At 08:21 PM 11/17/2006, Charles Greene wrote: >Jim and All, > >There are phase modulated signals, and there are phase modulated >signals. The phase modulated signals used in PSK31 do need a linear >amplifier, as the phase modulation produces RF with different >amplitudes. I think the modulation index is 90 degrees which does >have a spectrum different than the space signals.
For deep space operation, the mod index varies with how the spacecraft is being operated. There's always a tension between the navigators, for whom data is just noise corrupting their observables, and the scientists, who would like all data and no carrier for nav (the radio scientists use nav type data, so they're a bit different). In a low rate, safe mode, type operation, the mod index might only be 45 degrees. You want lots of carrier to make acquisition and carrier tracking easy. But for a short range link (e.g. UHF from Mars surface to relay orbiter), fully suppressed carrier is available (mod index = 180). These days, too, with software radios like Electra using vector modulators, there's a lot more flexibility to change things, as opposed to older designs using analog circuitry driving a linear phase modulator. Since this list is focussed on a software radio, readers might be interested in a new book out on autonomous radios for space. Of particular interest might be the chapter on Electra: http://descanso.jpl.nasa.gov/Monograph/series9_chapter.cfm?force_external=0 It's chapter 2. Electra is somewhat unique in that it's the first ever software radio that has been used in deep space. And, it has been reprogrammed in flight. The Rf and signal processing architecture will look very familiar to SDR1000 users.. a tunable RF converter tunes 390-450 MHz and the final down conversion and detection is done in DSP. The radio needs to accomodate 10 MHz bandwidth signals, so we don't use an 11 kHz IF like the SDR1000 does, but other than that, it's pretty similar. Jim, W6RMK _______________________________________________ FlexRadio mailing list FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archive Link: http://www.mail-archive.com/flexradio%40flex-radio.biz/ FlexRadio Homepage: http://www.flex-radio.com
