Problems because of filter switch during CW operation K2/ Filter FL4

[Renardy, Martin]

Dear all,

I ran through the K2 filter aligment considering all your tipps and
tricks. I also adjuest the reference oscillator
on the control board again. My filter bandwidths are 1.5 kHz, 700 Hz,
500 Hz and 200 Hz (Filter FL1 to FL4).

I have used Spectrogram to align the filters. The settings are matching
with zero beat tests on a carrier.

Now Fliter FL1 to FL3 are working perfectly. I can switch through the
filters and the tuned signal stays with
maximum amplitude in the filter with minimum frequency change.

The strange thing: Filter FL4 was not working. So I have set it's
bandwidth to 2 kHz and analysed the passband
By tuning over a carrier observing the peak in spectogram. I have seen,
that the center frequency of FL4
is different from FL1 to FL3!

How can that be, because the Filter settings FL1 to FL4 are controlling
the same electrical filter?!?

Regards

Martin
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[Don Wilhelm-3]

Martin,

There are 2 parameters to consider when making the filter settings - 1) the
width of the filter, and 2) the BFO setting.
It is the BFO setting that will change the position of the passband.  You
apparently have the BFO setting incorrect.

Use a noise source rather than a signal when observing the position of the
passband with Spectrogram and set the BFO so that the passband is properly
positioned.  See my website www.w3fpr.qrpradio.com article dealing with K2
Dial Calibration for detailed information for setting the reference
oxcillator and the filter/BFOs.

73,
Don W3FPR

> -----Original Message-----
> I ran through the K2 filter aligment considering all your tipps and
> tricks. I also adjuest the reference oscillator
> on the control board again. My filter bandwidths are 1.5 kHz, 700 Hz,
> 500 Hz and 200 Hz (Filter FL1 to FL4).
>
> I have used Spectrogram to align the filters. The settings are matching
> with zero beat tests on a carrier.
>
> Now Fliter FL1 to FL3 are working perfectly. I can switch through the
> filters and the tuned signal stays with
> maximum amplitude in the filter with minimum frequency change.
>
> The strange thing: Filter FL4 was not working. So I have set it's
> bandwidth to 2 kHz and analysed the passband
> By tuning over a carrier observing the peak in spectogram. I have seen,
> that the center frequency of FL4
> is different from FL1 to FL3!
>
> How can that be, because the Filter settings FL1 to FL4 are controlling
> the same electrical filter?!?
>
>

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[Tom Hammond-3]

Martin:

The Elecraft K2 CW filters are a bit unusual in that, as you 'narrow' the
passband, only one side of the xtal passband moves much. Thus, as you
narrow the passband, the center freq will shift inward a bit with each
narrowing of the passband.

Take a look at the illustration of the upper/lower passband edges in your
manual (probably somewhere around page 107 in your manual, depending upon
which manual revision you have... Look for "Crystal Filters and BFO Settings").

In what way was FL4 "not working". Is it possible that you have the BFO for
FL4 set to the wrong side of filter center? That will make the signal
appear to be badly off-freq.

73,

Tom Hammond   N0SS

At 02:34 AM 6/17/2005, you wrote:

>Dear all,
>
>I ran through the K2 filter aligment considering all your tipps and
>tricks. I also adjuest the reference oscillator
>on the control board again. My filter bandwidths are 1.5 kHz, 700 Hz,
>500 Hz and 200 Hz (Filter FL1 to FL4).
>
>I have used Spectrogram to align the filters. The settings are matching
>with zero beat tests on a carrier.
>
>Now Fliter FL1 to FL3 are working perfectly. I can switch through the
>filters and the tuned signal stays with
>maximum amplitude in the filter with minimum frequency change.
>
>The strange thing: Filter FL4 was not working. So I have set it's
>bandwidth to 2 kHz and analysed the passband
>By tuning over a carrier observing the peak in spectogram. I have seen,
>that the center frequency of FL4
>is different from FL1 to FL3!
>
>How can that be, because the Filter settings FL1 to FL4 are controlling
>the same electrical filter?!?
>
>Regards
>
>Martin
>_______________________________________________
>Elecraft mailing list
>Post to: [hidden email]
>You must be a subscriber to post to the list.
>Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
>Help: http://mailman.qth.net/subscribers.htm
>Elecraft web page: http://www.elecraft.com

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