[sdiy] 4069 VCO to get started? My answer

[Scott Gravenhorst]

Richard Wentk <richard at skydancer.com> wrote:
>At 14:32 04/10/2004 +0200, Nicolai Czempin wrote:
>>Laurie Biddulph wrote:
>>
>>>Hello Nicolai,
>>>
>>>Although the 4069 is fundamentally a logic IC it is actually used in this 
>>>application as a linear device and utilises the fact that at lower power 
>>>rails the gates can actually be treated as linear/analogue devices. This, 
>>>therefore, does make the 4069 a suitable device for your initial requirements.
>>>
>>Okay, I'll accept it even if I can't quite follow...
>>What kind of "analogue devices"?

Check out the PDF data sheet for the 4069UB (UB means unbuffered and only the UB
version works this cool way).  These inverters are just two transistor
amplifiers.  These amplifiers can be used in a variety of ways, such as an
integrator or two together form a schmitt trigger.

>It's basically just a buffer, which gives you a neat way of controlling the 
>charging current of a capacitor.
>
>The analogue/digital dichotomy is a bit simplistic because digital circuits 
>are still analogue circuits being made to work in a specialised way. A lot 
>of the time you can ignore the fact they're analogue and use a 
>much-simplified digital model of how they work. But in some cases, like 
>this one, it's worth understanding the underlying analogue properties.
>
>If you ever have to interface digital to analogue - as you often do - 
>you'll again need to understand the analogue properties of supposedly 
>digital voltage and current behaviour.

Also, realize that digital circuits are, in fact, just analogue circuits with
high gain and nonlinear behavior.  You still must obey analogue rules in the
digital world.  For example, gate fan-out is not infinite, it's limited by the
amount of current the output driver can supply.  That's a very analogue thing.

>It's true some books start with digital because there's less to learn and 
>it's cleaner and simpler than analogue. But you can't do reliable circuit 
>design, even digital design, without a good grounding in analogue basics.
>
>And then once you have that you can go on to learn about RF, which is 
>different again. :-)
>
>Richard
>
>

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