[Elecraft] K3 Filter Configurations and Explanations
Joe Subich, W4TV
lists at microham-usa.com
Tue Apr 1 22:24:23 EST 2008
> - For RTTY, my default bandwidth is 200Hz plus the Dual-Tone
> Filter of additional cascaded 50Hz filters around each tone.
> In theory, the bandwidth for 170Hz-shift RTTY should be
> 250-300Hz, but I've found operationally that 200Hz works
> great in my RTTY contesting.
The two 50 Hz filters spaced 170 Hz would have a total bandwidth
of 230Hz. This is very close to the "published" value of 224 Hz
for the 5-pole filter (I intend to measure mine when it arrives
later this week). However, I wonder if it would not be useful
to modify the 200 Hz filter - or for Elecraft to offer a "real"
250 Hz filter using the values shown in the K3 schematic pack
(page 7).
Based on the published 224 Hz (and reports of 210 Hz) and 4:1
shape factor a "250 Hz" 5-pole filter would still offer much
better performance between -6dB and - 30 dB than the "250 Hz
8 pole" filter but a "real" 250 Hz wide filter should be just
enough wider to eliminate potential "sharpening" of the dual-
tone (DSP) filter.
My filter choices - and I'll see if I guessed right in a week
or so - were FM, 2.8 KHz, 500 Hz and 200 Hz. I'm leaving the
one slot open for 2.1, 1.8 or possibly a variable SSB filter
if/when that is available. The FM filter was chosen for general
non-critical listening (including AM) as it is still narrower
than the front end filter in any of the mid-grade "up-conversion"
receivers. I know I will want something "between" 2.8 and 500 Hz
and suspect the 1.8 will fill the bill for general CW on a quiet
band and for "click on the waterfall" PSK31 operation.
73,
... Joe, W4TV
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ed Muns
> Sent: Tuesday, April 01, 2008 2:33 PM
> To: 'Jim Miller'
> Cc: 'Elecraft Reflector'
> Subject: RE: [Elecraft] K3 Filter Configurations and Explanations
>
>
> Repeating what I've posted here previously, the first thing
> to get firmly in mind about the K3 crystal filters is what
> their purpose is. They are not used to determine the
> bandwidth you are using on any mode. (Yes, you need the 2.7
> or 2.8 for transmit on CW, RTTY and SSB. The AM and FM
> filters for transmit on those modes.) Your receive bandwidth
> is determined by the DSP filtering which is continually
> adjustable to any bandwidth you desire. The Icom 756Pro
> series radios are excellent performers and have a single
> 15kHz crystal filter. Narrower filtering is provided by the
> DSP just like the K3.
>
> The purpose of the five easily-installable crystal filters in
> the K3 are to pre-filter strong nearby signals so that the
> DSP can better do its excellent job of the primary filtering.
> INRAD makes add-on "roofing filter" kits for many radios and
> now has a 4-5kHz "roofing filter" kit for the Icom 756Pro
> series for those people who are mostly interested in CW, RTTY
> and/or SSB. This is a great improvement over the stock 15kHz
> filter for contesting and situations where there are lots of
> very strong close-in signals.
>
> You don't need any additional crystal filters in the K3
> beyond the stock 2.7kHz filter to listen to CW at a 600Hz or
> 350Hz or 200Hz or whatever bandwidth. Just dial in the width
> you want with the WIDTH, SHIFT, LO CUT and/or HI CUT
> controls. You only need to consider additional crystal
> filters if you operate in conditions where strong signals
> close to your operating frequency (inside the 2.7kHz filter)
> may deteriorate the excellent filtering of the DSP with IMD
> and other interference.
>
> Because my primary Ham radio activity is contesting, having
> crystal filters right at the bandwidths I use for each mode
> is useful. It is one of many reasons why the K3 is my
> favorite contest radio. But all these filters are not needed
> by many K3 users.
>
> - For SSB, my default bandwidth is 1.5kHz and that is saved
> as ALT-1 and as my standard SSB bandwidth on all my band-mode
> memories. Specifically, the DSP filter is set at
> 300-1800kHz. I'm currently using the 1.8kHz crystal filter
> which is actually about 1.9kHz at the -6dB points.
>
> - For CW, my default bandwidth is 400Hz although I'd be just
> as happy at 500Hz. It's just that I'm currently using the
> 400Hz crystal filters. I seldom go below that, but
> occasionally on 160 with heavy QRM I might go down to 250 or 200Hz.
>
> - For RTTY, my default bandwidth is 200Hz plus the Dual-Tone
> Filter of additional cascaded 50Hz filters around each tone.
> In theory, the bandwidth for 170Hz-shift RTTY should be
> 250-300Hz, but I've found operationally that 200Hz works
> great in my RTTY contesting.
>
> Note that the actual -6dB bandwidths of the "400Hz" and
> "250Hz" filters are 435Hz and 370Hz respectively. You
> definitely don't need both of those filters, although that's
> exactly what I personally have right now because I failed to
> consider the actual bandwidths when making my initial
> selection. Don't make that mistake.
>
> Finally, you can set the point at which each crystal filter
> kicks in as the DSP bandwidth is varied. I set my trigger
> points per the -6db bandwidths of the various filters, not
> the product name. Thus, the "250Hz" filter switches in at
> 350Hz because its actual bandwidth is 370Hz. Since I receive
> RTTY at 200Hz DSP bandwidth, a better crystal filter for me
> in that mode might be the "200Hz" (225Hz actual) for maximum
> strong signal protection.
>
> 73,
> Ed - W0YK
>
> > Thanks for bringing up the filter topic -
> >
> > This is one of my main questions now. I finally ordered mine
> > with no optional filters and plan on adding them to the order
> > before ship BUT I do not know which will be the best for me.
> > I suppose the answer to that is that it will be different for
> > everybody. I do expect to add the second reciever board at
> > some point, maybe not before initial ship.
> >
> > Some sub-topics:
> >
> > 1) Is there any point in ordering the 200, 250, 400 and 500?
> > Probably not.
> > Somebody explain how the filters are selected by the radio
> > and recommend a couple of reasonable configurations please.
> > I want to be able to run PSK, RTTY, CW and SSB and mainly I
> > want to not have to deal with nearby signals, I want them GONE.
> >
> > 2) a. Why would you order a 2.8 when you get a 2.7 with the radio?
> > b. Can you order the 2.8 instead of the standard 2.7 and
> > receive "some"
> > credit for the 2.7 you didn't want?
> >
> > 3) Will the 1.8 make the SSB hard to understand vs possibly
> > the 2.1? I almost ordered the 1.8 but wasn't sure so didn't
> > order anything yet.
> >
> > 4) What would be the best filter for RTTY?
> >
> > 5) Which filter will be best for PSK? A 6K filter or a 2.8
> > or narrower looking at just part of the band at a time?
>
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