[Elecraft] Re: Wayne's comment on Filter Selection

[wayne burdick]

[Note to all: We're planning to put any further information about 
roofing filters (whether philosophical or practical) into the FAQ, not 
on the reflector. Thanks for all of your great input and questions!]


Hi Doug,

DOUGLAS ZWIEBEL wrote:

> I want to use the narrowest roofing filter by
> mode that works for my operating style only when I am having IMD
> issues (some have said AGC issues also).  My use of the word
> "narrowest" is key.  What possible advantage is there to "trying" a
> filter a which is a little bit narrower?  If it doesn't work, I still
> get IMD.  If it works "this time," will it work next time under
> harsher IMD conditions?  During a contest, I don't have time or
> motivation to "play around."  I want works all the time (or 99%).

This is a multi-part answer -- bear with me  :)

As you adjust the DSP's WIDTH control (or hicut/locut), firmware 
automatically selects that crystal filter which is closest to your 
selected DSP passband. This means you don't have to worry about manual 
crystal filter selection at all, normally -- let the K3 do the work. 
The five filter icons (FL1-FL5) will indicate which crystal filter it 
has selected. You can continue to narrow the DSP bandwidth all the way 
down to 50 Hz in every mode, and the K3 will dutifully select the best 
installed crystal filter. Slight differences in filter passband loss 
are taken into account as well.

If you have a number of roofing filters, you'll have a three-point 
filtering system with ultimate rejection that's basically infinite 
(roofing filter, DSP I.F., DSP AF). Anyone who has suffered AGC 
pumping, desense or other problems when using wide roofing filters will 
notice the different the first time they use the K3 in crowded band 
conditions.

You can select crystal filters manually if you wish using the XFIL 
button. When you do this, the DSP passband is set to match the selected 
crystal filter's passband. You can then alter it with WIDTH or 
HICUT/LOCUT from there.

There are two other controls of interest that really simplify operation:

   NORM  --  This normalizes the overall system passband (crystal filter 
and DSP filtering) to a per-mode value, e.g.
             2.7 or 2.8 kHz on SSB. This is a quick way to get back to 
initial conditions; it also removes any SHIFT.

   I/II  --  This toggles between two "presets", i.e. two 
previously-established combinations of DSP control settings
             and crystal filter selection. If you have two settings you 
typically use (say wide/narrow for CW tuning),
             toggling between the two presets makes it very fast and 
easy. In CW mode, I usually have preset I set
             for 400 Hz, and II set for 100 Hz.

All of the DSP/filter settings are per-mode and per-VFO. So if you have 
the subreceiver installed, you can for example listen to CW at two 
separate bandwidths in the two receivers. The audio can be mixed, or 
one receiver's audio routed to the left earpiece/speaker and the other 
to the right. This is why we wanted two high-performance receivers with 
identical filter complements: versatility.


> I guess the price of the variable passband roofing filter will add a 
> lot to the story.

Yes, these will in effect give you 4 to 8 filters in a single slot. 
Since crystal filters are selected automatically as you adjust the DSP 
controls, you can think of the variable-passband filters as 
DSP-tracking crystal filters -- unique to the K3.


> One last thought.  I have not heard the DSP output.  Perhaps there are 
> so many artifacts that using selectable "roofing" filters is a better 
> option?????  I kinda doubt it.

The DSP subsystem has extremely high fidelity and low IMD. Until 
signals reach a fairly high level, the DSP's own filtering could handle 
the entire job. The roofing filters, if installed, will remove very 
strong signals outside the communications bandwidth being used. There's 
also pre-DSP hardware AGC that comes in at a high level to protect the 
A to D converter when very strong signals are present in the crystal 
filter passband.

73,
Wayne
N6KR


---

http://www.elecraft.com