On Aug 26, 2009, at 11:08 PM, Jeff Blaine AC0C wrote:
> I think this helps
> to explain why there is quite a bit of satisfaction with the K3
> using this
> filter on RTTY.
Jeff, bear in mind when you select filters the following two points:
1) optimal receive filtering has nothing to do with FCC 47 Part 2.202
definition of occupied bandwidth (which is over 400 Hz in the case of
steam RTTY).
2) keying sidebands from nearby stations can never be removed with a
receiving filter (comments below).
To see what keying sidebands of an RTTY signals are like, you can take
a look at (you can see that this topic rears its head every couple of
years :-):
http://homepage.mac.com/chen/Technical/FSK/sidebands.html
In the above, I had looked at FSK (where you switch between mark and
phase with no regards of any continuity), phase continuous FSK (where
the transition between mark and space are continuous, but is not slope
continuous -- this is why FSK (vs AFSK) sidebands are wider than
neccessary), an AFSK signal that is well filtered and still provides
the receiving matched filter with something that is still pretty
decent, and finally an AFSK signal that is too narrow (this is the
same case as a receive filter that is too narrow).
Now, when you pass all that through a practical transmitter, the
behavior is not as obvious, but still visible. I took some samples
last night of the FT-1000MP's transmitter. The FT-1000MP feeds a
dummy load through a directional coupler, and I used transformer
coupling from the directional coupler (to minimize any ground loops)
into the antenna input of a K3, from which I recorded the following
two plots.
The first plot is that of the FT-1000MP running in FSK mode.
http://homepage.mac.com/chen/Technical/FSK/foxFSK.png
The data is a bunch of repeated "The quick brown fox jumps over the
lazy dog" with lots of spectral averaging (each FFT bin is
individually exponentially filtered).
The second is the FT-1000MP running in AFSK mode (no ALC, naturally)
with audio from cocoaModem that uses a moderately tight Blackman-
Harris windowed filter.
http://homepage.mac.com/chen/Technical/FSK/foxAFSK.png
Notice that the transmit phase noise predominates in the spectrum
beyond 100 Hz away from the FSK carriers. Even with that, you can
still see that the AFSK signal is a bit narrower (look at the 600 Hz
region and the 1600 Hz region and you can tell that the FSK signal is
wider than the AFSK signal, even though both are using 170 Hz shift.
The difference would be more obvious with a rig that has better
transmit phase noise.
In any case, take a look at both FSK or AFSK signals 200 Hz away from
the FSK carriers. They are only about 35 dB down from the carriers
themselves. I.e., an S9+35 dB signal will clobber an S9 signal even
if it is 200 Hz away. No filter is going to fix this problem. I.e.,
there is no need to be so anal about filter bandwidths, and in any
case, let your software demodulator do the heavy lifting in the
filtering department. The crystal filter only needs to provide a
protection against clipping the sound card.
[Note that you can also "just" see the difference of the keying rate
differences between the two plots. The FSK plot is keyed at 45.00
baud (MicroHAM microKeyer II's FSK generator) and the AFSK is keyed at
45.45 baud.]
From the last picture in the html page, you can see what a filter
that is too narrow would do to a matched filter. In the presence of
noise, noise would also go down by a little with a reduced bandwidth,
but not by as much as the peak of the matched filter goes down by
(this should not be a surprise, all the textbooks on matched filtering
tells you that :-). That last picture has a filter cutoff just beyond
the third harmonic of the keying signal, i.e., 22.725*3 Hz; in other
words, a perfectly tuned RTTY signal inside a 306 Hz brickwall filter.
Conclusion: if there is a strong RTTY station close by, a 306 Hz
brickwall filter will not be able to get rid of of QRM if it is more
than 35 dB louder than the signal you are trying to copy, and in the
meantime, you are losing a bit in the ability to copy a weak signal
because you are cutting off too much of the keying sidebands.
Fortunately, even Inrad 250 Hz crystal filters don't really cut away
much at 300 Hz.
You might want to use the captured spectra (the PNG files) to
determine your personal rule of thumb of how far you want to be away
from a nearby loud station during a contest, both for your sanity and
the sanity of the nearby station (if he is loud at your QTH, you will
likely be equally loud at his QTH :-).
73
Chen, W7AY
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|
