On Dec 2, 2010, at 12/2 4:32 AM, Barry N1EU wrote: > Are you saying that wide sidebands measured with the rig at 4msec > rise-time are probably not going to be even wider with the rig at 1 > msec rise-time?
Not that much, since the really far off keyclicks are mostly from higher order discontinuities. Remember that the RF spectrum for CW is approximated by a rectangular pulse that is convolved by the impulse response of the waveshaping filter if the filter is linear (i.e., none of the silly diodes and so on used to attempt to tame keyclicks). The short way of saying it is that the pulse is filtered in the time domain by the waveshaping filter. The "convolution theorem" thus states that the Fourier of the resultant filtered signal is just the *product* of the Fourier transform of the CW pulse train with the Fourier transform of the waveshaping filter. If you assume that the unfiltered pulse train is completely unfiltered (i.e., perfectly sharp edges and so on), a pulse train of dits will have a Fourier transform that is a sinc(f) (i.e., sine of f divided by f) function in the frequency domain. The spectrum of the waveshaping filter is simply the stuff you can see at the Wikipedia page. Multiply the two to get the RF spectrum of a waveshaped CW, and you will find that the sinc(f) is predominated by the waveshaping filter for far off frequencies. Since the power of the unfiltered CW keying signal (i.e., sinc(f)*sinc(f)) dies down very slowly, all the far off spectrum is determined by the "tail" of the spectrum of the waveshaping filter. Without waveshaping, the power of CW keying pulses die down asymptotically as 1/( f*f), since the spectral envelope of sinc(f) dies down as 1/f . The same scenario holds true for RTTY. You can think of an FSK signal as two pulse trains, when one is on, the other is off (i.e., really no different from a CW signal :-). With randomly generated FSK, you have the same sin(x)/x spectral envelope. If you now make sure that the mark-space phase transition of FSK is smooth -- what many people call "phase continuous FSK"," RTTY improves a lot (this is what the K3 and many other rigs and software do). "Phase continuity" is basically saying there is no first order discontinuity of the temporal waveform, but if the Mark and space frequencies are different (of course), there will be second order and higher order discontinuities unless all mark/space switching are done when their carriers are right at zero -- not very practical). Just like CW, you can also make your RTTY signal friendlier to your spectrum neighbors by waveshaping the FSK signal some more. You can see this process here (those are actual recorded AFSK signals): http://homepage.mac.com/chen/Technical/FSK/Sidebands/sidebands.html Instead of two peaks of the FSK signal, you can visualize in your mind a single peak for a CW signal that is represented by the one sided spectrum, and the spectrum for first oder discontinuitity, second order discontinuities, and waveshaped FSK (I had used a simple Blackman in the Web page above) pretty much holds for the CW case also. I don't know of a single FSK rig today that is as friendly to neighbors as the last two spectra shown in the web page. The Omni V and VI does a little of waveshaping of the keying signal that is applied to the varicaps to generate the FSK signal. I also don't know of any software that applies RTTY waveshaping, although it is easy to do for all AFSK software. You need to be careful, of course. Since any waveshaping will cause a slight overlap between the mark and space carriers, too much waveshaping and you again run into that pesky transmit IMD problem that John (juergen) mentioned. For that reason (and also not to degrade SNR at the end of the matched filter that is discussed in the above web page), cocoaModem doesn't go too far off the deep end when wave shaping. It actually survives pretty well through the K3's transmit IMD. You can see that in the last two spectra on this page: http://homepage.mac.com/chen/Technical/K3/Digital/digital.html The K3 native (i.e., using a paddle) FSK signal (measure by a separate receiver) is in the second last image, and cocoaModem's waveshaped RTTY (using K3 DATA-A) is the last image on that page. You can actually see IMD spikes dues to the intermodulation between the now overlapping mark and space signal. But overall, the QRM is still below the spectrum from the K3's FSK signal. But if the K3 transmit IMD can be improved, you can squeeze more RTTY stations in during a contest. The FSK ops will then get the same reputation as the FT-1000D CW ops :-) :-). The RTTY spectra in the second web page has random (well, LTRS Baudot characters) bit modulation and I had apply a pretty per-bin filtering of the spectra to remove the noise from the receiver and sound card, so it appears smoother and less serrated. But you can see the difference between something that has first order discontinuities, something that has no first order discontinuities, and something that also try to suppress higher order discontinuities. Not much happens in the nearby spectrum. But as you go further away from the keyed carrier, the difference between a "good" waveshape and a "very good" waveshape is quite large. And, at some point (as seen by the IMD spikes in that last plot), the transmit IMD becomes the "weakest link." 73 Chen, W7AY ______________________________________________________________ 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
