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http://www.weaksignals.com

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an experimental Software Defined Radio, with AM, ECSS, SSB and FM demodulators, with a highly graphical interface

 
Jason a keyboard-to-keyboard communication program, tailored for very low S/N ratios, based on the IFK technique, using only 4 Hz of band
For an explanation of the IFK technique, you can visit this link, on the Web site of Steve Olney, VK2ZTO
ARGO a QRSS/DFCW viewer, especially tailored for LF, but usable also for EME
For a primer on QRSS, look the link above, on the very informative pages of Rik, ON7YD or 'QRSS and You' from Clint, KA7OEI
 
SPECTRAN a spectral analysis program, with audio filtering, originally born for EME usage, 
but useful for many other purposes. An example of use for NDBs reception
can be found on this page by KA7OEI
 Spectran was used during the last successful attempt to establish the world DX record on the amateur 241 GHz band. Look here for details
 
  Two Tone Tester (T3) , a combination of spectrum analyzer and audio generator (with optional quadrature outputs), useful for many purposes

 
HAMVIEW a DOS program, which *requires* an old ISA original SoundBlaster card,
similar to SPECTRAN in functionality, though not quite as powerful 
 
DSPBOX a DOS program that interfaces with a DSKC50 DSP board, implementing
an audio filtering unit, with denoising, variable bandpass, auto notch , AGC