[Elecraft] K3 Filter NORM

[Richard Ferch]

K6IAM wrote:

> The DSP width can be adjusted within the range of the currently selected
> filter, though. The issue is that the NORM function reverts back to the
> default filter for the current mode, rather than reverting the DSP to
> the normalised width for the currently selected filter. I find this
> unintuitive too.

I believe the reason you find it unintuitive is that your mental model
("paradigm") for how the filters work is different from the designers'. Your
model, as stated in your first sentence above, appears to be that you pick
the hardware filter first, and then adjust the DSP filter to narrow down or
shift bandpass from the basic crystal filter values. This is the model that
best matches older non-DSP radios, where you choose the hardware filters
first and PBT/VBT controls make relatively minor adjustments.

In this mental model, "NORM" just means the normal DSP shift/bandwidth
setting for the chosen hardware filter, as you have described. This in turn
implies up to five different values of "NORM" for each mode, depending on
how many hardware filters you have installed. Since there are also "NORM
I/II" and "I/II" settings available, as well as the 5 basic settings
obtained when the XFIL button is used, this would seem to imply a total of
up to 30 different directly selectable filter settings per mode(!). 

However, the model underlying the description of the filter passband
controls and presets in the owner's manual pp. 23-24 is that the DSP filter
is primary, and once you select a DSP filter bandwidth, the hardware filter
is chosen automatically to correspond. This model matches the way the
controls actually work, e.g. if you increase the bandwidth setting beyond
the current crystal bandwidth, the crystal filter changes without any other
action on your part. This capability does not exist on pre-DSP radios, and
your first sentence suggests that you are not aware of, or at least do not
use, this capability. To say that "The DSP width can be adjusted within the
range ..." is inaccurate - the DSP width can be adjusted beyond that range
just as easily as within it.

In this DSP-first model, "NORM" (or "NORM I/II" or "I/II") just means a
pre-determined DSP filter setting, with the corresponding hardware filter
determined directly by the DSP bandwidth. There are between 6 and 10
selectable settings per mode - five selectable by pressing the DSP filter
controls, and from one to five basic (non-programmable) settings obtained
using XFIL.

Note that the XFIL button appears to use the opposite paradigm to the DSP
filter controls, i.e. you use XFIL to choose a hardware filter and the DSP
is adjusted to follow. However, there is another, perhaps less intuitive way
to look at XFIL, and that is that it chooses among a predetermined group of
DSP settings that exactly matches the available hardware filters.

73,
Rich VE3KI
K3 #1595