Topband: PSK-31

[Joe Subich, K4IK]

> Your point is well taken. I just looked at the output of the computer
> running full gain from the PSK software. The third harmonic (the
> strongest spur) is 24 dB below the fundamental. Not hard clipping, but
> not great either. What we used to call "communications quality."  I
> dropped the output gain 3 dB and the third drops to -45. I dropped it
> another 3 dB (that is, 6 dB below full gain) and the third 
> drops to -60 re: the fundamental. 

Unfortunately, when the user of one of these cards sets up with a 
100 Hz tone, turns the output up all the way, inserts an attenuator 
between the sound card and the radio and then increases the mic gain 
until the radio shows ALC action, one can almost guarantee a "worst 
case" scenario with harmonics/spurious products no better than 
-20 dB.  

Since the average power with PSK idle is about 50% of the "key down" 
or single tone power, the proper way to set up PSK (without a scope 
and spectrum analyzer) is to back the sound card output slider down 
to 50% (about 10 dB), set the mic gain to normal SSB levels, turn 
off any RF processing, and adjust the attenuator between the sound 
card and rig so that the RF output is 25% (- 6dB) of the key down 
power (25 watts for a 100 watt rig).  

That process generally assures that each stage of the system is 
operating near its "optimum" level and should keep the finals 
from being driven into clipping and/or to that point short of 
clipping at which IM products start to increase faster than the 
fundamental. 

Similarly, as Tom has pointed out, PSK users should be making 
use of the narrowest IF filters available (250 Hz) rather than 
a 2.4 KHz bandwidth for the AGC loop and then complaining about 
a strong signal 1.5 KHz away.  A "waterfall" tuning display is 
the worst thing that could have happened to PSK ... it encourages 
bad (less than optimum) operating practices. 

73, 

   ... Joe, K4IK