Dumb question .....

[Michael Heit]

This may seem like a really dumb question but I have to ask....

What are the "roofing filters" options for the K3 radio?  I understand
filters but the term roofing filters really confuses me and why they just
aren't built right into the radio from the start.

Mike Heit AD7VV

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Iain MacDonnell - N6ML]

Michael D. Heit wrote:
> This may seem like a really dumb question but I have to ask....
>
> What are the "roofing filters" options for the K3 radio?  I understand
> filters but the term roofing filters really confuses me and why they just
> aren't built right into the radio from the start.

Hi Mike,

You might want to start here:

http://www.elecraft.com/K3/Roofing_Filters.htm

HTH,

     ~Iain / N6ML
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Don Wilhelm-4]

Mike,

Since you understand 'filters' - consider that the ultimate selectivity
of the K3 is determined by the DSP.
Now think about the input to the DSP - if one restricts the number of
signals entering the DSP (actually the ADC front end for the DSP), the
DSP will not have to deal with as much 'garbage'.  The more narrow the
roofing filter, the less 'garbage' will be present at the DSP front end.

Consider also that the DSP can handle an S-9 +20 signal input from an
unwanted signal - above that level it will be overloaded.  In situations
where signals greater than that level are encountered within the
passband of the roofing filter, the Hardware AGC will limit the signal
level presented to the DSP.  Even though the DSP may be cranked down to
hear only the desired signal, the signal gets through to the DSP front
end and will reduce the gain of the receiver because the Hardware AGC is
being activated by the strongest signal within the passband of the
roofing filter - the result is AGC "pumping" as a result of a signal
that one may not even hear.  Using a more narrow roofing filter can
eliminate this unwanted signal and eliminate the AGC pumping.

Think of the roofing filter as a roof on a house - the wide filter
allows the DSP (and hardware AGC) to respond to all the rain falling on
the roof.  To reduce the amount of rain falling on the roof, a more
narrow roofing filter is required.

If you are interested in contesting and/or heavy DXing, you are liable
to encounter nearby unwanted strong signals within the passband of the
roofing filter which will cause the hardware AGC 'pumping', and you
should invest in narrow filters that will eliminate that effect.  For
those whose operation is more casual and do not wish to dig for weak
signals in the presence of stronger nearby signals, a wide roofing
filter will do the job nicely.

73,
Don W3FPR

Michael D. Heit wrote:

> This may seem like a really dumb question but I have to ask....
>
> What are the "roofing filters" options for the K3 radio?  I understand
> filters but the term roofing filters really confuses me and why they just
> aren't built right into the radio from the start.
>
> Mike Heit AD7VV
>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft   
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
> ------------------------------------------------------------------------
>
>
> No virus found in this incoming message.
> Checked by AVG - http://www.avg.com 
> Version: 8.0.176 / Virus Database: 270.9.17/1845 - Release Date: 12/12/2008 9:02 AM
>
>  

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com