Subject: TNC Deviation setting

Proper radio-to-TNC volume level is necessary to prevent distortion of the
audio signal which would interfere with decoding of received packet signals.
In general, the lowest possible audio output level from the radio that will
allow for copy of packet is the proper level.

On the other hand, how hard your TNC drives the radio with audio it inputs
to the radio is equally important.  Most TNC's have an internal control for
this kind of adjustment.  If you try to set it by ear then you are likely
to set it incorrectly and others will have trouble with copy of your traffic.
Of course, the best method is to use a deviation meter when making adjustment
of audio input to a radio.  But if you have an o'scope and another rig you
can use as a monitor then by observing the o'scope that is connected to the
audio output of the monitor you can get close to proper deviation setting.

First, connect the o'scope up to the monitor rig output audio (across an
external 8-ohm speaker is a good method).  Adjust the o'scope controls for
a good picture of received packet signals, then decrease the volume output
of the monitor rig for minimum useable o'scope picture.

Now temporarily set your TNC 'TXDELAY' parameter to a long time period, such
as 2 or 3 seconds.  Initiate a connection to a station called TEST.  Your
monitor rig should detect your packets and the o'scope will display them now.
(Of course, you should use low transmit power and an unused frequency.)

Look for any squaring or distortion of your transmit signal as shown on the
o'scope.  You want to see nice clean sine-wave type signals on the o'scope.
Adjust the TNC deviation control for decreased signal level, then back up
until further increase produces no increase in signal amplitude or you see
distortion just starting to occur.  Now again use the TNC deviation control
to reduce your transmit amplitude by at least 25 and no more than 33 percent.

Best results are obtained by having a friend at a distance be the one who
monitors your transmissions on an o'scope and who can compare your signals
to others as you adjust the TNC deviation control.

While this method is not as good as using a deviation meter, it is superior
to just using your ears to try to adjust the TNC deviation control.  Don't
forget to put your TXDELAY parameter back to its original setting!

A word about HT's.  If you observe the audio output of one on an o'scope
when the antenna is shorted, you will usually see much more noise than
you would with a decent rig.  Worse, many packet signals will be received
within that noise level, and your TNC has a hard time pulling them out
(after all, reception is the job of the rig, not the TNC).  This is due to
noisy audio stages in many TNC's.  The human ear can pull intelligence out
of a noisy signal much better than can most TNCs.  If you really want to use
a HT for packet you really need good filtering between the rig and the input
to the TNC.  Sufficiently good audio filtering will let you use 'open
squelch' operation, and improve reception of all packet signals, and good
audio reception becomes more important as you increase your transmit power.
Simply adding a preamp won't necessarily overcome reception noise troubles.
If you cann't detect stations that hear you, you may tromp all over them!