[Elecraft] Roofing Filter Usage

[Ron D'Eau Claire]

That's why the Elecraft doesn't call the first I.F. crystal filters "roofing
filters". Unlike some receivers, the first I.F. crystal filters in the K3
are there for selectivity to isolate the desired signal. That is further
enhanced in the second conversion with the variable DSP filter, but the
crystal first I.F. filters are a key part of the selectivity equation. 

I suspect in this situation it is a case of the signals actually overlapping
(assuming he's listening to SSB). As you know, an SSB signal occupies a
swath of the spectrum - typically about 2.7 kHz but may be more if it's
someone running ESSB or someone overdriving their rig.

There's no effective way to separate the two signals completely if they
occupy the same bit of spectrum. Narrowing the selectivity down to exactly
what the desired signal needs will help by reducing any signal outside the
essential pass band but the overlapping energy is there. 

There's one other obvious possibility. If the interfering station is very,
very strong, it will still pass through somewhat. The Elecraft filters are
very good, but no filter is really a "brick wall". Any filter only
attenuates out of pass band signals. So if an signal outside the pass band
is strong enough, what remains may still be strong enough to compete with a
weak signal inside the pass band. But, in the Elecraft design, that takes a
huge signal. I suspect it's a matter of two signals occupying some of the
same spectrum.

Ron AC7AC



-----Original Message-----
The purpose of the roofing filter is to reduce receiver generated 
distortion products, not to provide selectivity.  Are you sure that that:

1) the strong signals are completely out of band;
2) the only distortion products are those generated by the K3 itself, 
and not the transmitter.

pastormg2 at verizon.net wrote:
> I purchased a K3 with the standard 2.7 Khz SSB filter plus the 1.8

+ I was experimenting around listening to a weak station that was rather
+ close to a real strong station and when I activated the 1.8 Khz filter I
+ was still getting some carryover from the strong station on the weak
+ station. Are there other controls that I need to use that would enable
+ me to knock out the real strong station so that I could clearly hear the
+ weak station? I'm sure there is help in the manual but I certainly can't
> 


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