[Elecraft] K2 CW and DSP

[Don Wilhelm]

John,

That is not a consequence of any construction error, but a consequence 
of your filter alignment.

When you align a wide filter for either CW or SSB, do NOT CENTER it. 
Use Spectrogram and align it so that the low frequency edge of the 
passband cuts off above zero frequency (I prefer 200 Hz at the lowest 
corner).  In other words, an 1800 Hz wide CW filter should have an audio 
response from 200 to 2000 Hz.

Aligned that way, you will not be able to hear a signal go through zero 
frequency and come out on the opposite sideband (like a direct 
conversion receiver).  I would wager that you have that wide filter 
aligned so it receives from perhaps 900 Hz down to -900 Hz (the -900 Hz 
is on the opposite sideband, so you hear both sidebands for pitches less 
than 900 Hz).

If you like a really wide CW filter, try using the OP1 filter instead of 
the variable filter (assuming you have the KSB2 installed).  It is 
flatter and more 'listenable' than using the varaible filter at a wide 
setting.  I usually recommend setting the variable filter to 1200 Hz wt 
the widest although some filters are quite usable out to 1500 or even 
1800 Hz, but on some, the passband ripple becomes quiter pronounced at 
wider bandwidths.

Spectrogram will show all that to you visually.  If you have not used it 
to set up your filters, I recommend that you download it (version 5.17 
is available free from www.n0ss.net) and try it out.

Once you have the CW filters set the way you want, then you can use the 
DSP to augment those filters and improve the ultimate rejection and 
provide a sharper filter slope, but operate with the most narrow CW 
filter as your first course of action to combat QRM - if you do not, 
strong undesired signals in the CW filter passband will capture the 
receiver AGC and de-sense the receiver making all signals in the 
passband (including the weak one you are trying to copy) weaker - 
operating with a narrow CW filter is the only way to combat that (other 
than turning off the AGC).

For additional information on filter alignment, check the info on my 
website http://w3fpr.qrpradio.com or Tom Hammond's site www.n0ss.net or 
on the builder's Resource page of the Elecraft website.

73,
Don W3FPR

John Graf wrote:
> 
> I've been experimenting with the DSP filters on my K2 and I have found 
> several combinations of DSP and crystal filtering that work very well on 
> CW (at least for me).  However, I have run into a strange phenomenon.
> 
> Here is how I reproduce it:  I set my crystal filter at its widest 
> setting (1.8 KHz).  The DSP filter is set for "low pass" with noise 
> reduction on.  With this combination, I am able to find two identical CW 
> signals on either side of a center frequency.  It operates just like a 
> direct-conversion receiver.
> 
> So here are a couple of questions for the group:  First, has anyone else 
> seen this happen?  If not, do you think it may be a construction error 
> on the DSP board?  How about a screw-up in the filter alignment or 
> programming?
> 
> Thanks and 73,
> 
> John, WA6L