Look at the Tx spectrum plots of radios tested by ARRL. CW signals defintely occupy bandwidth! If they didn't, we could do CW contests in a 10 Hz slice of each band and accommodate every Amateur on earth with no QRM! And have essentially 10 Hz left over!
In fact, a CW signal might have no width if you never keyed it. The "C" in CW. But in fact we turn the carrier on and off, and this creates sidebands. If the signal on/off keying is properly shaped, these sidebands will occupy little spectrum. And, if you think about it, the total occupied spectrum width will be very nearly the same no matter your keying speed up to the limit allowed by the "waveshaping." If the waveshaping, for example, is a raised cosine and you key it at a rate such that an element just allows the complete waveshaping to occur, you will have a 100% modulated AM signal. So, if the waveshaping were, say, a raised cosine of 5 msec, and you keyed on for 5 msec, then off for 5 msec, then on... you'd be generating a AM signal 100% modulated with a 100 Hz tone. You occupied bandwidth would then be 200 Hz. Thus, to a first approximation, 5 msec waveshaping will result in a signal at least 200 Hz wide. As you reduce the keying speed (increase the length of the elements, or the "dwell time" for carrier on and carrier off, the energy in the sidebands will be reduced and the energy in the carrier will be increased. But the signal will still "occupy" 200 Hz of spectrum. You can get narrow spectrum at these lower keying rates by doing more complex waveshaping, but in the extreme (look at a sinc function) you'd need to modulate the carrier BEFORE the key is closed and for a similar amount of time AFTER the key is opened. This translates to latency, since the DSP can't look ahead in time to see what you will be doing in the future (if it could, I'd apply THAT signal processing algorithm to the stock market!). We're restricted to filtering that attempts to minimize occupied bandwidth while operating with absolute minimum delays/latencies. We CAN apply this sort of waveshaping to other digital data modes -- see, for example, James Miller, G3RUH, classic article on "The Shape of Bits to Come" < URL:http://www.amsat.org/amsat/articles/g3ruh/108.html > and think about CW rather than PSK while reading it. We'll never get to zero bandwidth if we want to convey information. I suppose if Shannon hadn't written his paper, we'd be able to communicate with zero bandwidth, but he did and now we can't... :-) Enjoy! 73, Lyle KK7P > ... a CW signal has no width, ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
