RE: [SI-LIST] : RE: 2.5 GHz in FR4

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From: Sreejith Varma (varma@blr.paxonet.com)
Date: Tue Apr 03 2001 - 04:53:42 PDT


Hello Patrik,

thank you very much for the info..

>* Do not use stripline. The discontinuities presented by
>the vias are most often killers. Avoid all vias in the high
>speed nets and use microstrip (i.e., top-surface only).

>* Use wide traces. FR4 is lossy at 2.5 GHz, so try to compensate
>for the high dielectric loss by decreasing the metal loss. Use
>wider traces which will reduce the resistive losses in the metal.

the general rule i am planning to give is 5 mils track with 5 mil spacing.
i have few 155mhz / 622mhz signals also and due to constaints of
BGA and microstrip, i was planning to put them also in microstrip with
5 mil track and 5 mil spacing.

how much more do i need to increase? is 8 - 10 mils ok?
as with different trace width the impedance varies, in order to get a
uniform
impedance across the board, i may need to make two power planes, one which
references the 155 mhz signals (5 mils) and one which references the 2.5 ghz
signals. will this be ok? or is it better to put the 155 mhz signals onto
strip lines and use 2.5 ghz as microstrip and manipulate the planes?

>* Use short traces. If you need to route 2.5 GHz signals
>on nets that are 10 inches (25 cm) long, I doubt if your
>system will function. Keep the traces *short* (say, less
>than 1 inch [2.5 cm]).

Best regards
sreejith

> HI All,
>
> i am doing a 2.5Ghz PCB design. i was wondering whether FR4 material
> is sufficient for such high speed signals? or i should look at some
> other material for these desings? and if FR4 is sufficient is
> there any
> special care i have to take (other than impedance matching
> and differential
> routings)
>
> thanks in advance,
> Sreejith
> Paxonet Comm. (I) Ltd.
> Blore, India

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