On Jan 17, 2009, at 1/17 3:00 AM, hb9ari wrote: > I can imagine that digimodes software generate ± complex audio > signals, > then we can adjust this audio level to allow TX chain, including > PA, to work in a > linear zone.
Out of the common HF digital modes, I believe that BPSK31 and QPSK31 are the most sensitive to nonlinearities. RTTY, MFSK16, Olivia and Domino are M-ary FSK (M=2 for RTTY) and are less sensitive to nonlinearities since their crest factor is 1. However, with poor linearity, you can no longer use envelope waveshaping to help reduce the FSK keying sidebands. BPSK31 is differential phase shift modulation, with either a phase change of 180 degrees or no phase change, between successive bits. The envelope is also waveshaped so that during a non-zero phase transition, the amplitude is modulated in a sinusoidal manner. The envelope modulation, what W4TV had called "predistortion," it is simply the waveshaping of the keyed signal with a sinudoidal window, easily achieved in the I and Q components of the PSK31 generator. (A digression: a PSK31 signal can be easily generated by starting with baseband I and Q signals that swing between +1 and -1. Depending on whether you want 0 or 180 degrees (and the addition of 90 and 270 degrees for QPSK31), you either keep I or Q, or both, unchanged and slew any component that changes between +1 and -1 with a half a period of a cosine function). When no information is being transmitted in PSK31, it puts out the "idle" Varicode, which in PSK31 simply causes a 180 degree phase transition at every bit period. The dibit representations in QPSK31 are chosen so that an idle Varicode also puts out a 180-degree phase shift per dibit period. When a PSK31 signals goes into idle, it therefore creates the familiar two tone signal, with only two sidebands (the center frequency is suppressed) that are 31.25 Hz apart. This causes a clean PSK31 to look like two parallel "rails" on a waterfall display. Any deviation from a perfect sinusoidal modulating envelope will cause an idling PSK31 signal to develop higher order keying sidebands, just like what a two tone test signal does with voice mode SSB. The keying sidebands at 2.f0-f1 and 2.f1-f0 are measured, compared with the "fundamentals" at f0 and f1 and this ratio is reported as the IMD number in PSK31. An IMD somewhere around -28 dB is considered decent, with many good stations putting out signals that are cleaner than that. People do differ in what they consider to be "good enough" until they get QRM'ed by a loud, but not so clean, signal :-). With the 100 watts PA turned off, I have measured an IMD of -34 dB on my K3, running at 10 watts peak (average idle signal of 5 watts). However, with the PA turned on and running above 25 watts average on idle (50 watts peak), the IMD from my K3 is only around a -24 dB. (IMHO, very mediocre.) With the PA turned off, the K3's IMD also starts to rise slowly when power is decreased below 500 mW. Please note that this was measured on my K3 that was built from a kit (SN 1432). Your K3's performance may be different from mine. For rough comparisons, my FT-1000MP running below 5 watts and with no ALC action whatsoever, is capable of putting out an IMD that is cleaner than -40 dB IMD both on the bench and with on-the-air tests. I have also copied a local who was using QRP levels on his TS-2000 with a tad better than -40 dB IMD. The IMD of my FT-1000MP does degrade to -30 dB dB when I raise the average PSK31 power to 25 watts, but still adequately clean. A clipped modulating signal will create something like -11 dB IMD, and I have seen even be worse than that on the air -- I suspect that the signal was not just clipping but had overshoots and undershoots in the envelope. 73 Chen, W7AY _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
