----- Original Message -----
From: "Bill Coleman" <aa4lr@arrl.net>
To: "Jim Lux" <jimlux@earthlink.net>
Cc: "Towertalk" <towertalk@contesting.com>
Sent: Thursday, May 12, 2005 8:40 AM
Subject: Re: [TowerTalk] Comments on Array Solutions "dissimilar antennas"
phasing application note.
>
> On May 11, 2005, at 10:35 AM, Jim Lux wrote:
>
> > 4) Measuring phase on a scope to an accuracy of a few degrees is quite
> > challenging. Spread one cycle out over 10 divisions, and each
> > division is
> > 36 degrees. By eye, you can probably estimate to a fifth or sixth
> > of a
> > division on a noisy signal. On a real clean signal, a tenth of a
> > division
> > is possible, so you're potentially in the 4-5 degree range,
> > accuracy wise.
> > With a good scope that has a X10 expansion, and with VERY stable
> > triggering,
> > you can do better, but you are really depending on the triggering.
> >
> > Don't forget that you're making a measurement between two signals,
> > so the
> > overall uncertainty is actually 1.4 times the uncertainty in
> > measuring one
> > signal.
>
> Wouldn't it be easier to measure the phase difference by feeding one
> signal to the horizontal amplifier and the other to the vertical and
> measuring the angle of the resulting scope trace? In phase signals
> should follow a 45 degree line, sloping low on left to high on right,
> out of phase signals would present other angles.
That's what I started with..
Two big problems: If they're 90 out of phase, you get a circle. What
you're really trying to do is measure minor and major axes of an ellipse,
which is harder than seeing zerocrossings or peaks of sine waves.
And, if there's any noise at all, it's really, really hard.
However, I recommend the lissajous pattern approach for checking for
distortion. We had someone testing out a breadboard to measure phases (at
about 9 MHz, as it happens). They were getting weird results with the zero
crossing approach, even though the sine waves looked fine. However, put it
in XvsY mode, and it was anything but elliptical. It was a lumpy figure 8,
indicating significant harmonic content. Turns out the output amplifiers in
the receiver chain (after the IF filter) were compressing and producing
distortion. There were harmonics all the way up to the 15th, when we
checked on a spectrum analyzer.
I was impressed... A relatively small harmonic content (i.e.
indistinguishable in the waveform display) was really, really obvious in the
Lissajous. We added that check to the test process.
>
> You still have the same S/N problems, but measuring the angle should
> be easy.
The angle is more related to the relative gains, than the phase shift.
>
>
> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
> Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 19
>
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