Further I noticed that launching another application while running Spectran
caused a kind of 'frequency shift' (see screenshot at my webpage).

Alberto found this same problem, but he says that debugging will be difficult,
and will probably take some time. Incidentally, he is releasing a new "beta"
version (beta3).

Just to straigthen things a bit up :  we are talking of two different problems :

1) The frequency error Marco is talking about, is known to me since some time,
   and it is caused by a wrong initial design assumption of Spectran,  i.e. 
that the
   frequency ruler scale must start at an integral multiple of 1 Hz. This 
because at
   that time Spectran was meant for EME use, where sub-Hertz resolutions are not
   needed. Now that with beta 3 we can do down to 21 milliHz, this must revised.

2) The frequency shift observed and reported by Rik and Alan, is a completely 
new
   thing, for which, at the moment, I have no explanation. I saw, one time 
only, a shift
   of 0.5 Hz, which I attributed to my RX. I wasn't launching or using any other
   application at the moment. As this shift has been signalled by more than one 
person,
   and on signals locked to some standards, then I must investigated into it.

Thanks for those reports, and don't be shy to send others. The very first 
reason for
putting out a beta before the final release was just to receive this kind of 
reports.

73s
Alberto    I2PHD
-----------------