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
