[Elecraft] AGC Setting for PSK-31

[Kok Chen]

On Jan 22, 2009, at 10:16 AM, Simon (HB9DRV) wrote:

> A good soundcard will not suffer from intermod - just like a good  
> radio.

Not just with intermod, but many mediocre sound cards also suffer from  
bad second harmonic distortion that could be more troublesome than IMD.

I have measured poor ones that have second harmonic distortion as bad  
as -55 dB, and I have seen traces of the second harmonic of a strong  
signal from them on a waterfall. I.e., similar to the problem with the  
K3 Line Output before applying the K8ZOA modifications.  Fortunately,  
unlike IMD, you can nudge the VFO knob a little and get the second  
harmonic to fall at a different spot in the audio spectrum from a weak  
signal that you are trying to copy.

Simon is right.  The reason for using a better quality sound card with  
narrow band rigs is not necessarily to get a lower noise floor.  It is  
to get lower IMD and harmonic distortion.  Even the worst sound card  
that I have encountered in the past half dozen years had a dynamic  
range which beats the analog gold standard that we use on HF RTTY, the  
HAL ST-8000 modem.

If you are using filters that only lets through a single signal to the  
sound card, none of this matters -- you should be able to live with 50  
dB of dynamic range if only a single signal reaches the sound card,  
and the rig's AGC can keep the amplitude of that signal moderately  
constant.   But many of us digital mode operators use at least a 2 kHz  
bandwidth; that is where good and poor sound cards can make a  
difference.

However IMHO, it is always better to use even a poor external output  
sound card; to prevent system chimes, music, etc, from getting out on  
the air.

73
Chen, W7AY