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[Linrad] Re: Reversed tuning?

Hi Andrew,

> I'm trying out linrad with a softrock 80 board, and on comparing the
> signals between it and a Kenwood receiver, I find that the direction of
> tuning is reversed, so that signals above the crystal frequency appear
> below on the linrad display, and vice-versa.  There is undoubtedly  a
> simple reason for this, I wonder is anyone can suggest it?
Yes, you have two audio signals I and Q. If you exchange them the frequency
scale would invert.

Whether I is channel 1 and Q is channel 2 or vice versa depends on
the hardware. In Linrad the user is supposed to edit users.c and rename it
to users_hwaredriver.c to adapt Linrad to the hardware in use.
At line 172 you would find:
// This routine is responsible for setting
// int fg.passband_direction = -1 or +1 depending on whether the
// hardware local oscillator is above or below the desired signal.
// This routine is responsible for setting
// int fft1_direction = fg.passband_direction

With a softrock you always would have the same center frequency
so you can set -1 or +1 to fit your hardware. You might set a sign
that depends on the center frequency because you might use converters
that reverse the received frequency band by having an LO above the
signal frequency.

Once you have changed the source code it is trivial to recompile Linrad
from source code as long as you use Windows XP or earlier. It might be
possible to use Vista in XP compatible mode but I do not know for sure.


Leif / SM5BSZ

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