Posted by
Alan Bloom on
Sep 16, 2010; 7:18pm
URL: http://elecraft.85.s1.nabble.com/K3-Diversity-Reception-and-Antenna-Directivity-tp5537531p5540035.html
On Thu, 2010-09-16 at 11:29 -0700, David Gilbert wrote:
>
> Well, I was thinking mostly in terms of CW so I think delay and phase in
> the context of a single frequency are essentially the same. I did give
> a bit of thought to what it would take to make it work for SSB, though,
> and while I am the last person on earth you'd ever want to consult on
> math, it seemed to me that maybe adding the shift in the frequency
> domain would work. I.e., perform an FFT, add the shift, and then
> convert back to the time domain. Does that have the correct effect?
> I'm just kind of guessing here.
Yes, that would be another way to do it. Determine the phase of each
complex FFT frequency point (arctangent of imaginary/real), add the
proper phase, and then do an inverse FFT to get back to the time domain.
To get a continuous signal you have to choose the proper windowing
function and the amount of overlap of the sample sets in the time
domain.
Yet another way to do it is to design an all-pass network with a nearly
constant phase shift over the frequency band of interest. That could be
done with either an FIR or IIR digital filter, but the coefficients
would have to be re-calculated for each desired phase shift.
I'm thinking it would be easier to do the method where you heterodyne to
"RF" using LOs of different phase and then downconvert back to baseband.
That can be done all in software.
Alan
> LMOP (larger matter of programming ... but less hardware)
>
> Thanks for humoring me on the discussion!
>
> 73,
> Dave AB7E
>
>
>
> On 9/16/2010 10:47 AM, Alan Bloom wrote:
> > Yes, that should work, just switch the sub receiver antenna over to the
> > main receiver to calibrate the phase. I think you'd get the best
> > accuracy by looking for a null and then subtracting 180 degrees.
> >
> > By the way, I think rather than an adjustable delay between the two
> > channels you need an adjustable phase. (If the phases of the two local
> > oscillators in the K3 differ by X degrees, than all audio frequencies
> > also differ by X degrees.) There are a number of ways to do that, but
> > probably the most straightforward is to re-convert each audio signal to
> > "RF" (a few kHz) using local oscillators of the same frequency but
> > different phases, and then convert back to baseband with a single LO.
> > That could be done either with hardware or in software.
> >
> > Just a SMOP. (Small matter of programming :=)
> >
> > Alan N1AL
> >
> >
> > On Thu, 2010-09-16 at 10:08 -0700, David Gilbert wrote:
> >> Thanks for the reply, Alan.
> >>
> >> That's interesting. I didn't realize that the two receivers in the K3
> >> had a random phase difference between them even when locked. I noticed
> >> differences in phase delay when I changed frequencies during my tests
> >> with the two horizontal antennas on the tower, but I mostly attributed
> >> that to different arrival angles for the different stations being
> >> monitored, and different phase delay of the feed lines (expressed in
> >> wavelengths) at the new frequencies.
> >>
> >> However, that random phase difference between the two receivers could be
> >> adjusted out by momentarily feeding the same antenna into both receivers
> >> at each new frequency. That routing could be accomplished within the K3
> >> if there was a quick and easy way to control whether or not the sub
> >> receiver was switched to the AUX RF antenna when diversity mode is
> >> active. If that routing option took the form of a command, the whole
> >> process could be done in software (either external or internal to the
> >> K3) each time the frequency was changed ... albeit of course with some
> >> settling delay while the phase difference was determined and adjusted out.
> >>
> >> 73,
> >> Dave AB7E
> >>
> >>
> >>
> >>
> >> On 9/16/2010 8:42 AM, Alan Bloom wrote:
> >>> Yes, I think that would work fine. There are two issues that I can
> >>> think of: While the main and sub receivers are phase-coherent, the
> >>> actual phase difference between them is random. I believe that if you
> >>> change frequency it is not guaranteed that the phase difference will
> >>> be the same. So every time you change frequency you may have to
> >>> re-adjust the phase delay to get the antenna to "point" in the right
> >>> direction.
> >>
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