Sarmad you describe a complex design involving many tradeoffs that is probably
beyond the scope of this forum to fully analyze. I will offer a few comments
and
hope that others will do the same.
sarmad Albanna wrote:
> Hello everyone,
>
> I really apprecaite any inputs on that subjects, thanks in advance:
>
> This is problem, I hope that I would not bore you will very much details.
>
> We are designing a 12 layer board that has a @ 2.5 Ghz circuit. The circuit
> contains a source crystal oscillator a ~ 78 Mhz PECL signal ,this feeds into
> a PECL/TTL converter (plastic with no metal case), then the output of the
> converter is feeding into a CLK multilayer (plastic with no metal case)that
> generated a differential output of 2.5 Ghz. All the power and Clk ( ref and
> others) are on the first layer (however very short ( electrically, the
> maximum length of 5 mm). The GND for all those chips are done through the
> voltage return pins ONLY for each one to Digital DGND ground that is on the
> 2nd layer. There is no shielding or localized grounding for those three
> components. Emission is measured at 2.5 Ghz is quite high, and we relaying
> the board by doing the following:
>
> 1. Introduce a localize GND on the 1st layer that is a solid Cu plane, all
> three components will sit on that GND. The GND will have vias connected
> to the DGND in the 2nd layer.
>
> 2. All the traces that going to and/from those chips will be buried in the
> forth layer ( layer 3 is Voltage plane). So effectively will be slightly
> longer than 5 mm ( <6mm)).
>
> 3. we are putting a metal SMT can on top this circuit will completely Seal
> the components and preventing them from radiating.
>
> Q1. Do I need to barry those traced or can I run them through the localized
> GND in the 1st layer?
I'm not sure how you would do this but it sounds like you would cut up your
ground plane if you ran traces through it. It sounds like your may already be
floating. You have not mentioned decoupling. It is essential you decouple
the RF energy from the power supply traces if you are going to keep the
whole assembly from becoming an antenna.
Impedance inverts every quarter wave length. Using the formula:
lambda = 300M / f(MHz) we get a wavelength of 120 mm. A quarter
wavelength would be 30 mm. This means a short (0 Ohms) would
have infinite impedance 30 mm away. Practically, ground trace lengths
of over 5 mm will be ineffective at 2.5 GHz.
>
>
> Q2> If I need to use a heat sink on the multilayer, How can use it without
> defeating the purpose of the metal RF can?
What is the purpose of the metal can? Perhaps you are thinking it
will serve as a shield? Not likely. With effective grounds you may be
able to contain the electric fields but it will do very little to contain
magnetic fields.
>
>
> Q3> Did I miss any thing interms of the layout, for better EMC?
>
Electromagnetic fields are best reduced by keeping 1) current low,
preventing 2) impedance discontinuities, and 3) preventing antenna
like constructions.
Practically there is very little you can do to control current (1) because
to prevent discontinuities (2) and antennas (3) you must use matched
transmission lines. (Transmission lines limit impedance (and thereby
current) to a relatively narrow range.) I suggest you address these
issues in your design. Transmission line rules are well embodied by
microstrip or stripline construction techniques found in many texts .
You do not mention any of these terms in your proposed solution so
it is unlikely your changes will produce good EMC.
>
> Q4> how much should I expect of reduction in EM radiation, when adding that
> change?
It sounds like your changes are a start in the correct direction but it is
not clear you will have any reduction. It may be better or again it may be
worse.
Fred Townsend
Consultant
f...@poasana.com
>
> Any helpful inputs will be very appreciated.
>
> THanks very much for your responses.
>
> Sami Alkar
> Compliance Engineer
> samii...@hotmail.com
>
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