[Elecraft] K2 - Tip using CAL FIL to get good sounding audio on SSB

[Bill Coleman]

I wanted to pass this along. My K2 is "finished" (until the KPA100 comes 
out), and I've been tweaking it.

One thing I didn't like was different pitch of the background noise when 
switching from USB to LSB. My TS-430S does this, and I compensate with a 
bit of IF shift when in LSB. Also, signals seem to shift around a bit as 
I picked different filtersmodes. Here's some things I discovered to get 
everything to sound good.

* Get the SSB option. The variable-bandwidth filter is awfully 
asymmetrical at wide bandwidths. The OP1 filter is much nicer.

* Do the 2nd XFIL mod. Cleans up the symmetry of the OP1 filter.

* Get the KAF2 option. Even if you never turn AF1 or AF2 on, the default 
low-pass filter takes out a lot of hiss.

* Use Spectrogram or similar program. The default setup suggested in the 
manual sounded really wacky. I used MacCRO (Think Different!). These 
programs can really help you get things close, although there is a bit of 
leeway where several BFO positions will produce very similar spectrum 
plots.

* In CAL FIL, listen to the audio in LSB from a noise generator like you 
used with Spectrogram. Switch to USB (use CW REV for this). Go back and 
forth and pick the BFO frequencies that give you the least pitch change. 
Note that the "color" of the noise will differ slightly, due to filter 
asymmetries. But you should be able to get them close.

* Get a radio with an AM detector. Tune to a strong 40m SWBC station. 
Tune the K2 to the same station in LSB. Tune so that the audio pitch from 
the K2 matches the AM detector. Switch to USB (use CW REV). If the audio 
pitch differs, go back to CAL FIL and adjust until you get the same audio 
pitch from both modes to match the AM detector on one frequency.

* Check the variable-bandwidth filters on SSB. Make sure you don't have 
any audio pitch changes when selecting these filters. Adjust the BFO 
frequency accordingly.

I used these techniques to tweak up my filters, and I'm real happy with 
the results. The BFO changes were really minor, and didn't make a whole 
lot of difference in the spectrum plots, but it certainly sounds great.





Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901