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From: <>
Date: Sun Feb 27 13:22:22 2005

how to build things!

> told me that the board would start with them all in.
> Then they would be removed until the board stopped
> working. Then they'd put that one back in! Seriously,

Words are about to fail me....

Whether or not decoupling is effetive depends on how the capacitors are
placed, not just how many there are. Consdier a board of 100 ICs, with a
0.1uF cap across the power pins of each one. Do you honestly believe that
an extra 10uF of capacitance (because that's the total value of 100 0.1uF
caps in parallel) across the PSU output terminals would do the same job
as all those individual decoupling capacitors?

Of course not. The _reason_ that decoupling capacitors are needed in the
first place is that the power supply wires/traces have impedance
(predominately inductive at the frequencies we care about). Which causes
voltage drops at the power pins of the ICs when they switch.

This means, of course, that the position and layout of the decoupling
capacitors is often critcal. One per chip, with the shortest possible
connections, is often good enough. Often, but not always!

> they would look at the power supply and use just

This is imposible!. You can't just 'look at the power supply' and
determine how good the decoupling is. Because of the impedance of the
power connections, the PSU output can look almost perfectly flat, even
though the power lines are bouncing all over the place at one of the ICs.

> It's my understanding that electrommagnetic deflected
> vector displays take very, very high-power deflection
> coils and drivers, and this is where the real money is

THis has nothing to do with decoupling or good/bad design. It is possible
to produce a well-designed vector display. HP did it. DEC did it.

> in these units. I don't know if the Imlac is
> electrostatic or electromagnetic deflection
> (electromagnetic, I suspect).

The only problem with using an electromagnetically deflected CRT in
vector mode is that you have to be able to quickly change the current
(which is considerable) passing through the deflection coils. This, in
turn means you need a high voltage amplifier (to be able to overcome the
back EMF of said coils when the current through them is changed). Said
amplifier is likely to have to handle considerable power in the output stage.

This is not magic, it's been done many times. It's been done _properly_
many times...

Received on Sun Feb 27 2005 - 13:22:22 GMT

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