From: gurunath vinayakrao kulkarni [mailto:gurunath.kulkarni@xxxxxxxxx]
Sent: Friday, August 09, 2002 3:18 PM
To: cadpro2k@xxxxxxxxxx; SI-LIST
Subject: [SI-LIST] Re: PCB tracks
Hi Mitch,
I meant 45 deg and not 90 deg. If you have 90 deg , there will problems when
the current flows through the trace, the steep edge ( 90 deg) will act as
antenna and there will some EMI problems( emit radiations). When the
electrons hit the edge , there will sudden change in the direction of the
flow of the current, so this creates the emission effects. Please anybody
correct me if I am wrong.
GVK
----- Original Message -----
From: <cadpro2k@xxxxxxxxxx>
To: "SI-LIST" <si-list@xxxxxxxxxxxxx>
Sent: Thursday, August 08, 2002 11:00 PM
Subject: [SI-LIST] Re: PCB tracks
> Hi GVK,
>
> I think you misspoke. I'm sure you meant to say "usually 90 deg is OK"
> CAD tools make it so easy to do the 45s, and we, the PCB designers, have
> been asked to make them all 45 for the longest time, since the
> engineering community drove the CAD tool vendors for the 45 degree
> option. Still, so much time is wasted day in and day out on "cleansing"
> the board ("NO! I can't have 90 degree bends on my board with a clock
> speed of 100Mhz!"), and making all the buried layers look appealing. I
> still contend the 45 degree theory is a fallacy, and I think it's been
> proven time and again (see http://www.ultracad.com , notes section).
>
> Ranting over.
>
> Mitch
>
> ---------Included Message----------
> > Date: Thu, 8 Aug 2002 17:45:38 +0530
> > From: "gurunath vinayakrao kulkarni"
> > I agree with Philippe Robert as I have not seen the PCB with
> curved traces
> > usually 45 deg is O.K
> >
> > GVK
> _____________________________________________________________
>
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