Topband: PSK-31

[Jim Brown]

On Wed, 28 Apr 2004 22:54:33 -0400, Joe Subich, K4IK wrote:

>ince the average power with PSK idle is about 50% of the "key down" 
>or single tone power, 

That's -3 dB.

>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), 

Huh? how does 50% power turn into -10 dB? According to my math it's -3
dB.  BTW, I measured the relative gain of the slider on my sound card.
The first notch down from the top is 3 dB below the top. The second
notch is 3 dB below that. As you get closer to the bottom, the changes
are considerbly greater (but it does not appear to be voltage linear). 

>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. 

Just a minute here. Yes, we want to stay below the ALC. But since when
is a decent ham rig generating lots of distortion 3 dB below full
output?  If it is, we ought to be screaming at every SSB operator (and
every CW operator) who operates his/her radio at rated power!  
>
>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 

Maybe, but maybe not. Remember that the phase response of a system with
sharp filter cutoff can get pretty nasty, and it is the phase
modulation that we are detecting! So let's not go too far with this.
2.5 kHz may be excessive receive bandwidth, but 250 Hz may be too
narrow. 

Jim