The use of narrow receiving filters has cropped up consistently on the
reflector.
Andy K0SM and I have been looking at narrow transmit filters, but there is
something from that study which can be "translated" to narrow receive filters.
After all, transmit filters and receive filters are cascaded (with the
ionosphere in between). A filter that is too narrow for transmit is also too
narrow for receive.
To answer the question of how narrow can you make a transmit filter, I have
written this article:
http://www.w7ay.net/site/Technical/RTTY%20Transmit%20Filters/index.html
Pretty much only modem developers are interested in transmit filters, together
with perhaps rig manufacturers who are interested in applying DSP transmit
filters to their FSK transmitters. So, if you are only interested in receive
filters for RTTY, just scroll down the web page until you hit Figure 2.2.
There, you will see a plot of the absolute practical minimum bandwidth for any
filter that you want to place in the path of a 45.45 baud RTTY signal. The
absolute minimum bandwidth for an "ideal" data filter is of course the 261 Hz
Raised Cosine that I often cite.
Notice from the figure that for no loss in Character Error Rate, the perfect
practical filter will need to be at least 270 Hz to 280 Hz wide. By perfect, I
mean that the filter has to (1) be flat to a fraction of a dB, and (2) is phase
linear (i.e., zero group delay), within that 270 Hz to 280 Hz.
If a filter allows more Mark energy to go through than Space energy, you will
suffer the equivalent of sustained selective fading. And group delay will show
itself as something similar to a sustain multipath (you *really* don't want
that!).
So, if you are going to place a crystal filter in between the antenna and the
modem, you need to make sure that whatever filter you use has 280 Hz worth of a
flat passband that has no group delay. If not, you will incur extra errors.
How much error? Notice from the plot that when you narrow the "perfect" DSP
filter by just 60 Hz, you will double your error rate. And it climbs much
steeper as you narrow further.
This does not include the need to widen the filter when propagation conditions
deteriorate and you have more Doppler scattering. Count on perhaps even
doubling the filter bandwidth when you hear audible flutter.
73
Chen, W7AY
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