[Elecraft] Re: roofing filters - when to use them

[Don Wilhelm]

Rich,

The analogy that crops up in my mind is that in the K2 the FL1 ... FL4 
bandwidth is both the 'roofing filter' and the ultimate filter - 
assuming no other filtering is present.

If either the KAF2 or the KDSP2 is installed and active, that filter can 
be the 'ultimate filter' while the FL1 ...FL4 becomes the 'roofing 
filter'.  With the K2 and KDSP2 or KAF2, that ultimate filter is at 
audio frequencies, but with the K3, the ultimate filter is done at the 
15 kHz IF.

Yes, with the DSP and KAF2, a very narrow filter can be developed even 
if the IF filter is set to 2.4 KHz (SSB filter width), but when 
operating, a strong signal within that 2.4 KHz passband can (and will) 
activate the receiver AGC and de-sense the receiver.  All is well if 
that strong signal is the desired one, but if it is an undesired one, 
you may find the desired signal is no longer Q5 copy because the 
undesired signal has reduced the gain of the receiver.

That effect in the K2 is most noticeable when operating digital modes 
with the filter set to the full SSB filter width - when a strong signal 
comes on, the signal that you are trying to copy becomes weaker because 
the strong signal activates the receiver AGC.  Switching to a more 
narrow filter reduces the AGC effect of the strong adjacent channel 
undesired signal and will allow copy of the desired signal - in this 
particular case, the K2 filter is the roofing filter and the digital 
mode software in the computer is acting as the 'ultimate filter'.  The 
K3 is not significantly different in this respect because the analog AGC 
is developed after the roofing filter and before the DSP filtering.  I 
am told from the FAQ page that the DSP does offer some additional AGC, 
but I do not have any details.

73,
Don W3FPR

Richard Lim wrote:
>>
>> I can't count how many times K2 customers have told me that their 
>> "other" rig (I won't list them) was useless in the presence of strong 
>> signals, while the K2, with its narrow roofing filters, completely 
>> eliminated this effect. The K3 also has this advantage.
>>
>>
>> 73,
>> Wayne
>> N6KR
> 
> Wayne for reference, what is the width of the roofing filter in a K2? 
> Are we adjusting the roofing filters when we set FL1, FL2 etc in the K2? 
> This may shed more light on the subject for those of us who have a K2.
> 
> Rich
> KQ9L