Roger- thanks for an excellent set of comments. With my rig (FT-920), my best filtering is in the audio, which limits me. I do have a narrower (INRAD) filter than the stock one and that helps. I can dream about a tuneable RF filter, though, can't I? Thanks- Bill-W4BSG
At 05:37 AM 10/1/2007, you wrote: >Bill Aycock wrote: > > Frank- I think that there is MUCH confusion in our ranks on this > > subject. For instance, I set my rig to one frequency (usually > > 14,070.00) and leave it there. I tune to different signals by moving > > the "marker" that shows the offset from the base frequency on the > > waterfall. The radio bandwidth is many times as wide as the signal > > width (for PSK31), and many signals can be accommodated in the > > passband. I have a tuneable Digital filter, and one of the most > > educational tricks is to shift the upper and lower audio limits of > > the filter, and watch the result on the waterfall One of the sources > > of the confusion is the ambiguity in the meaning of "best". I think > > that it is highly improbable that we can get a clear definition. Good > > luck- Bill-W4BSG > > > >In my opinion there are two "classes" of radios for digital use, and >which type you have dictates how you handle the center frequency >question. Older "legacy" radios do not allow use of narrow crystal >filters (originally intended for CW) in the digital modes. The >designers of these radios either ignored the digital modes altogether >(requiring, for example, interfacing the radio through the mic >connector) or simply didn't care much (my FT-900, an otherwise good >radio, falls into this category). These radios can be used for digital, >but lack the most important QRM-fighting tools--the crystal and >mechanical filters. For these radios you can get away with tuning by >leaving the VFO alone and simply "moving the marker" on the waterfall to >the signal you wish to receive. But you will miss receiving many, many >signals if the band is at all crowded if you tune using this technique. > >Newer radios generally all allow use of the narrow crystal and >mechanical filters in the digital modes and typically have a special >"DIGI" mode setting for this purpose. For radios of this type, it is >*very important* to tune the station that you are working to the center >frequency rather than simply moving the "marker" on the waterfall and >not touching the VFO. The reason is that otherwise you cannot use the >crystal and mechanical filters of your rig effectively. The optimal way >to tune a station with a modern rig is to place the received signal in >the center frequency passband, typically either 1000hz (most Yaesu >radios, for example) or 1500 hz, and then utilize the rig's narrow >filter. For PSK31, literally the narrower the better -- for example, my >Mark V's 250hz filters are super for PSK modes, and the 500hz filters >are great for MFSK, 500hz Olivia, Domino, and MT63. Doing this also >makes the IF width and shift controls much more effective, and often you >can use these controls to eliminate even a QRMing signal that is inside >the narrow passband "corridor." > >Placing the received signal in the center frequency passband often makes >a huge difference in your ability to receive a signal. Often if there >is a strong PSK signal elsewhere on the band, it will desensitze your >receiver through AGC action to the point where you are not receiving the >weaker signals at all. Kick in the narrow filters, and this problem >will disappear. This is true even with higher-end rigs such as the >FT1000MP/Mark V. Sure, you can work stations by simply moving the >"marker" to the station you want to work rather than tuning the same >station to your rig's center frequency, but this technique is >sub-optimal and makes the signal you wish to receive subject to QRM and >AGC desensitization even from signals 1Khz or more away. > >You can usually readily see how important 1) tuning the received signal >to the center frequency and 2) kicking in the narrow filters actually >is, by watching the waterfall. Often the waterfall on the rig will be >dark when the passband is wide open, and much lighter on the received >signal once the filters are kicked in. This is because some stronger >signal outside of the filter passband is desensitizing the receiver. I >can often copy signals with the filters in the circuit that are >completely invisible on the waterfall without the filters. > >The best digital programs recognize the importance of tuning the >received signal to the center passband frequency, and make this easy to >do. DM780 has a center frequency marker, and with one click of an icon >it automatically tunes the received signal to the center passband (moves >the received signal "marker" to the "C" center frequency marker). One >more click and you can kick in whichever narrow filters your rig has >that are appropriate for the mode, e.g. 500hz for MFSK, much narrower >for PSK. MixW also enables the user to do this with a few clicks by use >of an easy-to-program macro function. > >As the band conditions start to improve, with more signals on the band >(remember, we are presently right at the solar minimum) using the narrow >filters on digital modes will become more and more important. > >de Roger, W6VZV > > > >Announce your digital presence via our Interactive Sked Page at >http://www.obriensweb.com/drsked/drsked.php > >Yahoo! Groups Links > > > We batter this Planet as if we had someplace else to go.
