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RF ground planes layout

RF ground planes layout

RF ground planes layout

(OP)
May somebody provide me with any theoretical-practical application note (articles, web pages ...) regarding RF ground planes layout (up to 1.2 GHz)?

I certainly know about the characteristic PCB layout guidelines. A continuous ground plane on the back side of a printed circuit board as an ideal arrangement, the RF signal traces should be well terminated and kept short, careful decoupling of the power supplies, etc

Even though I am confused about the topside ground planes requirement and disposition. I believe for sure that a number of different strategies could be adopted. That there is not a unique answer for the best. In my design I am considering three synthesizers involving a couple of mixers and filters.

I have observed in some designs a different disposition of topside ground planes. Off course these grounds should be connected to the bottom side ground using plated through holes.
In some cases these topside planes looked as big as possible. Getting almost “indiscreetly” all the given leads under the common ground on that plane.

At the same time I have seen other layouts where the topside ground planes configured little islands. On these ground islands the leads of several components of particular functional network were placed

I believe that the last "top islands"strategy considering as many vias as possible could be really convenient .Is this correct?
 
 I do not have at this moment any simulation tool to test a given layout configuration.
 
I would sincerely appreciate any hint.
 
Thanks,
 
JB
.


RE: RF ground planes layout

Hi!

It strongly depends on the design.
 Sometimes, when the board is very big, ground plane on top side is used to make the whole board more rigid. In this case groung vias are necessary and placing them is also very important.

Buding.

RE: RF ground planes layout

JBE,

Good question.  I have played around with this a bit.  Are you still interested in the question?

Alva

RE: RF ground planes layout

Please tell us what you were going to say Alva.  I am very interested.

Sabe

RE: RF ground planes layout

(OP)
Alba, yes I keep on being interested in this subject. I believe that your "ground playing around" experience would be welcome.

RE: RF ground planes layout

Hi Alva,

I'm also really interrested about ground planes layouts and design for RF PCBs... Do you have some links to suggest?

Thanks,

Thomas K.
http://www.emcrf.eng.br

RE: RF ground planes layout

Hi.

From my experience, ground vias should be placed about every 1/10 lambda. Also, at frequencies aproaching 1GHz, the traces should be impedance matched to 50Ohms, or whatever system impedance you are using. AppCAD works well for this. You can download it, or request a CD from Agilent. Connection of components to ground should be as short and as rugged as possible, all components should be surface mount to avoid parasitic inductance. Hope this helps.

Victor

RE: RF ground planes layout

Those are some great links Toulouse, thanks!

RE: RF ground planes layout




    Ok i have worked an many a rf gen most were lower freq 13.5mhz to 40mhz and have also worked hf right on thru micro in the military. Now the higher your power the better isolated your dc must be this is very important that rf chokes (large gauge and due to the fickleness of rf you will have to toy with how many turns are needed) are used to keep the rf from backing up into the dc which would be very bad of course (as you know). Ground placement is a biggie never place them close to or below or above the rf plane this is inviting the rf to couple or just plain burn thru. It is normally best to place grounds around you PA and with many vias if going thru board. You have probably noticed how placement of the PA's effects the tuning of the rf. Most rf gen companies use almost the same basic spacing and shape for thier PA's because at different angles they tend to interact differentlly. Side by side or completely seperate seem to be the norm. well i hope that helps fell free to contact me if you need anything else i have some friends whom work as rf eng's and they could explain more prolly.


                        sorry it was so long,
                          Morg

RE: RF ground planes layout

Ground planes are troublesome!  

If you want a ground plane on top AND bottom of a PWB, beware that you are not fabricating an excellent rectangular waveguide resonator.  The two ground planes and low loss PWB dielectric will resonate at some frequency, and for big boards it might be as low as 800 MHz or so!  Put lots of ground via holes to push the resonance frequency far higher than your operating frequency.

A big problem I see routinely is where board layout guys cut the backside ground plane with a power or signal trace.  They often love to do this right where a chip meets an output transmission line.  Of course, the huge ground loop set up then causes all sorts of havock--emi radiation, filter mistuning, amplifier oscillation, and other works of the Devil.  Don't cut your groundplanes, and if you do, stich over the cut with MULTIPLE traces!

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