[Elecraft] roofing filters - when to use them

[Don Wilhelm]

Brett,

While there are DSP processors that *CAN* operate at higher frequencies 
(like the receiver 1st IF), they are very expensive - the downconversion 
to the DSP IF at 15 kHz is a much more economical approach.

If you want to look at it that way, the filters in the K2 act both as a 
roofing filter and a final selectivity filter - but in that context, the 
term 'roofing filter' is a misnomer.

73,
Don W3FPR

Brett gazdzinski wrote:
> 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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