[RTTY] A Different View--K3 Reduced-bandwidth RTTY Analysis

[aflowers at frontiernet.net]

>K3 FSK has always had a small amplitude difference between mark and space.
>The transition has always been abrupt, Photo 1. With the new beta software,
>the transition is much more gentle, Photo 2. Bravo!
 
Hank,
 
As you are aware the only reason you can see that transition in the RF envelope is because your tones are unequal by about 1 dB, assuming the vertical scale is linear on your scope pictures. That's a real issue since the amplitude difference between the tones because that is going to look like a fixed selective fade to whomever you are communicating with.  That's going to make a noticable difference in character error rate for weak signals, especially if the other person's demodulator is not using an ATC.    I've seen some of this same kind of thing when someone is using AFSK tones near the edge of their TX passband, and the roll-off of the filter causes this issue.  You probably know all this already.  It sounds a little strange, but I wonder if it could be caused by a nasty passband ripple in the TX IF filter.  One dB ripple over 170 Hz seems like a bit much for a filter that is nearly 3KHz wide.  
 
I don't see that amplitude difference on my radio with either of the firmware versions, which makes me suspect it might be something in your hardware.  (I can measure a difference, but it's consistent with <0.1dB ripple at the center of the IF filter).  I have the 8-pole 2.8 KHz filter in this radio, for reference.
 
Anyway, that's a datapoint and some speculation for you if it is helpful.  
 
Andy K0SM/2