[Elecraft] [K3] Roofing filters are misunderstood

[Don Wilhelm]

Jerry,

Yes, the term has become corrupted and misunderstood over the years.

That *is* exactly what the K3 filters do (protect the downstream 
circuits from strong out of passband signals), but is done using a much 
more narrow bandwidth at the 1st IF.

If the operator never encounters signal levels in excess of S-9+30, 
there is no need for the roofing filter, but signals stronger than that 
are encountered commonly in contests and tuning through a DX pileup.

If it were not for the Hardware AGC, they would overload the A-D 
converter causing the entire output to become garbage.
With the Hardware AGC present, that is not going to happen, but strong 
signals within the 1st IF bandpass will cause "pumping" of the Hardware 
AGC as those signals come and go.  In the K3, *that* condition is what 
the roofing filters will prevent.

Bottom line, if you hear that "pumping" (and subsequent desensing of the 
receiver), you would benefit from a more narrow filter.

73,
Don W3FPR

On 5/12/2014 12:33 PM, Jerome Sodus wrote:
> Hello Bill,
> The term "roofing-filter" made sense back in the 1980's when I designed
> roofing-filters at 70 MHz.
> Bandwidths would be in tens of KHz.
> The purpose then was to protect downstream circuitry by rejecting very
> strong out-of-band signals that could cause overload; selectivity was not
> the purpose.
> Selectivity was done further downstream.
> So the term has become corrupted over the years.
>