> I've never seen a situation where one outlet in the house could have a
> different frequency than the the rest of the house when they were
> all fed by
> the same circuitry...
Just to clarify, John, the problem comes in with the circuit that samples
the line frequency. Since transformers are expensive and line regulation is
poor, the usual circuit is a simple resistor and diode circuit that reduces
the sine wave of the AC to a DC square wave. Effectively, this circuit
looks only at voltage crossings, when the AC line crosses the diode voltage.
This is used to clock a counter chain, that forms the clock; 60 pulses
equals one second. All it takes is a little noise on top of the AC, and at
just the right time, to cause an extra crossing and hence false clocks to
the counter chain. For instance, a positive going noise pulse just after
the AC line falls below the diode voltage can create a double pulse at the
counter input.
So it's not really a different frequency, just noise on top of the standard
60 Hz. For a poorly designed (cheap) clock, it might only take 1 volt of
noise on the 110 volt line, which isn't much at all.
As Steve said, the noise is being attenuated enough by the run back to the
load center that other circuits are not affected.
Randall
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