-----Original Message----- The recent thread on filter settings and hearing reminded me of a question I
would like to get an answer to. In the ARRL 2001 Handbook on page 15.7 we find: "The dots and dashes of a CW signal must start and stop abruptly enough so we can clearly distinguish the carrier's presences and absences from noise, especially when fading prevails. The keying sidebands, which sound like little more than thumps when listened to on their own, help our brains be sure when the carrier tone starts and stops. It so happens that we always need to hear one or more harmonics of the fundamental keying waveform for the code to sound sufficiently crisp." What is meant by "the fundamental keying waveform"? How do we take "the need to hear one or more harmonics of the fundamental keying waveform" into account when setting up the IF and audio filters? 73, Darrell VA7TO K2 #5093 \------------------------------ The fundamental keying waveform is what you'd see if you monitored the keying voltage from your keyer on an oscilloscope, or looked at the dots and dashes of RF the transmitter produces. The carrier is keyed on and off sharply. That is, it rises from zero to maximum quickly at the start of each code element, then drops back to zero quickly at the end of the element. To do that requires a bandwidth much greater than the fundamental. Picture it this way. Look at the leading edge of one dit. Now construct a sine wave whose rise time matches the leading edge of the dit. Typically you will find that the frequency of such a sine wave will be in the range of 100 Hz or more. In that case, your cw transmitter will produce sidebands 100 Hz above and below the carrier frequency. When you key it, you are "amplitude modulating" the signal at the fundamental keying rate AND at the frequency defined by the steepness of the leading and trailing edges of the elements. For example, if you are transmitting at 7050 kHz (7050000 Hz) and the edges of the keying waveform has a shape corresponding to a 100 Hz sine wave, your transmitter will be producing a pulse of RF at 7050000.1 Hz and 7049000.9 Hz at the start and end of each code element. If your receiver bandpass does not allow that range of RF frequencies through, the keying waveform cannot be reproduced and the keying will sound soft. If the waveform is too sharp, those pulses (or clicks) will be widely separated from the carrier frequency - several hundred Hz or even more! That sharp of keying is never needed and only serves to produce QRM on the bands. But a reasonable amount of sideband energy is required for intelligible CW. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
