KX1 signals peak at low audio beat frequency

[Mike Scott-7]

First post to this list

The KX1 has a fixed BFO designed for 600Hz offset; this is where I expected
signal audio to peak. I validated what I thought I was experiencing that
signal audio peaks closer to 400 Hz than 600 Hz, lower than expected.

I put an audio spectrogram on the audio out of my KX1 (Visualization
Software PC version). Input to KX1 was the XG2 signal generator at 7.040
MHz. I set up so that I was showing a good S/N and then manually swept the
end-to-end response of the entire RF-to-audio path by slowly tuning the
receiver across the signal plotting response peaks.

End-to-end response shape seems normal falling off 20 dB per octave far
above the peak and rising faster far below the peak with a nice rounded peak
that is obviously sharper at the lower filter settings.

Results tabulated as follows picks off the peak and -6dB points of the end
to-end-response for four different filter settings:

Filter setting Peak Audio -6dB High -6dB Low
Maximum 592 Hz 1214 Hz 159 Hz
1.0 500 Hz 697 Hz 259 Hz
0.5 442 Hz 554 Hz 321 Hz
Min 444 Hz 516 Hz 337 Hz

As you can see with narrower filter settings the maximum signal tune point
is a beat-frequency tone just north of 440 Hz. The signal is already more
than 6 dB down by the time I have tuned to the design 600 Hz offset point,
where I would zero beat another signal.

Have other KX1 owners noted signals peaking at a low CW tone? I find myself
listening to some low notes on weak signals and this means I do a rather
poor job of zero beating.

I don't have an RF frequency measurement tool but am suspecting that the BFO
could be set more optimally in my case.

Does anyone know if there is a way that I might trim the BFO frequency
100-200 Hz lower? Or, possibly there is something else going on and I am
looking in the wrong place?

Mike Scott
AE6WA
Tarzana, CA
DM04



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[Martin Gillen]

Hi, Mike.

I noticed exactly the same thing here.  In fact the
passband ripple of my KX1 is pretty bad, 600Hz is
actually in a trough not a peak!  And when I narrow
the filter to the narrowest setting it centers on
around 480Hz.

Neither of these is a fairly significant issue in
practice though.  If I have occasion to use the filter
in its narrowest position then I zero beat first then
use the RIT to move the signal down to 500Hz before
closing the filter to its narrowest position.

Most of the time my filter stays in one of three
positions:

5 oclock - fully open - Quiet band.

12 oclock - halfway open - Fair amount of QRM.

9 oclock - about 2/10 open - In the 9 oclock
position the audio peaks (to the ear) at 600Hz
so this is the most useful position for zero-
beating.

I almost never close it right down (7 oclock position)
however I did last weekend when I had a station come
on 100Hz away from my QSO and he couldn't hear me.

In this situation I use the RIT control to send the
wanted signal to 500Hz after zero-beating at 600Hz.

Does that help?

By the way Michael VE3WMB mentioned before that he
has the same ripple pattern in the ten-tec scout
and it may be a feature of the Jones filter design.

After all the component count in the KX1 is tightly
controlled and there are tradeoffs because of that.

http://www.ac6rm.net/mailarchive/html/elecraft-list/2004-06/msg00906.html

73
Martin.


               
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