Independent Sideband Transmitters

Discussion in 'General Electronics Chat' started by KL7AJ, Nov 13, 2008.

  1. KL7AJ

    Thread Starter AAC Fanatic!

    Nov 4, 2008
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    One of our posters recently brought up a good question about Independent Sideband methods, which really fired up some old nostalgia glands. Generally speaking, ISB transmitters were very expensive military and industrial devices, as conventional methods required essentiially two transmitters driven with a single local oscillator, with duplication of most of the other components. The older phasing method was actually more viable for amateur radio applications of ISB, as it only required duplicating the audio shift network. However, few people in this day and age realize what it took to make a 90 degree phasing network exhibit a uniform phase shift over SEVERAL OCTAVES of frequency range! Amateur literature of the era was replete with countless experimental methods, of varying success, to achieve the holy grail of the perfect audio phase shifter.
    All that has changed with DSP, where creating a frequency independent phase shift is a trivial matter.

    Since IQ modulators are nowadays pretty much commonplace in both analog and digital communications devices, I expect to see a resurgence in the interest in independent sideband transmission. We almost saw this during the AM Stereo fiasco of the early 1980s. The Kahn-Hazeltine methode used leflt channel audio on lower sideband, and right channel audio on upper sideband. It had some performance issues, but certainly not enough to throw the baby out with the bathwater, which unfortunately is how the FCC dealt with the situation. As a result, we still have NO AM stereo broadcasts (at least in the U.S.)

    However, there's no reason ISB can't become a very popular mode on the amateur bands...one of the most useful applications would be for slow scan television (SSTV)....pictures could be on upper sideband, and voice on lower sideband, for instance.

    All I can say is....stay tuned for late developments!

    Eric
     
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