TNC2.15

Hello fellow packeteers!

I just discovered that the sensitivity of the XR2211-DCD circuit (built into
many of the modern TNC2 clone) depends somewhat on the NF-level, especially
on the noise in it.

Playing around with the NF knob of my transceiver while listening to the
signals on the channel, I noticed that very weak signals were not detected
by the DCD. As I increased NF level, this even got worse. So I turned the
NF down to the half of the previous level and the XR2211 DCD just worked
fine!

The TCM3105 modem chip seems to have an automatic NF level compensation,
as it workes ok with almost any NF signal (as long as it's not distorted :-)
while the XR2211 does not have a compensation.

Of course, you shouldn't give it too few NF, as even the 3105 has its
limits and there also might be stations with strong signal, but weak modu-
lation..

Probably this is irrelevant to people who work duplex digis where the NF is
almost always perfect, but it might help those who work on simplex channels,
where weak signals are likely to occur.

I also noticed that it is not trivial to adjust the frequency at which the
XR2211 DCD senses a carrier. The filter seems to be somewhat narrow and
it's difficult to adjust it for 1200Bd when there is only a trimmer poti
to do the job with. Lots easier to adjust it for 300Bd!

73's and happy packeting..   Rolf (dg4scv@db0sao.de.eu)

73s from Miami Ross...
Lionel,KC4CLD
SEE ALSO TNC2.16 FOR ADDL INFO