> [A large chunk of this response is mostly directed at invisible]
>
> Right. Filtering the signals. Caps are used a lot of time in digital
> circuits to prevent switching noise from corrupting signals via ground.
> When some digital circuits (TTL in particular) switch they create a lot
> of noise on the ground plane that must be dealt with. Too much
> fluctuation between power and ground, and the circuit begins to behave
> erratically. Also, its a good idea to limit current. Resistors are used
> to stabilize signals, pull up or pull down an input or output, bleed off
> excess voltage so FETs don't retain a charge, etc... You need RC
> networks to run a clock crystal, etc... Just connecting together a pile
> of digital components won't work. You need supporting components to make
> sure everything is stable in the system, especially when dealing with
> large-scale digital systems like an iPhone. Keep in mind, that device is
> also going to require a oscillators and filters for the RF parts, as well.

Well, the iPhone runs on battery power, so yeah, the power supply is 
probably a little unstable. I'm thinking more about desktop PC 
motherboards. Why do *they* need so many million capacitors?

> It gets even more interesting when you start dealing with designing
> circuit boards, where you actively need to avoid traces from running
> parallel to each other for too much distance, lest they build a
> parasitic capacitance between them. In systems with very tight voltage
> and current tolerances, the length of traces and thickness of traces
> becomes very important as well. Traces can also act as inductors.
>
> Some of the discrete components may be in there to also counteract some
> of these effects caused by the traces themselves on the board. You'll
> notice caps and resistors will often be very close to the pins of an IC
> to minimize any effect the circuit trace may have.

Meh, well, fortunately I don't have to care about such things. 
(Presumably a few attofarads only starts to matter once you're operating 
into the GHz range anyway...)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

Post a reply to this message