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.
I did a bit of research. PSK31 isn't just phase modulation, it uses amplitude shaped pulses, to reduce the sidelobe level. Straight BPSK at the rate would have the familiar sin x/x with the first sidelobe down 13dB. Until recently in the deep space business, we didn't much care about sidelobes, since we send the entire spectrum, and there wasn't a "spectrum crowding issue". Now there is a spectrum availability issue, so "spectral efficiency" has become more of a sticky point. The designer of PSK31 wanted better spectral efficiency, and so, chose a non-constant envelope modulation (raised cosine weighting) to get there. Since it was designed from the start as a "sound card mode" intended to be fed into a SSB transmitter (which has a linear amplifier) this wasn't an issue (assuming your drive is properly adjusted). Another approach to getting the same result would be using something like GMSK, which IS constant envelope, but in which the "phase transition" isn't instantaneous, but sloped. MFSK16 is another constant envelope (and phase continuous) modulation with good spectral efficiency. > A PSK31 signal in "idle" is a perfect two-tone signal, and as you > know, a two tone signal needs a linear amplifier. Actually, depending on the time scale, PSK31 doesn't really have two tones, although one can look at it as a DSB suppressed carrier with the two sidebands separated by a few tens of Hz. >There is a picture of the spectrum of a PSK31 single transmitting >"idle" in figure 9.29 of my latest Handbook, 2005. As you can see, >there are sidebands which get greatly increased and multiplied if >amplified by a non-linear amplifier. I don't know that it's actually IMD in the classical sense (that is, new components that are related to each other by things like 2fa-fb, etc.).. I think it's more that the gain compression removes the amplitude shaping, pushing it closer to nominal straight BPSK. In the limit, it would be like running the signal into a comparator, and then it WOULD be BPSK (after filtering the harmonics). The net result is that the spectral occupancy starts to look more like BPSK than raised cosine weighted BPSK. >73, Chas W1CG > > > >At 03:11 PM 11/17/2006, Jim Lux wrote: >>At 11:29 AM 11/17/2006, Charles Greene wrote: >>>Jerry, >>> >>>If Jim is correct, and I would have to think about it for awhile, >>>the answer is, It depends. The PSK31 signal has frequency and >>>phase information in it, and a RTTY has frequency in it only. So >>>a PSK31 signal needs a linear amp to preserve the phase >>>information, while a RTTY signal which has only alternating single >>>frequencies does not. >> >> >>No.. Phase modulated signals do not need linear amplifiers >>(although they may be nice for other reasons). All the current >>deep space probes send back their data using saturated amplifiers >>with some form of PSK (typically BPSK, with a mod index < 180 >>degrees so that there is residual carrier). Newer designs are >>using GMSK, which is still constant envelope and can be run through >>a saturated amplifier. >> >> >>There is a "gotcha" in that most devices have some AM/PM conversion. >> >>In any case, a FSK or PSK signal generated by the SDR1000 should be >>able to whip on through a saturated amp with no problem. And, in >>fact, with sufficient signal processing, one could do some >>predistortion of the waveform to compensate for non-ideal amplifiers. >> >> >> >> >> >>James Lux, P.E. >>Spacecraft Radio Frequency Subsystems Group >>Flight Communications Systems Section >>Jet Propulsion Laboratory, Mail Stop 161-213 >>4800 Oak Grove Drive >>Pasadena CA 91109 >>tel: (818)354-2075 >>fax: (818)393-6875 > James Lux, P.E. Spacecraft Radio Frequency Subsystems Group Flight Communications Systems Section Jet Propulsion Laboratory, Mail Stop 161-213 4800 Oak Grove Drive Pasadena CA 91109 tel: (818)354-2075 fax: (818)393-6875 _______________________________________________ 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
