John, N0TA, forwarded this question to me about why I listed the KX2
close-in dynamic range at 3 kHz rather than the more typical 2 kHz.
Some radios have limitations that make a measurement at 2-kHz impossible
or invalid. If a valid measurement cannot be made at 2 kHz, then it is
made at some wider spacing, such as 3, 4 or 5 kHz. Typically if a
measurement cannot be made at 2 kHz, it is because of filter leakage.
When a test is made at 2-KHz, for example, the third-order
intermodulation products are 2-kHz below the lower frequency test tone
and 2-kHz above the higher frequency test tone. If the radio cannot
reject the test tone that is 2-kHz away, then it is impossible to
measure the third-order distortion product.
In the case of the KX2, I could measure the low side distortion product
at 2 kHz with no problem using the standard 500-Hz filter bandwidth. On
the other hand, when measuring the distortion product on the high side,
the filter in the KX2 does not reject the test tone that is 2 kHz away.
Two different KX2 radios were tested, and both showed this identical
anomaly. If one reads the ARRL report on the KX2, you will see that the
reviewer mentioned the filter leakage when tuned above a strong signal.
Here are the odd details on the KX2 filter leakage. If the filter
bandwidth is 500, 400, 300, 200 or 100 Hz, filter leakage makes the
measurement at 2 kHz impossible. On the other hand, if the filter
bandwidth is 550, 450, 350, 250, 150 or 50 Hz, then the measurement can
be made. The League chose to publish a 2-kHz dynamic range in QST even
though the measurement cannot be made on the high side with the
standardized 500-Hz filter bandwidth (or any even bandwidth value).
Note: Both the League and I only publish the LOWER of the two
measurements. Let’s say the low side dynamic range of some radio is 95
dB and the high side is 90 dB. Both the League and I publish the lower
worse-case 90 dB number. In the case of the KX2, I couldn’t pick the
lower of the two readings since no measurement is possible when using
the standard 500-Hz filter.
Since the 2-kHz dynamic range value could not be made with the standard
filter bandwidth at 2 kHz, but could be made at a 3-kHz signal spacing,
I published that data. It may be of interest that the 2-kHz dynamic
range number with the non-standard 450-Hz bandwidth was only 1 dB worse
than the 3-kHz value. The point of listing the 3-kHz value, or any
other radio with a non-2 kHz listing is to point out the radio has a
limitation in its filter performance.
My long form report on the KX2, available in PDF format, goes into
detail about this issue, and has data on the two radios I tested, listed
by serial number.
At this point, Elecraft does not know why the KX2 behaves this way, or
whether there is a possible firmware fix.
Rob Sherwood
NC0B
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