[Elecraft] K3 250Hz Filter gain maxed yet low

[Bill W4ZV]

AB3CV wrote: 

>How do I center the filters (or not) I've not done any special setup
or played with EQ (in fact just made sure it was zeroed out) on this
unit. I went to the nabble site and searched on Lyle Johnson and could
not find the info you mentioned.

W7TEA replied:

>The thread was about the ARRL review and on Nov 27.  

        Here's the post W7TEA was referring to:

http://mailman.qth.net/pipermail/elecraft/2008-November/100401.html

Relevant excerpt from above post by KK7P:

>The second question has to do with the difference between the above and 
below values for a given filter when the signal spacing is less than the 
nominal width of the filter.

>This asymmetry is due to the way in which the K3 aligns the roofing and 
DSP filter passbands.  The signal is not necessarily centered in the 
roofing filter passband; rather, the signal is shifted towards an edge 
to maximize the use of the roofing filter's skirt selectivity.

>Why is this important?

>Consider the case in which you are using a wider filter in a crowded 
band while operating CW.  A huge signal appears very close by, perhaps 1 
or 2 kHz away.  The use of CW REV or CW may make a considerable 
difference on the impact of that signal on the receiver.  If the 
passbands were centered, this tool would be less effective.

>And if huge signal are on both sides?  Time to get a narrower roofing 
filter!

        To explain this another way, the K3 normally has the roofing filter
centered about the chosen PITCH in the CW passband.  For example, if you use
a 600 Hz pitch, the "250 Hz" filter (actually 370 Hz) would have 6 dB points
at 600 +/- 185 Hz or a passband of 415 to 785 Hz.   *HOWEVER* when using low
PITCH (as I prefer), the K3 may shift the filter passband so that its lower
end is no lower than 200 Hz.  For example, if using a 300 Hz pitch, the
above filter's passband would cover 200-570 Hz and therefore the chosen 300
Hz pitch is not centered in the passband.  The reason for doing this is to
reject the negative IF response which might allow signals from the negative
side of 0 Hz (if you've used older analog radios, you probably remember this
problem).

        There's another possibility that could be causing some apparent
loss.  Even though the 8-pole filters are advertised to have zero offsets,
in fact I believe some have discovered they may have up to 80 Hz offsets. 
This has negligible effect at the wide filter bandwidths but could cause
some problems with the 250 Hz filter.

        I would try the following test to troubleshoot the above problems. 
Se PITCH = 600 and apply a steady signal (e.g. XG1 or XG2).  Now tune slowly
and note the frequencies at which the signal drops by 6 dB.  You've
eliminated the passband shift issue by setting 600 Hz pitch and you should
be able to determine if the filter's actual offset is something other than
zero.  You can use the internal AFV/dBV meter to do this without any test
equipment other than a source (in fact you can even do this with the K3's
internal 49380 image as the source).  Turn AGC OFF, apply your test signal
at what you believe is the center frequency (i.e. 7040 the XG1).  Hold METER
and tune VFO B until you see AFV...you should see AFV #### in the display. 
Allow the #### numbers to settle, then rotate VFO B slightly clockwise and
you should see 0.0 dBV.  Now slowly tune above and below 7040 and note the
two frequencies at which you get -6.0 dBV readings.  This should tell you
whether the filter is correctly centered.  Of course you could simply have a
defective filter with high insertion loss, but doing the above will help
both you and K3support at elecraft.com determine what your actual problem is.

73,  Bill  W4ZV





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