[Elecraft] PSK31 -- ghost image:

[Rich Lentz]

Don:

My responses on the reflector were to someone else's question as to why
there are ghosts.  Several have stated that the ghosts are caused by the
original transmission.  I find this hard to believe.  How can the
transmission cause a ghost at 4.9 kHz?  A real signal here would be more
than 90 db down after going through the K2 band pass.  Also, If I =
re-center
the received frequency on this ghost - there is nothing there.  =
Therefore it
is caused by the soundcard.

As far as level, I have the slider for the aux input set at about 25%.  =
With
this setting and default MixW or Digipan colors and a divider pot in my
interface set at about 10% going to the PC, it takes the volume control =
on
the K2 set at 12:00 to get a red trace on the waterfall.  I really don't
think I am overdriving the PC sound card. And I can get ghosts with the =
K2
volume set to less than 8:00. =20

My opinion is that the sampling rate of the sound card generates false
artifacts of the original tones.  This explains why the ghosts are the =
same
distance apart as two of the stronger signals in the pass band. These =
are
rarely observed with a PURE sine wave.  However, when you have multiple =
PSK
signals these will introduce false signals caused by the nonlinearity of =
the
DAC.  You are only using 12 - 24 bits to represent the signals and these =
are
taken at an 11025 Hz sample rate.  Get out your college math book on =
Fourier
analysis and do the math.  (I am a math major)  You will find that the =
DAC
act as a nonlinear device and generates these false signals.  That's why
high quality CD's use 96 kHz (and higher) sample rates and 64, 128, or
higher bit DAC's.

Rich
KE0X