Re: [SI-LIST] : RF & Digital

Adrian Shiner (adrian.shiner@virgin.net)
Thu, 12 Aug 1999 19:47:47 +0100

I'll second that!. To a reasonable approximation the signal doesn't care whether a power supply line is the high voltage side or the ground side if the supply is designed correctly. It is the best of a non optimum situation

Adrian

"S. Weir" wrote:

> I think that the application needs to be understood. The problem that I
> see with the proposed stack-ups is what happens when a signal switches
> planes. The nearest reflection plane for several of the layers is one or
> the other of the PS voltages. This is going to interpose significant loop
> area and inductance as the return current will have to traverse some
> portion of one of the power planes and then finally return through the
> decoupling network. Given the fundamental lower inductance limits on both
> vias, and the decoupling capacitors, this is going to really limit
> crosstalk attenuation at very high frequency. My vote goes along with the
> suggestion of:
>
> S sparse
> S
> G reflection for L2/L4
> S
> V
> V
> S
> G reflection for L7/L9
> S
> S sparse
>
> With respect to the power decoupling and buried capacitance, I think this
> is better done by adding two GND layers if the design can possibly afford it :
>
> S
> S
> G
> S
> V
> G
> G
> V
> S
> G
> S
> S
>
> I am also a disciple of using plane cuts only with the greatest
> consideration. It is fundamental that where the digital logic interfaces
> to the RF that none of the digital signals cross a plane cut.
>
> I also agree with that consideration must be paid to which if either
> voltage plane is powering the RF, and whether it also powers logic. Once
> that is known, then a reasonable stack-up and placement rules can be
> generated. Without it, I think we are all firing arrows in the dark.
>
> Regards,
>
> Steve.
> At 04:35 PM 8/11/1999 -0500, you wrote:
> >Depends on what you are looking for, but the most likely effect is the
> >possibility of noise coupling directly between the two power planes. With
> >the stackup as shown here, 5V couples tightly to L6 and more loosely to L2,
> >giving enhanced plane-to-plane capacitance. If 2.5V needs to be the less
> >noisy, then switch L9 and L5 layers.
> >
> >Ron
> >
> >
> >
> >
> >Shayle Hirschman <shayle@mho.net> on 08/11/99 07:19:24 AM
> >
> >Please respond to si-list@silab.eng.sun.com
> >
> >To: si-list@silab.eng.sun.com
> >cc: (bcc: Ron F Parson)
> >
> >Subject: Re: [SI-LIST] : RF & Digital
> >
> >
> >
> >
> >An excellent suggestion, Mitch.
> >
> >Any benefit to switching L6 with L9?
> >
> >Shayle
> >
> >
> >
> >
> >At 07:59 AM 8/11/99 -0700, you wrote:
> >>Hi Lum,
> >>
> >>As Shayle states this is a rather odd stackup, and I would certainly not
> >recommend it unless I was under duress to comply. This will not get you the
> >impedance control you're after unless you take some severe steps to control
> >it
> >>layer by layer (i.e. differential pairings). I'd suggest the following
> >stackup design:
> >>
> >>L1 : signal
> >>L2 : GND
> >>L3: signal
> >>L4: signal
> >>L5: +5V
> >>L6: GND
> >>L7: signal
> >>L8: signal
> >>L9: +2.5V
> >>L10 : signal
> >>
> >>Good luck,
> >>
> >>Mitch Morey
> >>Sr PCB Designer
> >>San Diego, CA, USA
> >>
> >>Shayle Hirschman wrote:
> >>
> >>> Why a stack-up of 4 signal layers in a row (L4 to L7)?
> >>>
> >>> This seems a bit unusual.
> >>>
> >>> Shayle
> >>>
> >>> At 08:09 AM 8/11/99 +0800, you wrote:
> >>> >If I have a design that have digital circuit and a small portion of RF.
> >>> >There will be some signals going from digital into the RF and out from
> >>> >the RF to the digital portion. If the digital portion has to be
> >>> >impedance control at 50ohms, how should I go about doing such design to
> >>> >ensure that both RF and Digital requirements are taken care of, without
> >>> >causing any interference between these two circuits? What about the
> >>> >ground? This design will be a 10-layer with the following stack-up :
> >>> >L1 : signal
> >>> >L2 : GND
> >>> >L3 : +5V
> >>> >L4 : signal
> >>> >L5 : signal
> >>> >L6 : signal
> >>> >L7 : signal
> >>> >L8 : +2.5V
> >>> >L9 : GND
> >>> >L10 : signal
> >>> >
> >>> >I would appreciate any advise from you experts out there.
> >>> >Thanks and regards.
> >>> >
> >>> >
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