Filter selection question

[n7ws]

I received my new 400 Hz filter midway through the WAE RTTY contest and since I wasn't really contesting, took the time to install it.

I guess I'm a little underwhelmed so far, but to be fair I wanted to compare it under fire with the 2.8 KHz filter I have been using. (I expected less buckshot from out-of-the-passband adjacent signals with the narrower filter, although this may be a commentary on the lousy transmitted signals)

I continued to allow the 2.8 KHz filter to be used on data modes because if I decide to use the PSK modes, I might want the wider BW.

Problem is... when switching the crystal filters manually, the DSP bandwidth defaults to 2.8 KHz when I select the 2.8 KHz crystal filter and 400 Hz when I select the 400 Hz crystal filter, even though I may have previously reduced the Width setting to something narrower. Likewise, when in
 Data mode with the 2.8KHz filter selected, hitting "NORM" defaults to 400 Hz BW, which again selects the 400 Hz filter.

I suppose this is by design, but why can't the radio simply maintain some independence between the crystal and DSP filters?

Wes  N7WS



     
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[wayne burdick]

Hi Wes,

This is by design. Selecting a filter manually via XFIL normalizes the  
DSP bandwidth to the crystal filter bandwidth. Using any other method  
(WIDTH, SHIFT, LO, HI) selects a crystal filter based on the DSP  
bandwidth (which you can see by rotating WIDTH, or just tapping it  
twice).

If you want to quickly alternate between two bandwidth settings that  
may not be the normalized values, you can use the floating presets (I/
II switch, part of the HI/WIDTH knob). These presets are saved per-
mode. Typically I set preset I to 500 Hz and preset II to 100 Hz.

In addition to the "floating" presets, you can create two "fixed"  
presets in each mode. Please see NORM1/NORM2 in the owner's manual.

Regarding performance: There is absolutely no question that inserting  
the 400-Hz crystal filter will provide dramatically better results  
when large signals are present (> about S9+10). With the 2.8 kHz  
crystal filter selected, signals within the 2.8 kHz passband but  
outside a narrower DSP bandwidth could activate hardware AGC.

This reflects the K3's very crystal-filter-centric design philosophy:  
use the narrowest 1st IF roofing filter consistent with the present  
mode. Other manufacturers put less emphasis on this because they  
typically can't offer narrow 1st IF filters (3 to 6 kHz or wider, vs.  
the K3's minimum bandwidth filter, which is 200 Hz).

73,
Wayne
N6KR

On Nov 16, 2009, at 4:23 PM, Wes Stewart wrote:

> I received my new 400 Hz filter midway through the WAE RTTY contest  
> and since I wasn't really contesting, took the time to install it.
>
> I guess I'm a little underwhelmed so far, but to be fair I wanted to  
> compare it under fire with the 2.8 KHz filter I have been using. (I  
> expected less buckshot from out-of-the-passband adjacent signals  
> with the narrower filter, although this may be a commentary on the  
> lousy transmitted signals)
>
> I continued to allow the 2.8 KHz filter to be used on data modes  
> because if I decide to use the PSK modes, I might want the wider BW.
>
> Problem is... when switching the crystal filters manually, the DSP  
> bandwidth defaults to 2.8 KHz when I select the 2.8 KHz crystal  
> filter and 400 Hz when I select the 400 Hz crystal filter, even  
> though I may have previously reduced the Width setting to something  
> narrower. Likewise, when in
> Data mode with the 2.8KHz filter selected, hitting "NORM" defaults  
> to 400 Hz BW, which again selects the 400 Hz filter.
>
> I suppose this is by design, but why can't the radio simply maintain  
> some independence between the crystal and DSP filters?
>
> Wes  N7WS
>
>
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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