> I use 250, with an Inrad filter, all the time for RTTY. Any wider
> than that is too wide for me.
Most of the INRAD "250 Hz" filters are actually 325-275 Hz wide at
the - 6dB points as the "250 Hz" is based on cascading a pair of
filters in the IF (i.e., Yaesu or Kenwood style).
As documented by Chen, a 325 - 350 Hz "roofing filter" is a very
good design for 45.45 baud 170 Hz shift RTTY. It provides good
selectivity while preserving most of the signal energy in the
keying sidebands.
When talking about RTTY bandwidth, care needs to be taken in
defining terms ... even a 200 Hz crystal filter like the one
produced by Elecraft (a true 200 Hz, 5-pole design) can be
useful if the skirts eliminate QRM more rapidly than they
cut signal energy in the keying sidebands.
When talking about DSP filters, the "bandwidth" is more critical
as many DSP filters have much steeper skirts than those in
practical crystal filters. DSP filters can exhibit skirt
selectivity comparable to the 10 dB/100 Hz slopes in the
Elecraft implementation. Such sharp skirts would make a true
200 Hz DSP filter a poor choice for RTTY. On the other hand,
if the skirts were "softer" like those in some Icom implementations
(or Yaesu's "soft" filters), a 250 Hz DSP filter might work well
in heavy QRM.
73,
... Joe, W4TV
On 6/1/2010 12:48 PM, Tom Osborne wrote:
> I use 250, with an Inrad filter, all the time for RTTY. Any wider
> than that is too wide for me. Not sure about narrower as I can't get
> less than 250 cycles here. The number 2 radio here has a 500 cycle
> filter and that is WAY too wide. for contesting. 73 Tom W7WHY
>
>
> Don, AA5AU, wrote:
>> I think I remember someone, maybe Chen, saying a 200 Hz filter
>> would not work on RTTY or something to that effect. Maybe I don't
>> have the story just right, but it appears to be working great. Too
>> tight for contesting, but works when required.
>
> A passband narrower than about 280 Hz removes some of a 45 baud
> signal. OTOH, I have used a 200 Hz DSP filter a lot in contests and
> pileups because removing off-frequency signals is a bigger benefit
> than the little bit lost with the too-narrow filter. This requires
> precise tuning but helps separate signals. Even 150 Hz works pretty
> good, though I never go that narrow.
>
> 73, Ed -------------- Ed Muns - W0YK www.w0yk.com
>
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