substrate noise

can somebody tell me what is substrate noise ?
how does it effect coupling Cap of nets?

 

Substrate Noise:

( /Lecture mode on )
Substrate Noise is a component of supply noise caused by spurious signals
being injected into the circuit due to the conductivity of the substrate.

If the substrate were a perfect conductor ( and your "perfect supply" could
supply all of the required current at the supply voltage ) then substrate
noise would not exist.

Alas, we live in an imperfect world and the substrate has finite impedance.
This causes currents to flow in the substrate and the resulting current
injection and voltage bounce in the substrate cause "noise" in the circuit
under question.

Coupling Capacitance of a net to the substrate ( or shielding wires
connected to the substrate ) will add noise to the signal.
( Which is why a higher RHO substrate will cause less coupling ... ) but
this effect can often be overwhelmed by net-to-net coupling in modern
processes.
( /Lecture mode off )

-- Gerry Vandevalk www.ictooling.com

 

Thanx a lot 4 the info .....