RE: [SI-LIST] : Upper limit of interplane capacitance

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From: Barry Ma (barry_ma@altavista.com)
Date: Thu Jun 08 2000 - 11:16:21 PDT


On Thu, 08 June 2000, "Larry Miller" wrote:

> I think you are right.
> For garden-variety boards we see buried capacitance and ground planes start to give out (i.e., impedance starts to increase) at about 600-ish MHz when you measure them on a network analyzer. The measurement referred to is driving one point on a board, say near where the DC supplies connect, and measuring on another point some distance away. This is not to say the board does not work above this range. At somewhere in the 2 to 4 GHz range the dielectric losses for FR-4 start to get very bad as well.

> Somewhere in this range you have to start treating things like what they are-- microwave structures.
 
> Larry Miller

------------

Larry,

Thanks for the input. The measurement you described is interesting.

The usual way to measure the impedance through pwr/gnd planes is to directly solder a low inductance probe cable to two very closed bonding pads on the PCB. If two points are located separately on the PCB, some extra inductance might be involved. ...

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