[Elecraft] K3: Use of LP-Pan & Roofing filter interaction

[Edward R Cole]

At 07:28 AM 10/15/2012, Edward R Cole wrote:
>Looking into refining my use of the LP-Pan on the 1st IF of the 
>K3.  One artifact is that the spectrum of the IF has a gain dip at 
>the center frequency which apparently is due to interaction of the 
>2.8 KHz roofing filter that is also loading the IF.
>
>I am wondering if I might be able to program the K3 to use an open 
>xtal filter position when I am using the LP-Pan?  I do not use any 
>of the K3 beyond the IF so loss of use is not an issue; all signal 
>processing is via the LP-Pan and a Delta44 soundcard to the sw 
>running in my computer.  I might have to add a resistor to provide 
>proper impedance loading for proper gain levels to appear at the IF output.
>
>The dip in gain of the spectrum makes that frequency unusable in my 
>application so looking for solutions (that are not invasive to the 
>radio).  One option is inserting a pin diode switch to isolate the 
>roofing filter bank but that requires some RF designing and precise 
>surgery on the RF board.
>
>73, Ed - KL7UW

Found the solution.  Page 46 of the K3 manual:  Enabling Receive 
Filters.  I added FL3 for the DATA-A mode.  So now  I can switch 
between FL2 - 2.8 KHz or FL3 - no filter, as desired.

Testing with FL3 selected, I no longer see a gain dip in the center 
of the spectrum display of the LP-Pan output.  My guess it does not 
affect Tx filter which should be FL2.  I kept FL3 open for adding 
future filters: FL1 - 13 KHz, FL2 - 2.8 KHz, and FL4 - 400Hz.  FL5 is 
also vacant.  In the sub-RX only FL2 - 2.8 KHz is filled.

73, Ed - KL7UW