Topband: Polarity and Phase

[Michael Tope]

----- Original Message -----
From: "Jim Brown" <jim at audiosystemsgroup.com>

> No, it does not. But it IS one way that it is done. And my point is
> that the phase shift achieved in that line is DIFFERENT from changing
> the polarity.  I could also have used the example of a passive network
> (LC, RC, or RL) designed to produce a phase shift. The phase shift of
> all of those networks is a function of frequency. Yes, we can design
> all-pass networks with some constant delay or phase shift within a
> given range. But my point is that ALL of these are DIFFERENT from a
> polarity change. Not better or worse -- that depends on what you're
> trying to achieve with any given system/design/application. But they
> are DIFFERENT.

I think I see where you are going with this. Your video cable
example (in your other email) illustrates your point very well.
For narrowband systems, a 180 degree phase shift, Tau/2
time delay, or a polarity change, will all give roughly the same
end result. For broadband systems, they will not, so I guess
I can see why an audio guy such as yourself would cringe
when people use the terms interchangeably.

> You seem not to understand an important part of how many directional
> antennas work.

What you describe is and has been  my understanding of how
phased antennas arrays work.

> Take a horizontal dipole at some distance above a reflective earth. It
> is the phase (and amplitude) difference between the direct radiation
> from the antenna and the radiation reflected by the earth that provides
> the vertical directivity of that antenna!

Actually there is a "polarity" reversal of the electric field
when it reflects off the ground  - hence the null on the
horizon for horizontally polarized antennas :):)


> >Yes, everything you say is correct, but as Tom points out, you
> >can still null two "in-phase" signals by flipping wires
>
> Cummon -- say polarity. It doesn't hurt. :)
>

Uhhh....okay........"polarity". There I said it.


> There is a trig identity that shows that there is a beat note produced
> by this combination. I wouldn't call that a phase difference. Phase is
> the angular difference between two vectors (phasors) that are rotating
> at the same frequency. When they aren't at the same frequency, you've
> got some complex relationship between them, but it isn't phase.  In the
> above example, the carrier produces one beat, and each modulation
> component produces another. The result is akin to modulation.

I think we are arguing semantics here. My point is that
when the beat note is sub-Hz, you can actually observe
the baseband phasor rotate around the unit circle and
you can indentify the phase as it rotates. When the
beat note is larger, the phasor is rotating so fast, it
no longer makes sense to do that (in that case you
hear the beat note as a steady tone rather than as
a series of alternating peaks and troughs). In any case,
there is a very simply relationship between frequency
and phase (phase is the integral of frequency). I think
you are making it more complicated than it needs to
be.


> >Mathematically, flipping polarity is the same as adding a 180
> >degree phase shift to one of the local oscillators. The two are
> >indistinguishable. In other words if I have two phase locked
> >SSB receivers, flipping the "polarity" of the audio output
> >of 1 receiver has the same effect as shifting phase of one
> >of the two LO's by 180 degrees.
>
> ONLY if we are talking about a sine wave of a single frequency.
>

I am not sure that I agree in this case. If I shift the phase of
one of the LO's in the two receiver diversity system by 180
degrees, the phase of that downconverted signal coming from
the mixer fed by the phase-shifted LO will be inverted at all
frequencies. This 180 degree phase shift of the LO, will in
essence be translated into a polarity shift.  If we were talking
about adding a 180 degree phase shift to the RF path
(instead of the LO), then I would agree with you that baseband
polarity shift and the 180 degree RF phase shift would
produce different results.

>
> Again, it depends on the bandwidth of the signal and/or the system.
>

Yes I agree.

73 de Mike, W4EF...................