[Elecraft] K3: Peculiar BW behavior?

[Bill W4ZV]

Walter V. Gilles wrote:
> 
> 
> I used the utility to check things out before making any changes, and all
> was as it should be.  However I reloaded the current release firmware, and
> increased the insertion loss compensation on the 0.40 roofing filter from
> 1db to 2db.  I don’t use any other compensation.  Upon checking on air,
> the freq shift and fadeout experienced previously on 20M at 0.20 BW is
> gone, and a quick check on a few other bands indicates that problem is
> fixed. 
> 

A 2 dB gain setting for a 400 Hz filter sounds far too low.  I have two 500
Hz 8-pole filters in my base rig and one 500 Hz 5-pole in my portable rig. 
The 8-poles are 6 dB and the 5-pole is 5 dB.   I would expect more like 6-7
dB for the 400 Hz 8-pole based on the Inrad/Elecraft filter plot below :

http://www.inrad.net/product.php?productid=150&cat=140&page=1

You can check your gain setting by applying a steady carrier, zero beat with
your desired PITCH and gradually turn WIDTH from the widest setting to the
set point where the 400 Hz engages.  There should be no change as you
transition across the 400 Hz set point.  If you want to do this more
accurately you can use the internal AFV/dBV meter (remember AGC must be OFF)
to measure changes of 0.1 dB.  You should adjust CONFIG: FLx Gain so that
the transition is smoothest across the set point WIDTH.

If you have other filters besides the 400 Hz, you should start with your
widest filter, set that one to 0 dB gain and then adjust each filter, from
widest to narrowest in order so that the transition is smooth across the set
points.

It's been awhile since I looked at the manual and I see page 46 says:

"Use VFO A to set the gain in dB. In general,
you’ll want to add 1-2 dB for 400-500 Hz
filters, and 3-4 dB for 200-250 Hz filters."

In my experience (with several different 500 Hz and 200 Hz filters) the
recommended settings are about 4 dB too low.  They also do not agree with
published insertion loss figures by Inrad.  I would expect 5-6 dB is correct
for 400-500 Hz filters and 7-8 dB for 200-250 Hz filters.

Someone should write up a better description for how to accurately set the
filter gain settings using the internal AFV/dBV meter...hint hint to Ron
AC7AC.  ;-)

73,  Bill  W4ZV

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