Posted: 11/3/2013 1:51:43 PM EDT
Please educate me 
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FSK and AFSK are two means to the same end. These two modulation methods create the same RF signal. Using RTTY as a simple example, a true FSK modulated radio will cause the carrier to shift between the mark and space frequencies in accordance with the RTTY modulation scheme being used. Similarly, putting the radio into a USB modulation mode and applying the appropriate audio tones for mark and space causes exactly the same RF output. Allowing for slight differences in the spectral purity of the two modulation techniques, viewing either approach on a spectrum analyzer will appear identical in both cases.
The important difference between the two modulation methods is more about operating flexibility than anything else. To use RTTY as an example again, the "standard" frequencies for 170Hz shift RTTY are carrier. + 2295Hz and carrier + 2125Hz. Many radios with built-in RTTY modes, which may or may not use FSK modulation, utilize two narrow filters somewhere in the signal path at both of these frequencies. One simply tuned around until a signal "fell" into the filters. This is still a great way to go for maximum receiver performance. Using an AFSK software implementation of an RTTY modem, "standard" frequencies are right out the window, although the frequency shift must remain correct. The USB passband can be left wide open and the software simply pointed at the tone pair desired for demodulation. On transmit the software creates whatever tones are necessary to match up with the received signal. But this is all just a matter of implementation. One could use a radio with RTTY filtering built in with AFSK software and as long as everything is tuned properly life is good, indeed better, although the operating style will be a bit less freewheeling and more like old school RTTY. Clear as mud? |
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Quoted:
So on a FSK signal does the radio internally QSY back and forth between mark and space? Quoted:
So on a FSK signal does the radio internally QSY back and forth between mark and space? That's correct. Does that mean the left RTTY track is space and the right is mark? I actually had to look that up for you  . The answer is that, in the amateur standard, the higher of the two carriers is the mark. With the rig in USB, which is the sound card mode standard, then the rightmost frequency on the waterfall will be the mark.
Rob's point about overdriving is a very good one. |