> [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*

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Orchid XP v8 wrote:
> I'm thinking more about desktop PC
> motherboards. Why do *they* need so many million capacitors?

That's what *makes* them digital.

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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Darren New <dne### [at] sanrrcom> wrote:
> Orchid XP v8 wrote:
> > I'm thinking more about desktop PC
> > motherboards. Why do *they* need so many million capacitors?

> That's what *makes* them digital.

  Capacitors are not digital. You are confusing them with transistors
(which, technically speaking, aren't digital either, but can be used
for that purpose when their multiplying factor is ignored).

-- 
                                                          - Warp

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On 23/10/2010 06:42 AM, Warp wrote:

>    Capacitors are not digital.

Perhaps it would be more accurate to say that no single component is 
digital, more that a particular circuit design can be digital. (I've 
seen logic gates used to make an analogue circuit, for example...)

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

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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Orchid XP v8 wrote:
>>> I'm thinking more about desktop PC
>>> motherboards. Why do *they* need so many million capacitors?
> 
>> That's what *makes* them digital.
> 
>   Capacitors are not digital. 

Nothing is digital. Digital is an interpretation of an analog signal. To 
"make an analog signal digital" requires analog components. You have to 
drive the analog signal so it doesn't linger between the fully-on and 
fully-off states (however those may be defined).

And yes, capacitors can be digital in a voltage-driven circuit.

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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Darren New wrote:
> Warp wrote:
>> Darren New <dne### [at] sanrrcom> wrote:
>>> Orchid XP v8 wrote:
>>>> I'm thinking more about desktop PC
>>>> motherboards. Why do *they* need so many million capacitors?
>>
>>> That's what *makes* them digital.
>>
>>   Capacitors are not digital. 
> 
> Nothing is digital. 

As an example, even your keyboard needs debounce circuits, and power 
supplies have to be careful not to let the inductance fry things when you 
toggle the switch.

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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> Well, the iPhone runs on battery power, so yeah, the power supply is 
> probably a little unstable.

Not just that, but from a single voltage (the battery) several very stable 
voltages must be generated.  This involves lots of analogue components (eg 
how do you efficiently make accurate 3.3V, 5V and 8V lines from a LiPo 
battery that can be between 3 and 4.5V?).

> I'm thinking more about desktop PC motherboards. Why do *they* need so 
> many million capacitors?

Noise reduction, basically they allow everything to run correctly at much 
higher frequencies than would be possible without them.  It works two ways, 
the capacitors around each IC help to keep a smooth voltage level for that 
IC, ie shielding it from any noise on the power supply lines.  Also the 
capacitor then prevents a lot of noise being put back onto the supply line 
from the ICs.

There are also a lot of resistors on the signal lines, these in conjunction 
with the tiny parasitic capacitances cause a low-pass filter effect on the 
signals.  This also helps to reduce noise emission from the lines.

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On 25/10/2010 01:02 PM, scott wrote:
>> Well, the iPhone runs on battery power, so yeah, the power supply is
>> probably a little unstable.
>
> Not just that, but from a single voltage (the battery) several very
> stable voltages must be generated. This involves lots of analogue
> components (eg how do you efficiently make accurate 3.3V, 5V and 8V
> lines from a LiPo battery that can be between 3 and 4.5V?).

1. Why do you need several different voltages?

2. I'm not aware of any way to change the voltage of a DC circuit. You 
can use a resistor to limit current, but AFAIK there's no way to 
actually change voltages.

>> I'm thinking more about desktop PC motherboards. Why do *they* need so
>> many million capacitors?
>
> Noise reduction, basically they allow everything to run correctly at
> much higher frequencies than would be possible without them. It works
> two ways, the capacitors around each IC help to keep a smooth voltage
> level for that IC, ie shielding it from any noise on the power supply
> lines. Also the capacitor then prevents a lot of noise being put back
> onto the supply line from the ICs.
>
> There are also a lot of resistors on the signal lines, these in
> conjunction with the tiny parasitic capacitances cause a low-pass filter
> effect on the signals. This also helps to reduce noise emission from the
> lines.

Mmm, interesting. (And ridiculously complicated.) I wonder how much of 
this stuff is to do with electromagnetic radiation? (You don't want EM 
interference stopping your motherboard working, and you don't want the 
board to radiate EM either...)

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> 1. Why do you need several different voltages?

Different circuits need different voltages.  It's why the connector you plug 
from the PSU to your motherboard has 20-odd wires and not just 2 big fat 
ones.  In an iPhone I imagine you'd at least need different voltages for the 
digital circuits, the audio circuits, the display and the RF ciruits.

> 2. I'm not aware of any way to change the voltage of a DC circuit.

This is often used for battery powered devices as it's very efficient:

http://en.wikipedia.org/wiki/Charge_pump

> Mmm, interesting. (And ridiculously complicated.) I wonder how much of 
> this stuff is to do with electromagnetic radiation? (You don't want EM 
> interference stopping your motherboard working, and you don't want the 
> board to radiate EM either...)

I assume they must pass certain legal test standards for EM compatibility, 
but there are products that require stricter controls, especially ones that 
are used near other RF equipment where interference is unwanted (eg medical 
equipment, circuits in radio receivers, on planes, near to any 
safety-critical device etc).

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> 2. I'm not aware of any way to change the voltage of a DC circuit. 

I made an example using that Java circuit simulation applet:

http://tinyurl.com/2cplrbn

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