On 07/12/2018 02:46, Simon wrote: > What is the guideline or specification for audio frequency signal > attenuation outside of the 50Hz (plus and minus 25 Hz from centre) band > width? > > For example, signal should be xxxxx dB below carrier, at frequency plus > and minus 50 Hz from centre?
In the Protocol Specification in the User Manual, it says (in the case of FT8) that "The total occupied bandwidth is 8 × 6.25 = 50 Hz." It's just the number of tones multiplied by the tone spacing. The ITU defines occupied bandwidth (Radio Regulation 1.153) as being the bandwidth containing 99% of the mean power, with 0.5% above and 0.5% below. The same definition of bandwidth is used in the UK amateur licence, and probably those of many other countries. With the ITU definition, the occupied bandwidth will depend on the shaping of the keying waveform. To get the bandwidth low, Gaussian keying is used in some modes. FT8 does not use Gaussian keying. Instead, the tone frequency is switched instantaneously at a zero-crossing point. This produces a wideband transient between symbols. It's comparable to frequency modulating a carrier with a square wave. There's also a wideband transient from the amplitude keying at the beginning of the first symbol and the end of the last, where the tone switches suddenly between zero and full amplitude. This is comparable to a CW key click. This was explored by Neil Smith G4DBN in his presentation at the 2018 RSGB Convention. A video of the presentation is available online but only to RSGB members: https://rsgb.org/main/publications-archives/video/rsgb-convention-lectures/rsgb-2018-convention-lectures/ I've often wondered what the occupied bandwidth of FT8 is when measured the ITU way. As far as the WSJT-X waterfall goes, some of the apparent 'smearing' is owing to the FFT, but I suspect some may also be real and caused by the switching transients. 73, Richard G4DYA _______________________________________________ wsjt-devel mailing list wsjt-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/wsjt-devel
