Re: [K3] Diversity Reception and Antenna Directivity

Posted by Alan Bloom on Sep 16, 2010; 8:55pm
URL: http://elecraft.85.s1.nabble.com/K3-Diversity-Reception-and-Antenna-Directivity-tp5537531p5540402.html

Hi Dave,

For some reason I got several copies of your message.  I'll go ahead and
post my reply to the reflector since it may be of wider interest.

Yes, another way to get a constant phase shift at all frequencies is to
convert to an I/Q (in-phase/quadrature) signal.  One way to do that is
to take the audio signal and run it through an all-pass network with a
constant 90-degree phase shift at all frequencies.  That is then the "Q"
signal and the original non-shifted signal is the "I".  You can then
obtain any desired phase shift by adding the two signals together with
the proper weighting factor for each.  For example, if you weight I and
Q by the same factor you get 45 degrees.

But that still requires a wide-band 90-degree phase shift network.  It
can be done with a FIR or IIR digital filter.  I think there are some
free filter design programs on the web that can design an all-pass
network.  It may need to be a big filter (i.e. lots of coefficients) to
get good amplitude and phase accuracy over a 10:1 frequency range (300
Hz to 3 kHz).

The other way to get the I/Q signals is with an I/Q modulator.
Basically you run the baseband audio signal into two mixers whose local
oscillators are 90 degrees out of phase.  Now you have two RF signals 90
degrees out of phase.  If you convert them back to baseband with the
same oscillator you get two audio signals 90 degrees out of phase and
can combine them as before.  You then combine that signal with the
original to simulate rotation of the directional antenna.

I still think the easiest method is what I first suggested.  Like the
I/Q modulator except that, instead of a 90-degree phase shift, you
adjust the relative phase of the two oscillators.  When you convert back
to baseband the two signals are already at the correct phase, ready to
be combined to simulate the directional antenna.

Alan


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