On Apr 14, 2005, at 8:41 AM, Peter Laws wrote:
> I did read it - thanks for the link. I should know this, but since
> I'm at work, apparently I've forgotten: What is the bandwidth of a 170
> Hz-shift RTTY signal? It's more than 170 Hz, but by how much!?
With AFSK, you can experiment with different transmit bandwidth
rather nicely. I have found that a moderate brick wall output filter
with about 350 Hz bandwidth to not interfere with the performance of
RTTY.
With FSK, unless you have built the rig yourself, you are at the
mercy of what the rig does.
Remember that when you are using Fred Terman's rule of thumb, the
fundamental of 45.45 baud signalling is 23 Hz, not 45 Hz.
If you include the third harmonic, that makes the half-width of the
mark and space "lines" to be about 70 Hz. Quick back-of-the-envelope
therefore gives 170 + 2*(70) = 310 Hz.
One of the problems with making the bandwidth of an FSK/AFSK signal
too narrow is that you will start to get into inter-symbol smearing
-- the mark "bit" smearing into the space "bit" unless you also
waveshape the keying signal.
Take a look at Fig 16 of
http://www.wj.com/pdf/technotes/FSK_signals_demod.pdf
if you are technically oriented. Notice that the SNR needed for good
copy climbs rapidly when the bandwidth is reduced below the optimal
(to the left of the dip) -- that is the region where inter-symbol
smearing becomes predominant.
IMHO, this Watkins-Johnson white paper should be one of the required
reading material for anyone trying to understand RTTY demodulators.
They also have a pair of white papers on PSK, but this FSK paper has
in one place some good stuff which you have to dig hard to find
elsewhere. Lots of other goodies (especially if you are into
receivers and demodulators) can be found at
http://www.wj.com/technotes/index.asp
On a different topic, while we are on the WJ white paper, Fig 17
shows how much more power is needed in the presence of HF fading.
Remember the modem performance plots from Alex, VE3NEA? If you
compare the abscissas of his various plots, they basically tell you
the same story.
Alex's plots show that TrueTTY is 3 dB more sensitive than MMTTY when
presented with an RTTY signal in the simple white noise case. But
both demodulators require *much* more SNR when faced with the real
world conditions, i.e., "Fig 17." It is probably because of this
variability why most people don't notice that MMTTY is poorer than
TrueTTY, even though TrueTTY beats MMTTY by an even bigger margin for
the selective fade case.
73
Chen, W7AY
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