Re: [Elmer 101] Help for the tardy


From: Mark J. Dulcey (
Date: Tue Dec 18 2001 - 07:11:37 CST

Jim McGough wrote:

> In the notes to the power supply section, it is said that "C102 and C112
> provide decoupling on the power rail. They provide a low impedance path for
> any AC on the internal power system. This keeps the supply a clean D.C.
> voltage with a very small A.C. component".
> Evidently, the assumption is that we will use some sort of AC-transformer
> power supply instead of battery?
> If one wishes to use only battery power, are there components that could be
> eliminated?

Not those, in any case. Even if you're powering your radio with
batteries, there are other ways that AC could find its way onto the
power rails.

First, fluctuations in power consumption by circuits in the radio itself
generate noise that gets coupled onto the power rails. (And just about
all circuits generate such noise.) The decoupling capacitors help keep
that noise out of other parts of the radio.

Second, the radio can pick up stray RF noise from the outside. Possible
sources are all around you, including radios, TV sets, computers, PDAs,
automobiles, cordless phones, and other ham stations. (That's only a
short list; there are many more.) Decoupling capacitors help keep that
noise away from critical circuits.

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