[sdiy] Decoupling caps?

[Tony Clark]

   Hi Charlie,

> I have read often that when setting up PCB's one
> should always use decoupling caps (.1uF or so) for
> IC power pins.

   Yes, but there are a few options for you.

> Now, I know this may be too general a question, maybe too general, but,
> how important are the decoupling caps in a circuit like this?

   For a VCO I would say pretty important.  There are two chips that should 
definately be decoupled, and that's the LM311 (reset comparator) and 
the CA3140 integrator.  These two chips are responsible for generating 
spikes in the power lines at whatever frequency the VCO is operating.
  The other thing that MAY need to be decoupled is the power to the 
exponential converter (1M resistor tied to the power line).  This is 
where the VCO can _receive_ spikes in the power lines, causing 
potentially unwanted VCO syncing.

> Maybe I can put some decoupling caps away (bigger? 10uF?) from the IC's
> where there is a bit more room on the board?

   The other option, to decouple the whole circuit, but I'd use 
bigger caps, like 100uF or 220uF.  BTW, you don't necessarily have to put 
the caps directly next to the ICs.  It is _ideal_ to locate them next to 
the power supply pins, but they will work just as well at the outside 
edges of the board.

   Cheers,

   Tony

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