[Elecraft] adding 250Hz filer, any improvement?

[Scott Ellington]

I agree with both Don and Bill.  There are really two issues:

In the presences of just noise, the DSP with a wider crystal filter is adequate.  The additional noise reduction of a narrow crystal filter is negligible.  (QRM is another matter.)

The brain/ear system, at least for many operators, is very good at copying weak signals in noise, even with a broad bandwidth.  If there's no QRM nearby, I always find it easier to copy with a larger bandwidth, though anything above 500 Hz makes little difference.  (Except for quickly tuning across a mostly empty band.)  The 250 Hz bandwidth, in my experience, ALWAYS makes it harder to copy weak signals, unless there is adjacent QRM.  For that reason, I prefer to operate with 400 Hz bandwidth most of the time, switching to a narrower bandwidth only when necessary.

This seems to contradict signal theory, which says that a narrower bandwidth improves S/N ratio until the filter bandwidth is equal to that of the signal.  What signal theory fails to take into account is the matched filter in a CW operator's head.

73,

Scott  K9MA
 
On Feb 9, 2011, at 7:51 AM, Don Wilhelm wrote:

>  Ed,
> 
> If there are no strong signals on the band or nearby your receive 
> frequency, then any filter, even the stock 2.7 kHz is sufficient.  The 
> only purpose of the roofing filter is to keep strong signals out of the 
> receiver passband so they do not activate the AGC and de-sense the 
> receiver for the signal you are trying to hear.
> 
> 73,
> Don W3FPR
> 
> On 2/9/2011 3:07 AM, Edward R. Cole wrote:
>> Let me ask a filter question for a different situation.  I am
>> interested in receiving very-weak CW and currently have the 2.8 KHz
>> and 400-Hz 8-pole filters.  Will narrowing the DSP bw down to 200-Hz
>> work as well as using a 200-Hz filter?  There are no strong stations
>> either nearby or on the band.
>> 
>> BTW I am impressed with the 400-Hz filter on the K3 when compared to
>> the 400-Hz DSP on my FT-847 (no surprise).
>> I find trying to hear extremely weak-CW that narrowing down to 200,
>> 100 or even narrower makes the difference.
>> 
>> I left a blank filter space on the main receive for adding a narrow
>> SSB filter at a later time.  I also have the 13-KHz filter for
>> FM/AM.  Sub-Rx has 2.8 and 13-KHz.
>> 
>> 
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Scott Ellington
Madison, Wisconsin
USA