Hi Andy, There is no simple universal relationship between the "shift" and the transmitted "signal bandwidth", because there are so many factors other than "shift" that contribute to the bandwidth of an "FSK" signal:
1. Symbol rate 2. Shape of waveform 3. Symbol transition point 4. Filtering 5. Number of tone frequencies 6. Transmitter chain 7. Other factors related to modulation process 8. Noise 9. Transmitter oscillator spectral purity 10. Definition of bandwidth This is an especially complex calculation for multiple frequency FSK signals, commonly 4-ary FSK, 8-ary FSK, 16-ary FSK, 32-ary FSK etc. where the number of shift frequencies is greater than 2, or the number of carriers is greater than 1. The FCC rule says "maximum frequency shift of 1 kilohertz between mark and space." But, that FCC rule was written in the old days when common ham RTTY was Frequency Shift Keyed between only 2 frequencies, technically described by "mark and space". However, in modern multiple tone frequency shift techniques, with binary symbols there is no such thing as "mark and space". Thus, the rule became inapplicable to the new multiple frequency shifting keying modes. When the FCC was asked to convert from "shift limit" to "bandwidth limit", the FCC refused, and at the same time, FCC said it had chosen not to limit bandwidth because it is important for ham radio to have the freedom to innovate and develop new techniques. Thus, the mark and space shift limit became a mere footnote in history that largely does not affect most modern digital techniques used in ham radio today. If you wish to delve into the finer math points of relationship between bandwidth and shift, may I suggest reading Section 6 (starting on page 37) of this fine document: "Necessary Bandwidth and Spectral Properties of Digital Modulation" by David J. Cohen: http://www.its.bldrdoc.gov/pub/ntia-rpt/84-168/84-168.pdf 73 Bonnie KQ6XA > Andy K3UK wrote > -Bonnie, can you explain to this bozo what the > difference between a "shift" restriction and > "bandwidth" restriction would be? My brain > viewed them to be the same, that is that a > 170Hz shift would be roughly that amount > of Hz wide at the usual ham speed. >
