Topband: PSK-31

[Jim Brown]

On Thu, 29 Apr 2004 10:02:43 -0400, Joe Subich, K4IK wrote:

>
>
>> 
>> >Since the average power with PSK idle is about 50% of the "key down" 
>> >or single tone power, 
>> 
>> That's -3 dB.
>
>You're correct but so what?
>
>> 
>> >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). 
>
>That's the audio (subcarrier) drive, not RF power.  Your own
>measurements 
>showed that your sound card required operating at -6 dB before the 
>harmonic and spurious content became acceptable. 

Acceptable by what standard? -60 dB re: fundamental is 0.1% distortion.
-45 dB (which I measured only 3 dB below max out) is 0.3%.  In the
communications world, those are very low numbers. If I can easily
achieve them I will, but I won't call a radio or system broken if it
doesn't. 
 
>> 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!  
>
>SSB operators take advantage of the compression provided by the ALC. 
>With PSK (or other multi-tone digital modes) actual intelligence may  
>conveyed in the absolute/relative level of the specific tones.  Further 
>one does not want the compression characteristics of the system to raise
>the (relative) level of the spurious/harmonic content present in sound 
>card output.  Finally, without the ability to look at a spectrum 
>analyzer to see the actual transmitter behavior, 3 dB of additional 
>headroom (above the known peak to average) is a wise minimum to provide 
>for transient response.  Operating PSK31 at 3 dB down from transmitter 
>peak output almost guarantees that the final amplifier is pushed to the 
>limit and is operating at some level of gain compression (if not at the 
>very threshold of clipping). 

Have you done measurements to verify this?  The radio I'm currently
using for PSK31 is a TS850.  I also own an Omni V and a K2/100. The 850
is rated for 100 watts out on CW, SSB, FM, and RTTY (it was built
before PSK31 came into use), and I can drive it to 100 watts out
wiithout an indication of ALC. For my initial work with PSK31, I've run
it at 50 watts (by reducing the audio drive). ARRL tests of the Omni V
and 850 show worst case IMD at 100 watts at -30 dBc and -28 dBc
respectively. That's pretty moderate power compression, so my gut
feeling is that dropping the drive by 3 dB ought to reduce that IMD by
at least 10 dB and probably more. On the other hand, I haven't done
this sort of measurement, and I could be wrong. 

>> >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. 
>
>It depends on the phase characteristics of the filter.  Most 
>250 Hz filters have relative benign filter skirts (1.8 - 2: 1
>shape factors ... 2.5:1 for a mechanical filter).  Even a 250 
>Hz filter provides twice the "necessary" bandwidth for a PSK31
>signal ... without a brickwall transition from passband to 
>stopband any problems with phase distortion should be much less 
>than those from overload and/or AGC induced gain reduction.   

Have you done measurements that verify this? Have you used narrow
filters in a system decoding PSK31? A filter with sharp cutoff is well
known to have (wildly) non-flat phase response far beyond the point of
transition.  The K2 achieves narrow (1.8 kHz) SSB receive response by
stagger-tuning the IF filters designed for CW. The response is quite
ragged (+/- 7 dB with three peaks in the passband), and it's painful to
listen to on speech.  

Another example. A narrow sweepable (bridged-T) notch filter made by
UREI in the 60's used to be a very popular piece in the studio world..
It could put a 60 dB notch in the spectrum to eliminate a ring
(feedback on a live recording, for example). But if you used multiple
instances of it to get rid of 60 Hz buzz (i.e., the harmonics), the
phase distortion made speech unintelligible. I had this experience many
years ago when a surveillance tape was brought to me for cleanup. 

Another datapoint -- when not aligned for wide response, the IF filters
in the K2 are pretty good. (I've measured their amplitude response, but
not their complex response). The recommended setting of their filters
for PSK31 is for a 700 Hz bandwidth. Knowing what I do about the
interactive nature of the users of that radio, I suspect that the
recommendation is the result of testing by some pretty sharp users. 

73,

Jim K9YC