Posted by
Don Wilhelm-3 on
May 12, 2006; 5:06am
URL: http://elecraft.85.s1.nabble.com/DSP-XTAL-filter-settings-tp389470p389472.html
Bill and all,
I must disagree - when the SSB filter widths and corresponding BFOs are
correctly set, the variable XTAL filters can be quite usable for SSB
reception.
First of all, the lower -3dB frequency corner of the passband must be near
300 Hz to maintain intelligibility. The high frequencies may be cut without
sacrificing a lot of intelligibility. The filter width must be determined
by using Spectrogram or a similar means - the numbers given by the K2
display for the wide filter settings may be grossly in error (the actual
filter width is usually quite a bit wider than the numbers indicate at the
wider filter settings - it is close to actual at 400 to 500 Hz widths, which
is the design center for the filter).
If the SSB filters are set for FL1 = OP1, and FL2 thru FL3 are set for
progressively more narrow filters, then a properly tuned SSB signal will
remain intelligible as the filters are narrowed, and high frequency QRM will
be reduced.
Assuming the 2.4 kHz width is used for OP1, centering the filter passband
between the following Spectrogram markers will result in quite usable
filters:
FL1 width = OP1 -- 300 and 2600 Hz
FL2 measured width = 2200 Hz -- 300 and 2400 Hz
FL3 measured width - 2000 Hz -- 300 and 2200 Hz
FL4 measured width = 1800 Hz -- 300 and 2000 Hz
The DSP can be used as a supplement to these filters, but I find that the IF
filters are adequate in most cases, freeing the DSP to be used for noise
reduction and notch filtering purposes in SSB mode.
Again, I find the most prevalent 'error' in setting the variable filters for
SSB use is in assuming that the filter width is what is indicated on the K2
display - and that actual width may be quite a bit wider than indicated -
measure the width with the Spectrogram display and ignore the width numbers
shown in the display.
73,
Don W3FPR
> -----Original Message-----
>
>
> On SSB, there's basically one XTAL filter - OP1. The variable
> bandwidth filter is pretty asymmetric for use on SSB. OP1 also has a
> very modest shape factor.
>
> I use the DSP filter to clean up the shape of the OP1 filter. I have
> a couple of variants programmed that cut the high and low end of the
> filter. These are useful during contests or any time there's lots of
> QRM. Narrowing the filter may slightly reduce intelligibility, but it
> also cuts adjacent signals.
>
>
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