Re: [KX3] Re: KX3 a K3 killer - Sherwood RX Test Numbers Posted

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Re: [KX3] Re: KX3 a K3 killer - Sherwood RX Test Numbers Posted

wayne burdick
rich_ve3ki wrote:

 > This points out the importance of understanding what the test
 > results actually mean. The KX3's 104 dB dynamic range figure
 > reflects an idealized condition where there are only two signals on
 > the band, separated by exactly 2 kHz. Under those specific
 > conditions, the KX3's dynamic range is slightly better than the
 > K3's. However, if the separation of the two signals were changed to
 > 1 kHz (or with the 8 kHz shift turned on, 16 kHz), then according to
 > the data in the Sherwood table the KX3's effective dynamic range
 > would be closer to 65 dB, which would place the KX3 a long way down
 > the list.

This interpretation is incorrect.

There's an important distinction to be made between an opposite
sideband image and third-order IMD. An opposite-sideband image is a
completely linear response to one strong nearby station. An IMDDR3
image is a nonlinear product of two (or more) signals. So, comparing
the 104 and 65 dB numbers in Sherwood's table is a case of apples and
oranges. The 65 dB number simply reflects an inability to *measure*
the IMD number, because it masked by the opposite sideband image. Not
only that, the OSB image was measured at 1 kHz, while the other
numbers (104 and 97 dB) were measured at the offset indicated in the
table: 2 kHz.

A receiver that really does have a 2-kHz IMDDR3 number in the mid-60s
(say, an IC-7000, FT-2000, TS-520) will fare far worse than the KX3
under all conditions, because the IMD products can originate from many
combinations of nearby strong signals. In stark contrast, the KX3's
"65 dB" number only applies to one extremely strong signal that is
nearly on top of you and just happens to have its image at the same
pitch as the desired signal. If the pitch were even slightly
different, you could eliminate it just by narrowing the DSP passband
using the PBT control.

I can understand why Rob put the OSB image number in the table, but
it's important to clarify exactly what it means in the real world. It
is not the same as IMD.


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