Michael Black wrote:
And of course, one reason three terminal regulators were invented was
so small sections could have their own regulators.

They are cheap enough that it's hardly worth not having a separate
regulator for the VCO. And maybe even a separate regulator for the
phase detector (and any amplifier) section.

Absolutely. For example, if you need really clean 5V supplies in various
places, a good technique is to run a pre-regulated "semi-clean" 12V rail
around the system, and then regulate locally to 5V.

Don't rely totally on the IC to clean-up the noise on the input rail.
Remember that it's a feedback system, so there is always a
high-frequency corner, above which the regulator can't keep up. Above
that corner frequency, the regulator becomes increasingly transparent
to high-frequency input noise and sharp spikes.

If low output noise is important, include some good old-fashioned R-C
filtering between the 12V line and the input to the regulator IC. Don't
forget to leave more than the minimum required voltage at the input of
the regulator... and that's not the average DC level - it's the voltage
at the *bottom* of any noise spikes at the input, so you have to allow a
little extra input voltage.

Even so, it's still only R = V/I, like we used to do routinely for zener
diodes (I'll swear that half the attraction of IC regulators is that
they let people avoid that simple calculation). If you want to use two
poles of R-C filtering, simply split the R into two parts.

You're still faced with other sources of noise, for example noise
originating in the regulator itself, and things like induced voltages
and ground loops... but all those are actually separate problems.

In different applications, R-C input filtering combined with a voltage
clamp device is a good way to protect regulator ICs and their downstream
components from the very sharp spikes induced by lightning. The
pre-filtering reduces the peak amplitude of the incoming spike and also
attenuates its high-frequency components, down to levels that the
voltage clamp and the regulator IC can safely handle between them.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek