[Elecraft] Roofing Filters

[dj7mgq at muenchen-mail.de]

Zitat von Bill Tippett <btippett at alum.mit.edu>:

>          Filling K3 with filters as Toby suggested is
> both unnecessary and expensive IMHO.  I think he is
> forgetting that there is very good DSP filtering in

<< many lines removed >>

> Even a perfect receiver cannot prevent an adjacent TX
> signal's defects!

>From my layman's view, I am not sure that I fully agree with you. Here in the 
EU the CW segment of the 40m band is (still) only 40kHz wide, in other words, 
assuming that in a major contest there are 200 european stations on 40m, then 
you have an average spacing of 200Hz. In SSB the bandwidth is 60kHz giving an 
average spacing of 600Hz - assuming 100 stations. (To be honest the number of 
stations calling CQ is an educated guess.)

As you may be able to imagine, the 40m band can be a real zoo, and every bit of 
(clean) selection, as early as possible, helps. Some of the major contest 
stations go so far as to use a bank of switchable crystal filters in front of 
the mixer <http://www.yt6a.com/en/front-end-filters/index.php>.

As I see it the main job for the roofing filter in the K3 is to help "protect" 
the ADC and prevent AGC pumping. If I am listening to a weakish signal (S5) 
between two strong (9+40) stations with a 200Hz bandwidth DSP and a roofing 
filter of say 500Hz on 40m then either the AGC will densense the RX (AGC 
pumping) or the strong stations will "overpower" the ADC.

I contest mostly in SSB, so I need good early selection in SSB. At out contest 
QTH, where my K3, may end up living, there are many avid CW operators and they 
will need good selection in CW. Not to mention the RTTY contests. Also, when 
the K3 is not at our contest QTH, e.g. if I take it with me on vacation, then I 
do want to be able to listen to broadcast stations or when using a transverter 
work over relais (FM).

This why my personal dream(!) filter setup is so rich. It might be expensive 
but I doubt that it is totally unnecessary. 

As to the side effects of poor signals (TX phase noise etc.), I agree with you 
fully. It DOES make things much more difficult.

But the again, what do I know... ;-)

vy 73 de toby
-- 
DD5FZ, 4N6FZ
K2 #885, K2/100 #3248, K3 #<200