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?
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?
Q3> Did I miss any thing interms of the layout, for better EMC?
Q4> how much should I expect of reduction in EM radiation, when adding that
change?
Any helpful inputs will be very appreciated.
THanks very much for your responses.
Sami Alkar
Compliance Engineer
samii...@hotmail.com
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