On Oct 19, 2013, at 1:38 PM, John Reilly wrote:
> Is there a consensus on the best K3 roofing filter for RTTY? I currently only
> have a 250 Hz, which isn't very useful.
A modern RTTY demodulator only requires between 23 Hz to 45 Hz of bandwidth for
45.45 baud RTTY.
The RTTY demodulator has two of them, at 170 Hz from one another. This means
that a perfect bandpass RTTY filter will need to be no more than 215 Hz to 260
Hz wide.
However, to be this narrow, the filters need to have a very specific shape. In
practice, when you add a bandpass filter ahead of the modem is to use one that
is perfectly flat and with no group delay to about 280 Hz.
This does not mean that you cannot go narrower -- if you are willing to allow
the error rate to be double, for example, then you can even use a filter that
is only flat up to 210 Hz wide.
However, notice that I mentioned flat and no group delay. If the filter is not
perfectly flat across said passband, or it has significant group delay, then
the errors will also rise. High order crystal filters are notorious on both
counts.
With something like a superhet, the other extreme is no good either. If there
is no QRM, you would want as wide a filter as possible. But I as I pointed out
in the following article regarding specifically the K3, the receiver (at least
mine, serial number 01432) is very prone to overloading where there are more
than one loud station under the roof.
http://www.w7ay.net/site/Technical/K3/Content/imd.html
While the front end of the K3 may have a dynamic range of 100 dB, the entire
chain all the way to the K3's line output only has a dynamic range of no better
than 80 dB. The headphones output of the K3 actually fares better than the
line output, but you need to be careful when you use those (watch out for noise
and hum when the modem's ground is not at the same potential as the receiver's
ground).
Because of the overloading problem, when there are lots of loud signals close
by, a moderately wide filter will not work well. The K3 line output will
collapse even before a good sound card is overloaded (good 16 bit sound cards
have around 95 dB of dynamic range).
If you are a DXer looking for weak signals with no QRM, go for something that
is low order (to avoid group delays) if you have a choice, and something around
400 Hz or wider (I have 250, 400, 2.8k and 6k roofing filters in my K3). Under
flutter conditions, something wider than 500 Hz will be more optimal.
When there is no QRM that can overload the receiver (or the sound card), just
open the bandwidth to 2.4 kHz. The modem will thank you. When there is no
QRM, the worst thing you can do to optimal filters like the ones found in 2Tone
and fldigi is to ruin them with a narrow crystal filter.
I also cannot overemphasize the need to widen your filters when there is
flutter. Stop listening to what your ears tell you. Widen the filters and the
modem will prefer them for flutter.
In a tight contest environment, where the line output of the K3 can be
overloaded by adjacent QRM, something like 300 Hz might be better. I.e., the
ISI may end up causing less error than the mess from IMD. Your error rate will
likely be double of what you get from a wider filter when there is no QRM, but
that is still better than the error rate going sky high due to overloading (or
AGC pumping, if you use AGC).
(Because of all of these problems, I have abandoned superhets and gone to the
DDC variety of SDR (a HPSDR Hermes) to get good true dynamic range all the way
to the demodulator, with very flat, and wide linear phase filters all the way
to the demodulator. For RTTY, I am basically using a bandwidth of 96 kHz
today, with no AGC into a modem that has a pair of Matched Filters.)
73
Chen, W7AY
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|