K3: use of 1.8 filter - clarification

[DOUGLAS ZWIEBEL]

I've received emails replies (direct and otherwise) that, for me, indicate
that I should elaborate/clarify what I am referring to.

Let's assume that you have a 2.7 and 2.1 roofing filter.

If you set up your "narrower" SSB filter according the the manual, which is
what the majority of K3 ops do, as you transition from, 2.7 to 2.1 using
the width "knob," you are doing two separate, but linked things.  First,
you are reducing the DSP BW.  Sync'ed to that (you have no choice without
manual intervention, ie "faking out" the K3), as you get to 2.1 kHz in the
DSP BW, you simultaneously switch from the 2.7 to the 2.1 roofing filter.

Unless you "unhook" this coupling between the DSP BW and the Roofing Filter
BW, you simply cannot "tell" which one is influencing what you hear (what I
called "feel").  So when you get down to 2.1 (from 2.7) are you
appreciating the impact of the DSP or the roofing filter - or maybe both?
How would you know?

I am saying that unless you are operating in extremely crowded band
conditions (not with just one "interfering, very near-by station") you are
appreciating the impact due to the DSP BW and certainly NOT the roofing
filter.

It is possible to manually intervene to alter (even temporarily "turn off")
the (point of) sync between the DSP BW and the roofing filter (narrower)
BW.  And I am saying that unless you actually do this "de-linking," that
you won't be able to know/confirm the true source of what you are hearing /
feeling.  I'm betting that for the vast majority, especially non-contesters
and non-160 serious DXers, what you hear is due to the DSP BW.

The roofing filters are used for reducing IMD.  A receiver with poor IMD
will experience "phantom signals" due to the mixing of other very strong
signals (multiple) on the band and these phantom signals can potentially
cover up real, but weak, signals.  You can usually identify such IMD as
received "squeaks, pops, whines, groans, etc."  The band will sound like it
is covered with garbage - even somewhat "musical.  The addition of the
appropriate roofing filter can reduce or eliminate this.  This is esp true
(for me) on CW and especially (for me) on the low bands (lots of very
strong signals) during a contest.  In "the old days," I've had many
receivers that would simply collapse under the strain.  It made tuning
across the band painful and arduous when trying to pick out weaker signals.
After hours and hours of this, it is very fatiguing.  I use the 5 pole
500hz filter on CW for both of my K3s.  Is it "wrong" to get a narrow
filter?  Of course not.  But unless you are planning on operating in the
midst of multiple, super strong signals, then I do not see much "logic" in
getting a narrower CW filter...and certainly not a narrower SSB filter for
SSB use.

At least on CW, you can fit another signal inside the passband (very rare)
when going from 500 to 200hz, but on SSB, going from 2.7 to 2.1?  No way
that will "fit another signal" in that delta (which is half of the
difference or just 300hz.).

Now, if you have the sub-receiver and want to "match" filters, then I see
nothing wrong with buying one of the narrower (or even wider) SSB filters.
But that would be motivated by a different reason.

If you're spending money frugally or even just "being thrifty," (however
you want to define that) you can stick with the 2.7 stock filter.  If money
isn't an issue (the cost isn't that much different), you sure can't be
"hurt" by getting a narrower SSB filter, except as cited by others, when it
comes to "ease" of tuning in a station (though I have never experienced
such difficulty).  Lots of folks simply like to "load up" the open the
slots.  That is yet another motivation...not much to do with operating.

Hope this helped to clarify what I was trying to get across.

de Doug
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[Bill W4ZV]

Hi Doug,

DOUGLAS ZWIEBEL wrote

The roofing filters are used for reducing IMD.  

I agree IMD reduction is not a big issue on SSB but blocking dynamic range (BDR) is the primary reason I sometimes need a narrower filter.  If you have an S9+25 SSB signal CQing next to you (sometimes the case in crowded contests), a narrower filter will prevent it from de-sensing your receiver.  If part of the interfering signal falls inside your 2.7k filter, signals on your frequency will be gain modulated by his signal (commonly called AGC pumping).  In this case a narrower filter will prevent the problem.  Of course when this happens, we still have to contend with his transmitted "splatter" (typical SSB TXs run -35 dB IMD which would result in ~S7 splatter from an S9+25 signal), but at least you can still copy signals at S7 or above (and lower when he's not transmitting).

73,  Bill

[Guy, K2AV]

That really IS the kicker.  I guess the point is how bad do you want/need
the cost of an extra filter.  How important would it be in the 1% condition
where the signal up or down is INSIDE the roofing filter.

In my experience from home (not the monster antennas out at N4AF), in ANY
phone contest, changing the width from 1.8 to 1.9, thus changing to the 2.7
roofer, always is a large jump in crap inside the DSP.  This is because I
just went from DSP and roofer ALIGNED SKIRTS, that drop 100+ dB in a given
width (combined) to a composite skirt that only drops 50 in the same range.
 This means that the loudness of what is just up and down JUST GOT 50 DB
LOUDER at the roofer change, because the close in skirt is now ONLY DSP
instead of DSP+roofer.

The DSP is great, but hear this, at 400 Hz, the straight selectivity of DSP
plus a CAREFULLY ALIGNED 8 pole roofer is still less than my MP with INRAD
400 Hz 8 pole filters at the 8 and 455 IF slots.

The presumption with no tight filters is that the selectivity of the DSP by
itself is enough.  Personally, my experience is that the K3 DSP, by itself,
as very excellent as it truly is, is NOWHERE near enough to deal with
contests and situations where one is trying to listen to a 0.05 uV signal
next to a 500 or 5000 millivolt signal that is JUST BARELY OUTSIDE the
passband.  It takes the DSP, as excellent as it is, TOO MANY Hz to reach
the ultimate 100 dB.  I need the FASTER DIVING COMBINED skirts of a matched
roofer and DSP skirt to knock off the content just barely up and down.

While my MP has faster diving skirts at 400 Hz using the matched INRADs,
the K3 nearly matches the skirts AND has vastly superior IMD and internal
noise.  ***BUT***  without the matched DSP and roofer skirts, particularly
if the roofer is significantly wider, there will be so much stuff let in by
the missing 50 dB of skirt inside ultimate rejection that the advantages of
the superior IMD are quashed by all the stuff now let in, and in fact, in
that particular situation, the MP would be the superior rig.  Credit where
credit is due.

When I am running at NY4A on 40 with the overwhelming signals from Europe
on the monster quad, I use the 400 roofer and DSP at 450 and the "250"
roofer and DSP at 350 to get sharp aligned hard-diving skirts just above
and below.  Don't bother to tell me that some of those Italian signals are
running anywhere near 1.5 kW because I've rechecked the S meter countless
times, and something that tickles 40 over is an issue.

You COULD say, just reduce the width, and I would agree **IF** it weren't
for so very many people calling fairly well off frequency.  There are times
where I have to reduce the width from 350 to 300 or 250, where the up or
down signal is just too strong and too close and overwhelms the excellent
roofer/DSP combo at 350.  But I know I'm losing contacts.

I know this is more like a NASCAR take on tires, and some of you may be
driving the K3 like the family car.  But if you understand the NASCAR
(severe contest) issues regarding roofer selection, then you know what it's
about and can choose one way or another and know it's right for you without
being snookered.  If you occasionally take the K3 down to the race track
and want it to perform out there, you need to match DSP and roofer skirts,
and align them.

73, Guy.

On Sat, Jan 14, 2012 at 11:43 AM, DOUGLAS ZWIEBEL <[hidden email]> wrote:

> But unless you are planning on operating in the
> midst of multiple, super strong signals, then I do not see much "logic" in
> getting a narrower CW filter...and certainly not a narrower SSB filter for
> SSB use.
>
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[Arie Kleingeld PA3A-2]

`Guy,

That is a very true statement. Thanks.

73
Arie PA3A

Op 14-1-2012 21:22, Guy Olinger K2AV schreef:
> SNIP>  If you occasionally take the K3 down to the race track
> and want it to perform out there, you need to match DSP and roofer skirts,
> and align them.<SNIP
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[Guy, K2AV]

Pardon the misstatement in the paragraph below, I accidentally switched
units.  It should read:

On Sat, Jan 14, 2012 at 3:22 PM, Guy Olinger K2AV <[hidden email]>wrote:

> The presumption with no tight filters is that the selectivity of the DSP
> by itself is enough.  Personally, my experience is that the K3 DSP, by
> itself, as very excellent as it truly is, is NOWHERE near enough to deal
> with contests and situations where one is trying to listen to a 0.05 uV
> signal next to a 500 or 5000 ***[ uV ]*** signal that is JUST BARELY
> OUTSIDE the passband.  It takes the DSP, as excellent as it is, TOO MANY Hz
> to reach the ultimate 100 dB.  I need the FASTER DIVING COMBINED skirts of
> a matched roofer and DSP skirt to knock off the content just barely up and
> down.
>

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