[Elecraft] here is how to "get a feel" for a roofing filter

[Brett gazdzinski]

Is there no way to put dsp at/as the first filter?
What are the limitations of the dsp, frequency, filter quality?
Is there a problem getting dsp to work at say 455 khz?

What would be the result of a real good variable filter
at the 8.215 Mhz IF just after the mixer and NO dsp
for filtering? (K2?)


The 2nd IF of 15 khz is audio, is it not?
At what point does the audio spectrum end and
RF begin?
I notice that the K3 info says the dsp is in the 2nd IF,
but the 2nd IF is high pitched audio?

This modern stuff sure spins my head, the homebrew receiver
is a single conversion to 455khz, no rf amp, good filter
just after the mixer, wide IF chain, detector.
It seems to work real well, and is basically the old tube table top
radio (all American 5?) with a good filter, s meter, and a digital 
frequency readout.

Brett
N2DTS







> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Eric 
> Swartz - WA6HHQ, Elecraft
> Sent: Wednesday, May 02, 2007 11:12 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] here is how to "get a feel" for a 
> roofing filter
> 
> The confusion here may be primarily one of semantics. The K3's main 
> narrow IF filters -are- the first filters seen after the first mixer, 
> just like those in the K2. Our first IF at 8.215 MHz acts as 
> our primary 
> narrow IF filter stage and can also be considered our roofing filter 
> stage, since it is followed by the DSP IF and its additional 
> filtering. 
> We had so many questions about what our 'roofing' filters 
> were on the K2 
> and K3,  that we decided to refer to the 8.215 MHz narrow filters as 
> roofing filters. Since they are the first IF filter this is also 
> technically correct.  :-)
> 
> In our case, since the roofing filters ARE the main, narrow, 
> IF filters 
> (just like the 4.915 1st IF filter is in the K2),  dynamic range is 
> measured for signals OUTSIDE the filter b/w, not inside. The 
> K3's 1st IF 
> (8.215 MHz) can easily use a crystal filter down to 200 Hz. Higher 
> frequency first IF stages in other radios are limited to about 3 kHz 
> minimum and typically use filters in the 5 kHz to 10 kHz width range. 
> They only have one filter at their 1st IF (their roofing 
> filter) so it 
> has to be at least wide enough to handle SSB. Plus it is very 
> difficult 
> to make a filter narrower than 3 kHz economically at an IF of 
> 40-70 MHz.
> 
> On traditional up-converting IF radios with wide (>=3 kHz) roofing 
> filters at their first IF (40 MHz to 70 MHz) the interfering 
> signal can 
> easily be inside their first roofing filter but outside their 
> next stage 
> narrow (500 Hz etc) IF filters. This causes havoc with the stages 
> between their roofing filter and their narrower IF filters.  
> (The ARRL 
> test sweeps to 1 kHz away with a 500 Hz main IF filter.)
> 
> 73, Eric   WA6HHQ
> Elecraft
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