Re: K-3 Roofing filter comment

Posted by Don Wilhelm-4 on Jul 12, 2011; 6:33pm
URL: http://elecraft.85.s1.nabble.com/K-3-Roofing-filter-comment-tp6576237p6576271.html

  Richard,

You were not hearing the effect of the 2.8 MHz roofing filter, but
instead the DSP filter.
In order to hear the effect of the 2.8 kHz filter skirt, you would have
to widen out the DSP bandwidth to max or greater.  The DSP forms the
ultimate filter, and does have nearly vertical slope.

The purpose of the roofing filter is to keep adjacent strong signals
from activating the hardware AGC and causing the resulting AGC pumping.  
It is the DSP that sets the actual filter width.

73,
Don W3FPR

On 7/12/2011 2:22 PM, Richard Fjeld wrote:

> FYI,
> I don't want to open up a 'can of worms', and Rob Sherwood could better speak to this, but this is an observation I made using a P-3.
>
> Not long ago, there was quite a string of threads about the 5 pole 2.7 Khz roofing filter versus the 8 pole 2.8 Khz filter. Before I ordered my K-3, I had studied the filter skirts of each. I don't remember how I found the plot for the 5 pole filter, but I remember that the span of each was different. At the same span width, the 2.8 Khz filter skirts seemed to me to be considerably straighter when compared to the 2.7 Khz filter, so I ordered the 2.8 filter.
>
> The other day, I was on a qso with a group and while watching the SSB signal on the P-3, I noticed a carrier much taller appear right on the edge of the filter bandwidth as shown by the brackets at the bottom of the P-3. The carrier was on the far end of the filter, or 2.8 Khz away from the center frequency.
>
> I could not hear it, but expected to, so I tuned over slightly to get it within the filter bandwidth and heard it strongly. I seem to recall that a filter's width is rated at something like 6 dB down, so I thought I would have heard it within the skirts at their wider limits.  I still wonder about that, but I'm very pleased.
>
> (Just a comment; I'd hate to do without the P-3)
>
>
> Richard Fjeld, n0ce
>

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