[Elecraft] The Mysterious K3 Roofing Filter Scheme

[Ron D'Eau Claire]

Al asked:

Here's what I haven't determined yet: under what conditions it is necessary
to match (2) 5-pole filters. Is it when I have a 5-pole in the K3 and a
5-pole in the KRX3, of identical bandwidth? or different bandwidths?
Probably identical, based on my understanding of diversity reception above.
But maybe not.

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The 'offset' of the filters of similar bandwidth must be matched when doing
diversity reception. 

Keep in mind that the KRX3 is a completely *separate* receiver from the main
K3, so any filters in it have nothing to do with the K3, receive or
transmit. In order to avoid a slow beat sound in the audio when using both
receivers in diversity mode, the filters must be matched must be matched. 

You are correct about the offsets. The 8-pole filters require a zero offset
(no adjustment) while the 5-pole filters come marked with the required
offset so their band passes line up exactly. 

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Has there ever been a ham who experienced as much difficulty deciphering all
of this as me? Why not? What do you young whipper-snappers eat for
breakfast?

;-) Wheaties. But I'm hardly a whipper-snapper (darn it). 

If you are familiar with the receivers we've been using since Armstrong laid
out the first superhet, you know that a superhet takes its main selectivity
at the intermediate frequency (I.F.). That was the whole point of the
superhet design: convert all incoming signals to a single I.F. where a good
filter could be used. 

The K3 is no different. 

You may also recall that in years past we augmented the I.F. filter with
filters at audio to achieve even more selectivity when needed, or we went
'double conversion' with a very low I.F. with a second filter for more
selectivity. 

The K3 is basically the same. 

The difference between the older receivers and the K3 is that instead of a
second I.F. or an audio filter, a digital signal processing (DSP) system
converts the I.F. to digital data and manipulates the digital signal to
provide additional selectivity, noise suppression, and also demodulates the
signal. The output of the DSP system is audio. 

The K3 has the same issues with where the selectivity is present as we did
in older gear: every amplifier before the final selectivity opens up the
possibility of overloading the receiver with adjacent-frequency signals. 

The K3's "roofing" filters are simply the main first I.F. filters. To avoid
problems with strong signals near the desired signal, you can install a
variety of filters that are selected as needed, from 13 kHz for FM down to a
few hundred Hz for CW. 

The final selectivity is provided by the DSP system. 

You can select the I.F. ("roofing") filter to use manually or, when you
rotate the SELECTIVITY control the K3 will step through the "roofing"
filters automatically as it also changes the DSP system selectivity. That
gives you a "one-knob" selectivity control. 

Hope that's a little clearer than mud, Al! 

Ron AC7AC