Configuring the FT 991A for 9600 bps FM packet
The FT-991A is capable of 9600 bps FM packet radio - with the catch that is not possible to transmit using the USB interface, this have to be done via the RTTY/DATA jack. In this document I'll share my personal findings after investigating the feasibility of this operation mode.
This is due to a project choice. The FT-991A USB audio enters a low-pass filter (LPF) right after the digital-analog converter inside the radio (blue box). After this LPF, the audio enters the 'regular' audio transmit chain (red line).
FT-991A TX audio input stage block diagram
Conversely, the audio coming from the RTTY/DATA Jack is greeted at the Q1136, where in the presence of the 9K6 signal, diverts the signal to a different signal treatment chain (see green line).
Reinforcing- you will need to transmit your audio through the RTTY/DATA port.
For this test I did a very simple cable, with a 4.7 K resistor using the computer phone jack to feed the RTTY/DATA port, using pins 1 & 2:
FT-991A rudimentary PC-To-Radio cable - for DATA/RTTY port
If you will be using your computer to process the receiving audio, there's no need to fully wire the RTTY/DATA port - use a cable just to send the computer audio to the radio, and use the regular USB audio for the reception.
If you have an external TNC and will not be using the computer to process the incoming audio, wire it as the below diagram (from manual page 124):
FT-991A Packet Radio - DATA/RTTY External Audio wiring
- Connect your computer TX audio (or TNC) to
DATA/RTTYport, as per the above diagram - Set the MODE to
DATA-FM - Set the menu clause 076 FM PKT PTT SELECT to
DAKY. That will be good for either direct PTT in Pin 3 or CAT PTT command. - For 9600 bps mode, the menu 077 is irrelevant
- Set the menu clause 079 FM PKT MODE to
9600
It is of great help to have a nearby receiver to test the transmitted signal for proper gain. Define a suitable transmit gain and enjoy!
The transmitted 9600 audio is way more detailed, with wider bandwidth. There's a small video demonstrating the audio difference from the standard 1200 bps audio (which happens to be the same bandwidth as our voice) and the 9600 bps. Click here to watch.
Hope that helps; 73 de PY2RAF.
