[sdiy] 4069 VCO to get started? My answer

[Colin Hinz]

On Tue, 5 Oct 2004, Richard Wentk wrote:

> Digital circuits are just analogue circuits used in a special way. It's all
> still about voltages and currents. There's nothing clever about using a
> digital circuit in an analogue way, because really it's *all* analogue.
>
> The difference is that digital is simplified so you can build certain kinds
> of circuits without worrying exactly what those voltages and currents are
> doing. Digital includes a built-in fudge factor so everything gets rounded
> up or down to '1' or '0'. With analogue you need to be more precise.
> Internally though, both are made out of the same kinds of transistors and
> resistors and other basics.

Until you get to high-speed digital.........

High speed digital design requires "ordinary" logic design (with a really
sound knowledge of sequential circuits), as well as analog skills, and
especially a big dose of RF transmission-line theory. Oh, and the power
distribution can be truly fearsome -- multiple voltages (e.g. 3.3V, 2.5V,
1.8V, 1.5V.....) which all must power on and off in timed sequence, and
go to all chips over low-inductance "planes".

I do high-speed digital design for a living. I poke around with analog
design at home as a way to relax. I'm a long way from being an analog
wizard but I at least understand that the basic rules for the two are
somewhat different, both in terms of what's essential and what can
be safely ignored.

- Colin Hinz
  Toronto, Canada
  remembering when "33 MHz bus speed" was considered "pretty fast"....