[Elecraft] K3 Filter Configurations and Explanations

[Ed Muns]

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?