On Fri, 2007-06-01 at 09:31 -0700, Kevin Purcell wrote:
> You are confusing two usages of "out of phase":
>
> 1. having 180 degrees (pi radians) phase difference
>
> 2. not being in phase -- having a phase relationship of anything
> other than zero phase difference
>
> As Jerry said adding two signals that are precisely 180 degrees phase
> difference will result in a zero amplitude signal. Adding two signals
> with a random phase relationship will result in a higher amplitude
> signal.
Not exactly true. The sum will only go to zero if the amplitudes of the
two signals is EXACTLY the same and the phases are exactly 180 degrees
apart.
Adding two signals with a random phase relationship will result in some
amplitude between zero (and then ONLY when the amplitudes are EXACTLY
the same) and increased with all values in between possible. And in the
REAL radio world, signals rarely have constant phase difference so the
amplitude constantly varies. That variation is the "beat note."
>
> The latter case is "always" the case unless you have the two tones
> both phase-locked. Which you won't have in tuning two separate
> oscillators. Their phases will be just randomly related.
And constantly changing.
>
> When you zero beat something you are going for zero frequency
> difference between the tones -- the audible beat signal goes to zero
> FREQUENCY not zero amplitude sum tone.
YES!
>
> On May 31, 2007, at 7:33 PM, Gary Hoffman wrote:
>
> > That's what I said Jerry....or at least tried to say. Normally, in
> > applications I have experience with the signals are out of phase
> > and thus
> > cancel. This, in my personal view, is the most useful
> > configuration. But,
> > as I said, if someone does something else with the phase (i.e., in
> > phase
> > instead of out for instance) then they could add.
>
> --
> Kevin Purcell
> kevinpurcell@pobox.com
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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