[Elecraft] K3 #1456 - data modes setup

[Joe Subich, W4TV]

> But if you're running 
> PSK31 with a typical wideband filter, the low frequency components 
> (static, noise, other signals) could produce enough distortion to 
> degrade operation of the PSK decoder (in the computer sound card). 

Operating narrow band modes with wideband receivers is not good 
engineering practice.  If AGC is disabled the receiver and audio 
(soundcard gains) need to be backed off sufficiently to prevent 
overload with strong undesired signals.  This give up dynamic range 
on weak signals.  If AGC in enabled, strong undesired signals will 
activate the AGC and reduce gain - often to the point of blocking 
the weak signal.  

Narrow receiver filtering makes a big difference and protects the 
audio circuits from the unnecessary LF energy. 

As long as the user does not "push" the IF response below the K3 
standard 200 Hz low cut, the LF energy is not a significant issue. 
A 1V peak tone at 250 Hz has harmonic distortion at -78 dB (2nd), 
-49 dB (3rd), -81 dB (4th) and -65 dB (5th).  The 2nd and 4th 
harmonics may be lower as the measured values are the noise floor. 
Although those numbers are slightly worse than at 500 Hz or 1 KHz, 
-50 dB distortion levels are more than adequate for narrow band 
reception.

By the way, in order to get 1V Line Out levels with an XG2 (preamp 
on), I found it necessary to disable AGC or set the slope below 10. 
With AGC and slope settings used by most users, maximum Line Out 
will not generate a 1V Peak signal for S9 inputs.  With AGC, setting 
Line Out as high as 25 results in maximum audio levels at -15 to -20 
dBu which allows sufficient headroom before the onset of distortion. 

73, 

   ... Joe, W4TV 
  


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
> Sent: Thursday, August 28, 2008 11:46 AM
> To: Elecraft at mailman.qth.net
> Subject: RE: [Elecraft] K3 #1456 - data modes setup
> 
> 
> On Thu, 28 Aug 2008 10:51:57 -0400, Joe Subich, W4TV wrote:
> 
> > As long and Line Out
> >remains below roughly 1V peak (about .8V RMS) distortion levels 
> >will be minimal.  
> 
> Not quite. Because most of the distortion is caused by the 
> transformer, distortion rises rapidly with decreasing frequency 
> (that's the nature of transformers at the limits of their ratings). 
> If you're running RTTY at narrow bandwidths, using 2.2 kHz tones, 
> and paying attention to the AGC considerations noted below, you're 
> not likely to have problems at this level. But if you're running 
> PSK31 with a typical wideband filter, the low frequency components 
> (static, noise, other signals) could produce enough distortion to 
> degrade operation of the PSK decoder (in the computer sound card). 
> 
> >The specific Line Out setting will depend on your particular
> >AGC settings.  If AGC is off, an S9 signal (XG1/XG2, etc.) 
> >will generate 1V peak with Line Out as low as 3 or 4. 
> 
> Yes.
> 
> Jim Brown K9YC
> 
> 
> 
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