[Elecraft] Let's Try This Again -- was "[K3] ... A 750 Hz, 8-Pole ... Filter?"

[Guy Olinger K2AV]

I have to second EXTREME QRM.

One of the poison backlashes of really good antennas that have all
that wunnerful transmit gain, is that it works REALLY WELL on receive.
I have heard 9+30, 9+35, 9+40 and 9+45 signals on a calibrated K3.  I
have NO idea how they are doing that, or what magical propagation or
combination of prop and power is producing these levels.  But if you
get one of these parked up 350 or 400, you can either give up your run
frequency (which I think is why they do it) or you can GET SKIRTS.
Dropping down to a real 250 roofer will not do it because you will not
hear all the stations that call up and down more than that narrow
passband. I'd say that overall a third of all callers will not call
inside +/- 125 Hz.

The 330 Hz 8 pole is just the thing.  I have two in my dual RX K3 and
one in my MP. They all measure the same. In the MP the 250 8 Mhz job
and the 250 455 khz job ADD UP to a very clean sharp 250, which I
think is where this filter first got its name.

One of the tricks on the MP was to drop back from the 500 setting
(which was really populated with an INRAD 400 hz pair) using the 400
455 filter and the 250 8 Mhz filter.  This is about the same as using
the 250(330) as a roofer and the DSP.  No comparison on crushability
of course.  Back on the MP a 40 over up 350 meant you either gave it
up or staggered

Now those guys have K3's and they AREN'T moving.  So either solve it
with the 330 8 pole and 350 DSP width and NB on DSP 1-7 or 2-7, or
leave.  In ARRL DX went 4 hours and 290 contacts with one such up 380
Hz.  I know he had to be having his ears smoked by my signal, so I
also know he had a K3.

This is also the answer for hams who live 6 blocks from one another.
Get a K3 and live in peace.

73, Guy.

On Fri, Jul 16, 2010 at 4:52 PM, Ian White GM3SEK <gm3sek at ifwtech.co.uk> wrote:
> Wayne Burdick wrote:
>>It *is* difficult to build very narrow 8-pole filters with low loss.
>>That's one reason Elecraft offers a 5-pole, 200-Hz unit.
>>
>>5 poles is entirely adequate at these narrow widths. Our 2-kHz IMD
>>dynamic range using the 200-Hz filter is outstanding.
>>
> Of course it is; but this discussion is about QRM at much closer
> spacings, sometimes occurring within the passband of a 350-400Hz filter.
>
> It is also about obtaining the maximum possible performance from the K3
> under conditions of extreme QRM, which is sometimes strong enough to
> make the hardware AGC flicker on and off... at which point, the K3 fares
> less well. Under these extreme conditions, the DSP needs every possible
> protection from a tight roofing filter.
>
> I do have to keep emphasizing that word "extreme". Many people will
> never experience such levels of QRM because they have the good sense to
> run away... but contesters need to stay and tough it out.
>
>
>>We've had some requests to offer a 270 or 300-Hz 5-pole filter, which
>>would be better optimized for narrow-shift RTTY than the 200-Hz filter.
>
> It was I who initiated that request, for precisely the reasons outlined
> above (and further back in the archives [1]). Wayne kindly provided
> suggestions for a modification to increase the bandwidth of the 200Hz
> filter to about 270Hz, in order to accept more of the FSK sidebands and
> allow a few Hz more tolerance for calling stations. That filter has
> proved to be a true winner [2].
>
> However, contesters are always looking for an extra edge, and a true
> 250-270Hz 8-pole filter with a steeper transition from the passband into
> the stopband would be the next step to try.
>
> As Joe, W4TV so rightly says, it would fill a genuine unmet need.
>
>
> 73 from Ian GM3SEK
> http://www.ifwtech.co.uk/g3sek
>
>
>
> [1] http://lists.contesting.com/pipermail/rtty/2009-August/029397.html
>
> [2] The modification is to change both C3 and C5 in the KFL3-200A  to
> 830pF (680 + 150pf SMD, in parallel). The filter schematic is in
> Elecraft's K3 schematics file.
>
> Details of the wider passband are at:
> <http://www.ifwtech.co.uk/g3sek/misc/kfl3-200_mod260.gif>
>
> Although Wayne provided helpful advice, please note that this
> modification is not officially supported or recommended by Elecraft.
>
>
>
>
> --
>
>
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