To try and answer this question completely, I generated a single Spectrogram plot showing: 1) A typical dual peak RTTY filter response (expect the K3 looks like this). It was from a Timewave DSP-559ZX. Receiver IF filter is about 350Hz. MTTY looks the same. 2) A RTTY signal.
I haven't figured out how to post it here. Send me an E-mail and I'll send it to you. Bottom line: The DSP dual peak filter is 8-10 db down at a bandwidth of 260Hz. The actual signal fits completely within the minimally attenuated portion of the dual peak filter response. The concern about loosing information for weak signals is unfounded. The weak signal sidebands are so far in the noise that no information can be recovered. Theory about the sidebands being there is correct. It just doesn't consider this weak signal case. Thus the "250 Hz" filter is plenty wide enough for S&P and DXing. I'm not a believer in using AFC, even when running them during contests, but if you are, then perhaps a wider filter is more appropriate for that. 400Hz ought to be plenty wide enough. Guys who call you 100Hz or more off frequency don't deserve to be worked! I'll run this again when my K3 arrives. I don't expect the conclusions to change. More important is that the narrow filter response is centered within the passband. I note that available plots show 8 pole filters as much as 25 Hz from this. Five poles filters are probably even worse. Thus one needs to use the K3's ability to offset filters (of shift) for RTTY to achieve centering. Spectrogram is an easy to use, free tool to help sort things out. 73 de Brian/K3KO Axel Kaiser-2 wrote: > > > > > > To all you filter aficionados, > > first of all thank you very much for all your detailed answers to my > original question, which roofing filter ist best for RTTY operation of the > K3. Lots of very detailed and informative answers were given and even a > general discussion on this issue among several others was started. But to > tell you the truth, I am still somewhat uncertain, which filter to order > with my K3. > One of my theory books about HF technology, modulation, bandwidth and so > forth, tells me, that the necessary bandwidth for a 170 Hz shift RTTY > signal should be around 320 Hz. Now some of you guys write, that you are > using a 250 Hz filter for RTTY operation. A 250 Hz filter though has this > 250 Hz (ok, a few Hz more in reality) bandwidth at - 6dB attenuation, > which means that the actual bandwidth at about 0 db is even less. > So my understanding for a 170 Hz shift RTTY operation is, to use a filter > of at least 400 - 500 Hz (at -6dB) in order to get all the information > through the filter. This I can confirm also with my current OMNI VI+ which > does not allow me to copy RTTY signals with a 250 Hz filter . So please, > where is the point which I am missing with the K3, that you can work weak > RTTY signals with a 250 Hz roofing filter?? > > Best 73 de > > Axel, DL3ZH > > > > _________________________________________________________________ > Discover the new Windows Vista > http://search.msn.com/results.aspx?q=windows+vista&mkt=en-US&form=QBRE_______________________________________________ > Elecraft mailing list > Post to: Elecraft@mailman.qth.net > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > > -- View this message in context: http://www.nabble.com/RTTY-filter-for-K3-tp17300221p17301567.html Sent from the Elecraft mailing list archive at Nabble.com. _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
