[Elecraft] K3 Filter questions

[k.igor at comcast.net]

If it pleases you to use DSP set wider than roofing filter - by all means, go for it. 

I would not, however, clame objectivity based on my subjective experience  or somebody's valuable opinion and simplistic explanation , I perefer measurements performed on good lab equipment. K8ZOA did some of the measurements on 500 Hz filter, but most of them probably applicable for the 1.8kHz filter. See report here: http://www.cliftonlaboratories.com/elecraft_k3_noise_blanker_and_crystal_dsp_filtering.htm 

In his measuremetns, setting DSP wider than the actual roofing filter actually degraded the total response in the stop band. 



73, 

Igor, N1YX 

    

  



----- Original Message -----


From: "Jim Brown" <jim at audiosystemsgroup.com> 
To: elecraft at mailman.qth.net 
Sent: Tuesday, March 27, 2012 2:33:14 PM 
Subject: Re: [Elecraft] K3 Filter questions 

On 3/27/2012 11:12 AM, k.igor at comcast.net wrote: 
> I am pretty sure the DSP filter is effective only  if it is narrower than the roofing filter. 

Not at all true.  When reading K6LL's comments, bear in mind that he is 
a VERY good operator and engineer.  I value anything he has to say. 

I posted yesterday that I had done almost the same thing -- told the K3 
that my 1.8 kHz filter was a 2 kHz filter so that it switches in at 2 
kHz -- and that I was pretty pleased with the result. 

When there are multiple filters in any signal chain their responses 
combine (the proper engineering term is "cascade") so that the total 
filtering capability is that of both added together. This is most 
pronounced when two filters have approximately the same bandwidth and 
are set to the same frequency.  The way this works is that if filter #1 
is rejecting by 6 dB at a given off-frequency point and filter #2 is 
attenuating by 4 dB, the combined rejection will be 10 dB. This is 
approximately what K6LL and I are doing. 

73, Jim K9YC 

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