15 Feb 90

The frequency of operation for data modes has long been a  source
of  confusion if not a point of contention for both  Amateur  and
Professional radio operators.

All Mark/Space data mode modulation can be generated using FSK or
AFSK.   What  is  the difference?  FSK is  true  Frequency  Shift
Keying  of the transmitter's carrier.  This shift can be  applied
to  any  of the transmitter oscillators.  Audio  Frequency  Shift
Keying  is  generated  by  shifting the  frequency  of  an  audio
oscillator  that  is fed into the transmitter's  normal  transmit
audio input. Unlike FSK, AFSK can be used for FM modulation.

The  frequency of an FSK system is easily determined  by  reading
the transmitter's dial or crystal frequency.  Frequency is not so
easily  determined on a SSB transmitter modulated by AFSK  tones.
In this case audio tone frequency and sideband must be known.

For  discussion  purposes, let's assume that the AFSK  tones  are
1600 and 1800.  The following figure illustrates how to determine
the resultant operating frequency.


Figure 1:
                      |           |          Mode: LSB
                      |           |
                      |           |
                      |           |
                      |           |
                      |           |
                      |           |                  |
         -------------+-----+-----+------------------+-------
                      ^     ^     ^                  ^
        1800 Hz ______|     |     |______ 1600 Hz    |
                            |                        |
Virtual Carrier or Center __|     Actual Suppressed _|
   of Intelligence                    Carrier

Note  that in the diagram, three news terms are  mentioned.   The
Virtual  Carrier refers to an equivalent frequency to that  which
would  be used if the system was FSK.  This can also be  referred
to as the Center of Intelligence, or simply half-way between  the
highest and lowest modulating tone.  The Suppressed Carrier is  a
function of using a SSB transmitter and is its' dial frequency.
The difference between the actual and virtual carrier frequencies
is the potential source of confusion.  If Station A is using  FSK
and  tells  Station  B who is using AFSK he  is  on  28.113  MHz,
unless  they both "know the rules", is 28113 or (28113  +  1.700)
KHz the correct frequency?
.pa
To  make matters worse, what if there are two AFSK  stations  one
with  1600/1800 tones and the other with 2110/2310 tones. How  do
they  tell  each  other the frequency?  If they  use  their  dial
frequency,  their  virtual carrier frequency will not  match  and
they can not communicate.

The  FCC,  MARS, Military and  knowledgable  commercial  stations
solved  this by always refering to the Center of Intelligence  or
Virtual  Carrier.   That put the burden of tone  offset  on  each
station but made it unnecessary for each station to know or guess
about the other station's equipment.

I  recently became active on HF Packet radio and was  shocked  to
find  references  to  frequency being made  by  using  AFSK  dial
frequency.  One BBS even went as far to set his UNPROTO string to
NET113,   the  dial  frequency,  when  actually  the  Center   of
intelligence  was (28.113 - .001700) or 28.111300 MHz.   Since  I
own a PK-232 which uses a tone pair of 2110/2310, I have to  tune
my radio to 28.113510 MHz, still no relevance to 28.113!

I  suppose it too late or futile to try to get the  Amateur  Data
mode  community  to start talking Center of Intelligence  but  at
least  they  should be aware of it.  If they add notes  in  their
BBS's  online tech info etc., they should mention the  center  of
intelligence.

Another  useful bit of related information is the simple rule  of
thumb that the Bandwidth required is:

                     BW Hz = Shift Hz + Baud

For most HF packet this works out to be 500 Hz.

Knowing  the  BW  required and actual  frequency  is  helpful  is
picking an operating frequency.


In researching what Packet TNC's use what tone pairs, I  compiled
the following data you might find helpful:


         TNC Type                          Tone Pair
         ========                          =========

Kantronic's KAM (default settings)         1600/1800
AEA PK-232 (USA normal ham)                2110/2310
AEA PK-232 (Sitor)                         1615/1785
AEA PK-232 (European)                      1250/1450



73's

Jim Lill
WA2ZKD
WB2PSI-1 28.1113 (28.113)
GEnie Address: APPLE.CPM
RFCARC BBS 716-544-1863