[RTTY] 250 hz filter on RTTY

[Bill Turner]

ORIGINAL MESSAGE:

At 08:04 PM 5/22/2006, Rick Mintz wrote:

>Before I do the mod, a question for those that have a 250 Hz 
>filter.... Have you found this to be to narrow for RTTY contesting? 
>Any other comments?

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It depends on the steepness of the filter's skirts, but here's a 
quick check you can do, assuming you are running MMTTY with the 
crossed ellipses or have a crossed-ellipses scope monitor.

1. Using the 250 Hz filter, tune in a weak station to where the 
ellipses are crossed at the normal 90 degrees.

2. Note the amplitude of each ellipse.

3. Slowly tune off the correct point and see if either ellipse 
increases in amplitude. Tune in both directions, watching both ellipses.

4. If the filter is too narrow, one or both of the ellipses will 
increase in amplitude when mis-tuned. This tells you that with normal 
tuning, one or both of the received frequencies is partly outside the 
bandpass of the receiver.

5. If the amplitude of each ellipse stays the same or decreases, your 
bandwidth is ok.

It is important that you do this with a relatively weak signal so the 
AGC of your receiver and the AGC of MMTTY or your TNC does not 
compensate for the reduced signal outside the bandpass, if any.

Theoretically, a 250 Hz filter with really steep skirts is too narrow 
for 170 Hz shift, but I have used several receivers with a more or 
less "normal" 250-270 Hz filter and observed no ill effects. The 
theoretical ideal, according to the experts, is between 300-350 Hz, 
given a "perfect" filter. I suspect you'll find your filter is quite ok.

The effect of a too-narrow filter is to "round off" the corners of 
what should be a perfect square wave being fed to the RTTY 
demodulator. Some rounding is tolerable, but too much gives a flat 
portion of the top or bottom that is too short (both in time and 
amplitude) to demodulate properly. Strong signals will have enough 
amplitude to compensate for reduced amplitude on the filter skirts, 
but weak signals will be a problem.

Bill, W6WRT