[Elecraft] Best Digital Program for the K3?

[Kok Chen]

I had recently made some measurements of RTTY and PSK31 which shows  
that sound card driven programs using DATA-A can put out a cleaner  
transmitted signal than the K3's internal generators in the PSK-D and  
FSK-D modes.

Unlike many other rigs, the K3 is very forgiving about the precise Mic  
Gain/audio levels.

I'd found that with Mic Gain set to 6, audio levels anywhere between  
250 mV RMS and 350 mV RMS work equally well, with the RF output  
tracking the PWR knob setting and not needing to readjust the audio  
levels.  With many rigs, you need to adjust the RF output through the  
amount of audio drive, just so you can keep the ALC turned off.

With the "ease of use," together with lower IMD, there are few reasons  
to use PSK-D (or FSK-D, unless you are out in the boonies with just a  
Morse paddle) on the K3.

I've just spent a little time this morning and compiled the results  
into a web page here:

http://homepage.mac.com/chen/Technical/K3/Digital/digital.html

The write up includes cases where I found a power creep (with an  
attendant ALC creep and worsening IMD numbers from PSK31).  It appears  
that the power creep could be the result of insufficient audio drive.   
With the "good" fixed audio level mentioned above, I see no power  
creep at various RF power settings.

The measurements also show that the K3's transmit IMD is low enough  
that AFSK RTTY waveshaping by software can help reduce the keying  
sidebands when compared to a rig (K3 or otherwise) that uses FSK.

The bottom line (if you want to skip the nitty gritty details in the  
above web page) is that the K3's PA at 50 watts average PSK31 power is  
capable of better than -35 dBc IMD for close spaced carrier pairs.

By carefully tuning the audio drive, I can get -37 dBc, so I assume  
the PA itself is good to at least that.  The -35 dBc number is  
probably limited by something else in the chain.

73
Chen, W7AY