Correct, the picture is complex. The PCB is 2-layer with signal, 5V-power
and 0V-ref lines routed on both sides. There is no ground layer/plane. There
must be a large number of RF current loops because the 0V-lines are routed
up and down and around.
Beside trying to achieve a good decoupling I assume that reducing loop area
is the most important.

Amund



-----Opprinnelig melding-----
Fra: Cortland Richmond [mailto:72146....@compuserve.com]
Sendt: 18. april 2002 00:54
Til: am...@westin-emission.no; ieee pstc list
Emne: Re: Decoupling - capacitor values


Yes, it makes sense. But the goal here is preventing or reducing Vcc drop
during the time the microprocessor is switching. You need not only low
reactance, but *also* enough capacitance to supply the current needed
_while it is switching_. You have not given enough information here to tell
if 820pF is sufficient.

Regards,

Cortland Richmond

Amund Westin wrote:

>> Insert a SMD ceramic capacitor of value 820pF in parallel with the
existing
100nF. The reason for the low value 820pF is because the capacitor
self-resonance frequency is approximate 180MHz, and I believe it is
important to choose a Cap value with a resonance frequency higher than the
frequency we would like to decouple.

Does it make sense?

<<



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