[K2] question about crystal filters and DSP audio filter settings

[KK4LSG]

Hello from Kevin, KK4LSG, in Seaford, Virginia. I recently completed my
beloved K2 and have been enjoying getting on the air with CW as much as
time will allow. I am a new operator, receiving my license in the fall of
2012.  I built the elecraft KDSP2 audio filter a few weekends ago, and have
been playing around with the settings to find an optimal filter width. I
seemingly understand that the crystal filters eliminate much of the
unwanted RF from ever entering the circuitry, and that the audio filter
further refines the signals before the signals exit the speaker. I
currently have the crystal filters set at 1.5 khz, 700 hz, 400 hz and 200
hz. The DSP filters are set to kick in at 150 hz, 100 hz and 50 hz. I read
somewhere that the crystal filters do a great job down to 200 hz and that
the DSP should be set to go narrower if needed. My question then is whether
the collective here recommends similar settings or is there a better way to
ensure adequate audio filter functionality without duplicating the efforts
of the crystal filters? Is 50 hz too narrow. Are there better settings to
help dig out signals from a crowded band? Any help is appreciated as I have
found little online that addresses the questions above in a way that I can
understand.

Kevin
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[Don Wilhelm-4]

Kevin,

Usually the KDSP default filter widths work fine,  I would suggest that
you leave CW AFIL1 set for LoPass and normally use the IF filters for
general use.  When you encounter QRM, then first narrow the IF filter,
and then if still needed, start down the AFIL steps to resolve the signal.
In other words, there is only marginal benefit in using a 150 Hz KDSP2
bandwidth if your IF filter is set to 1.5 or 0.7 kHz.  The offending
strong signal will still take over the K2 AGC and you will hear
'pumping' because the strong offending signal is controlling the AGC.

The KDSP2 filter skirts are steeper than the IF crystal filter skirts,
and that is the main advantage.
Narrower bandwidth is not always the best cure, but the choices are
entirely yours to make.  Experiment a bit while listening to DX Pileups
or during a "hot and heavy" contest where the bands are crowded, and
that experience will give you the information you need to set the KDSP
filters the way they will be of the most advantage to you.  Your current
setting may be quite adequate.
Your goal should be twofold - first is to reduce QRM from adjacent
signals, and the second is to be able to pull weak signals out of a
crowded band.  More narrow filter widths are not always the way to make
that happen, but escaping from QRM with steeper filter skirts may work
better.  A lot depends on your tuning skill as well as selection of the
proper filters and the best way to develop that skill is to try it out
on crowded band conditions.

It may be that you simply want to set the KDSP bandwidth to the same
widths as the IF filters to take advantage of the steeper skirts. In
other words, use AFIL2 in conjunction with IF FL2, AFIL3 in conjunction
with IF FL3, etc.  It is rare that you will need less than 200 Hz of
filter width, but YMMV.

73,
Don W3FPR

On 1/30/2013 1:57 PM, Kevin Crossett wrote:

> Hello from Kevin, KK4LSG, in Seaford, Virginia. I recently completed my
> beloved K2 and have been enjoying getting on the air with CW as much as
> time will allow. I am a new operator, receiving my license in the fall of
> 2012.  I built the elecraft KDSP2 audio filter a few weekends ago, and have
> been playing around with the settings to find an optimal filter width. I
> seemingly understand that the crystal filters eliminate much of the
> unwanted RF from ever entering the circuitry, and that the audio filter
> further refines the signals before the signals exit the speaker. I
> currently have the crystal filters set at 1.5 khz, 700 hz, 400 hz and 200
> hz. The DSP filters are set to kick in at 150 hz, 100 hz and 50 hz. I read
> somewhere that the crystal filters do a great job down to 200 hz and that
> the DSP should be set to go narrower if needed. My question then is whether
> the collective here recommends similar settings or is there a better way to
> ensure adequate audio filter functionality without duplicating the efforts
> of the crystal filters? Is 50 hz too narrow. Are there better settings to
> help dig out signals from a crowded band? Any help is appreciated as I have
> found little online that addresses the questions above in a way that I can
> understand.
>

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[RobertG]

Kevin...

Don's advice is excellent and reflects his many-faceted expertise with
the K2. He repaired mine a while ago after I had reached the end of my
line with the unit, and his advice here is always spot on.

I would offer a comment on Don's very last bit that less than 200 Hz is
seldom necessary. Generally true. I have found that, when the QRN is
high, either natural or human generated, bandwidths down to the K3's 50
Hz DSP limit can sometimes be useful, even allowing for lost gain and
the ringing and signal artifacts. Just a thought.

...robert

 > On 1/31/2013 01:57, Don Wilhelm wrote:

> It is rare that you will need less than 200 Hz of
> filter width, but YMMV.
>
> 73,
> Don W3FPR
>
> On 1/30/2013 1:57 PM, Kevin Crossett wrote:

>> Is 50 hz too narrow. Are there better settings to
>> help dig out signals from a crowded band?

--
Robert G Strickland, PhD ABPH - KE2WY
verizon.net.usa
Syracuse, New York, USA
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[KK4LSG]

Thanks to Don and Robert for the quick and thorough responses about setting up the DSP filters. I will experiment with the settings now that I understand a little bit better.


On Jan 30, 2013, at 8:57 PM, Don Wilhelm wrote:

> Kevin,
>
> Usually the KDSP default filter widths work fine,  I would suggest that you leave CW AFIL1 set for LoPass and normally use the IF filters for general use.  When you encounter QRM, then first narrow the IF filter, and then if still needed, start down the AFIL steps to resolve the signal.
> In other words, there is only marginal benefit in using a 150 Hz KDSP2 bandwidth if your IF filter is set to 1.5 or 0.7 kHz.  The offending strong signal will still take over the K2 AGC and you will hear 'pumping' because the strong offending signal is controlling the AGC.
>
> The KDSP2 filter skirts are steeper than the IF crystal filter skirts, and that is the main advantage.
> Narrower bandwidth is not always the best cure, but the choices are entirely yours to make.  Experiment a bit while listening to DX Pileups or during a "hot and heavy" contest where the bands are crowded, and that experience will give you the information you need to set the KDSP filters the way they will be of the most advantage to you.  Your current setting may be quite adequate.
> Your goal should be twofold - first is to reduce QRM from adjacent signals, and the second is to be able to pull weak signals out of a crowded band.  More narrow filter widths are not always the way to make that happen, but escaping from QRM with steeper filter skirts may work better.  A lot depends on your tuning skill as well as selection of the proper filters and the best way to develop that skill is to try it out on crowded band conditions.
>
> It may be that you simply want to set the KDSP bandwidth to the same widths as the IF filters to take advantage of the steeper skirts. In other words, use AFIL2 in conjunction with IF FL2, AFIL3 in conjunction with IF FL3, etc.  It is rare that you will need less than 200 Hz of filter width, but YMMV.
>
> 73,
> Don W3FPR
>
>

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