This page is currently a WIP. Please revisit for updates! Kevin G7BCS

This is going to get TL;DR, however, it might be interesting… Once I've arrived at some conclusions I'll create a less wordy summary, the aim being to make 9600 BAUD packet more accessible by discovering some of the issues around using 9600 BAUD packet with commercial off-the-shelf rigs.

I've been on a bit of a journey trying to get 9600 BAUD packet working, especially with off-the-shelf commercial rigs allegedly 9k6 packet capable via their “data” connector. This has been resolved, for me, thanks to Tom M0LTE, who sourced a Tait radio which “just worked”. Such radios are hard to find and costly, however, so I feel that, if we can better understand the problems with commercial rigs on 9k6 packet, we can increase uptake of this mode and speed our packet network up considerably.

9600 BAUD packet, whether using the legacy G3RUH mode or IL2P, uses direct FSK modulation of the carrier as opposed to the more usual 1200 BAUD AFSK modulation whereby audio tones transmitted by the radio are modulated in frequency. The upshot of this is that the audio path through both transmitter and receiver needs to be pretty much flat from almost DC to at least the maximum modulating frequency of 4800Hz, and probably beyond this to around 6 KHz, to work well. It's no longer just carrying the 1200 or 2200 Hz AFSK tones but the complete spectrum resulting from the FSK modulating waveform. This isn't going to work via the rig's usual audio signal path, which will be band limited to 300Hz - 3kHz and include some pre-empasis and de-emphasis.

It used to be commonplace to modify rigs to directly modulate the transmitter using a varactor, and to take the demodulated signal directly from the receiver's discriminator for this reason. Later, manufacturers started adding a DATA connector (usually a 6 pin mini-DIN) to bring out 9600 (and 1200) packet capable signal paths. There doesn't appear to be much standardisation of the characteristics of this connection, however.

I had tried at least 3 commercial rigs for 9k6 packet using a Nino-TNC and a simple connection to the DATA connector. None of these even remotely worked. I got the odd packet through, but nowhere near enough to hold a L2 connection, let alone exploit the speed of 9k6 packet properly.

I set about measuring the performance of each of the rigs I had at my disposal, including the Tait, hoping to find something to explain the poor performance, perhaps even something that could be fixed with a simple modification.

I had a think about what factors might affect a rig's compatibility with 9600 BAUD packet. This gave me some ideas on the things I needed to measure to find, and, hopefully, resolve, blocking issues for 9k6 packet:

1. Modulation frequency response (on Transmit)
2. Demodulation frequency response (on Receive)
3. Receiver phase response and group delay
4. Receiver sensitivity to frequency error
5. Transmit delay and settling time
6. Receive delay and setting time
7. Modulation polarity
8. Demodulation polarity

To be continued…