There is a need for automated HF digital data links that don't require active operator intervention. An example would be a HF APRS station broadcasting GPS position data. Unfortunately, most amateur HF data links are quite narrow band (few hundred Hz channel) and require fairly precise tuning (10 Hz) for decent error performance.Typical synthesized consumer HF transceivers don't have very good frequency accuracy: a typical spec of 100ppm would be good, and at 10 MHz, this is 1 kHz . I think the use of narrowband schemes and tight frequency tolerance is mostly for historical reasons, since they evolved from RTTY (45 baud 170 Hz shift) and HF Packet modems based on Bell 202 standard. Certainly, there isn't any good performance or legal reason to use such a narrow bandwidth. For a manually established link, the operator just tunes up and down, watching the tuning indicator on the modem. This is not acceptable for an automated link.
Several solutions are possible: 1) Use a better transceiver with better frequency stability (some transceivers have TCXO's as an option); 2) Use a modulation scheme that is wider-band and tolerant of mistuning (or that has AFC); and 3) determine the tuning error and correct it on the fly. Strategy 1 is expensive, or possibly not available. Strategy 2 has the problem of incompatibility with the existing installed base, and further, the high signal density in some areas and limited receiver instantaneous dynamic range means that you need narrow IF filters for adjacent channel rejection.
This page discusses a scheme for the third strategy, measuring the transmitter (and hence the receiver) frequency error, and then retuning to account for the error. The scheme is simple, and uses parts of the system that already are there for a typical APRS GPS tracker. The idea is to use the 1pps tick from the gps receiver as a timing gate for a frequency counter to measure the actual transmitted frequency of the transmitter. Typical commodity GPS receivers (that have a 1pps output) are accurate to 500 nSec, or 0.5 ppm, on the 1pps output, when moving. This is sufficient to get 10 Hz measurement (and hence) tuning accuracy on HF links in the 30meter band (10 MHz).
A simple frequency counter, which could be as simple as a 8 or 16 bit counter, measures a sample of the transmitted carrier. (you could also measure some significant internal signal from the transceiver, if available). The measured frequency is then used to change the transmitter/receiver frequency, either by changing the frequencies in the AFSK modem, or by sending actual commands to the transceiver. The counter can be very simple, because we are really only interested in the difference in frequency from that expected, since we have sent a tuning command to the transceiver (either explicitly, or as a channel select). For instance, a 16 bit counter would give a range of +/- 32 kHz to work with.
hflink.htm - 24 January 2000, Jim Lux