[Elecraft] roofing filters - when to use them

[Brett gazdzinski]

How does the below compare to the K2?
There are no roofing filters in the K2, right?

In my homebrew receivers, I have a high Q tuned circuit
at the antenna, then into the mixer, the output of the mixer
goes to my filters (KIWA), then the IF chain.

There is nothing to roof there, right?

I get artifacts of close signals, but not from the receiver,
from the transmitter...
The $50.00 KIWA filters are good enough so if its out
of the passband, its not there.

Now I wonder why you cant just go from the mixer
into the DSP...

Brett
N2DTS
 

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of DOUGLAS ZWIEBEL
> Sent: Tuesday, May 01, 2007 7:59 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] roofing filters - when to use them
> 
> Hi all:
> 
> Well, Bill (ZV) gave a nice (brief) summary and pointed to a nice
> reference.  Ten Tec had (maybe it is still there) some good info also,
> but specific to their radio...however, you can still use it to get a
> "feel" for a what a roofing is (and what it isn't).  If you're a
> member of ARRL, go to their web page and search on Roofing Filter.
> 
> Don (FPR) cited some uses which I am not familiar with (data), so no
> comment from me on that.
> 
> I've commented directly to the source about my "wonder" at why one
> would want a variable roofing filter.  As Lee said, one "wide" and one
> "narrow" is enough (at least for SSB / CW only operation).
> 
> If I'm  on CW and my radio is hearing squeaks, pops, and "garbage" due
> to lots of LOUD signals being in my passband (say 15 or 20kc wide on
> an "old" radio), I would not be able to do much...maybe cut back on
> the RF gain, add some attenuation, etc.  However, if I could narrow
> the passband (the roof) to say 1kc, the likelihood of two (or more)
> very loud signals being captured within that 1kc is greatly reduced.
> If I use a 500hz roofing filter (assuming all other parameters could
> remain unchanged), the odds of having two (or more) "other" very
> strong signals there (besides the one I want to copy) is even
> less...hence less chance of squeaks and pops masking the signal I want
> to copy.  BTW, the squeaks and pops = IMD Products.
> 
> Of course, if there were that many LOUD signals that close together,
> I'd simply move (qsy) to a clearer (relatively) spot.
> 
> Why in the world one would want to take the time to "carefully" or
> "slowly" adjust a roofing filter's bandwidth is beyond me.  I would
> simply dump in my "narrow" roofing filter and be done with it.  Either
> it works or it doesn't; why play around?
> 
> This topic is of major concern to contesters, especially (mostly) on
> the low bands.  I know...it is also of concern to Europeans on 40m
> even NOT during a contest...true.
> 
> If you're not a SERIOUS contester or Low Band DXer (or data guy ala
> Don's description?), I have a hard time understanding why you'd want
> these "narrow" filters.  Hey look, 2.7kc is pretty darn narrow
> compared to most of what is out there.  Again (as had already been
> cited), even the IC7800 and FT9000 have a 3kc roofing filter as their
> narrowest (which I don't think is good enough for some CW situation).
> 
> I would guess that for 95% of non-serious contesters, or non-low band
> DXer (and digital guys?), you'll be VERY happy with a single 2.7kc
> roofing filter for SSB and even for CW.  For those who are used to IF
> filters, this is just not the same.  You'll still have that with the
> K3....twiddle the bandwidth any way you like (narrow).  Remember,
> roofing filters are for CROWDED band condx FILLED with LOUD signals
> (with the digital proviso still there).
> 
> And NOW, I'd like to say, I have no idea exactly how much MAGIC the
> Aptos boyz have included in this radio and for all I know (not having
> the radio, the specs, schematic, code, etc., they may have come up
> with something that voids all of the above.  Wouldn't that be great!
> 
> de Doug KR2Q
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