On Sat, May 1, 2010 at 2:20 PM, Brian Lloyd <[email protected]> wrote:
BTW, if you look at how channels are defined in commercial SSB operation, > they specify the passband width (3kHz) and a center frequency for > thechannel as that clearly defines precisely where the RF ends up in the > spectrum. > But that's also simply convention - there's no more info there than saying the (suppressed) carrier frequency is 8080 Khz, and the signal is USB, 3 KHz. That's 8080 to 8083, just as saying the center frequency is 8081.5 KHz, and the signal is 3 KHz centered there, is *also* 8080-8083. Strictly convention. Not more, or better, information. So the question becomes, since these are not different from one another in terms of describing what's actually happening with the RF, is there are a reason to use one or the other at any other level? And there are several that I've identified, all of which fall on the carrier frequency side of the argument, rather than the center. I'm very interested to learn what benefit one might gain from center and excursion, instead, because that's kind of what the subject is. So far, nothing. > > Ok. Here's the crux of the matter. Why does "the operating ham need to > know > > this"? > > Because he needs to know precisely where his RF is going so that he > doesn't operate outside the band. Here in the US the FCC doesn't give > a rat's patootie what the dial frequency is, only if the RF goes > outside the spectrum allocated to the class of license held by the > operator. > So, you have a 3 Khz USB signal, you don't go beyond 3 KHz of near the band's upper edge. Are you telling me you don't think hams should know that? Heck, if they don't know that, they're not even qualified to operate voice. Further, if your radio is on 14.349 KHz, and you intend to transmit 170 Hz around that (presuming you were allowed to), but the cat pulls your AF wire, you're all of a sudden sending noise from 14.349-14.253 KHz... No, I'm not buying that one. You should be tuned to the suppressed carrier frequency, MAX at 14.247, so you KNOW you won't be sending out of band. That's the correct way to handle OOB tuning. Next? > You are thinking too much from the point of view of doing things for > the radio, not the radio doing things for you. > The radio isn't going to re-tune the if shift, or correct the AF output for external hardware, or eliminate AGC signals from a wide-open passband, is it? So this isn't a valid point. > > Please enlighten me here. What *is* fldigi's display of RF, as opposed > to > > AF, frequency good for, and why does the operator "need to know"? > > Uh, because the operator might not want a pink ticket? > No, that's already been disposed of. It's not sensible. If the op doesn't know where he's allowed to operate, and is setting his radio so that the transmit capability overlaps the edge of the band, what's he doing on a ham band in the first place? > > On the other hand, if the waterfall is set to AF, and the modem is set > > correctly (let's say that RTTY mark is at 2125 Hz where it belongs), > > Why does it belong at 2125? 1) Because every chunk of hardware designed for RTTY expects that frequency (and there are a lot of great AF filters and other devices, such as displays or teletypes, out there); 2) because the higher in frequency it is, the sharper the mark-space transitions can become 3) because that puts your radio, when its in the proper sideband, on the same frequency to transmit back > I never use 2125 for RTTY. I happen to use 1500 because that is the center > of my passband and I have my filters centered on that. I just switch the > width to match the emission mode but they are still centered on 1500 Hz low > IF. > Well, then you're doing it less well than you could be. Generally speaking, you'd get better performance at 2125. Not to mention better compatibility with other hardware and direct rx/tx frequency correlation for AFSK users (ie, most of us.) > You mean by mentally setting your radio dial 2125 Hz below the > frequency where you actually want to operate. No, I don't mean that at all. I simply slot the signal where the modem says it should go, then check the radio to make sure that's inside the band limits so I don't get a ticket if I transmit. There is no other step. I'm tuned, ready to go, AGC is tight inside the signal bandwidth, IF shift is correct, external hardware is set perfectly, nothing else to do but enjoy everything working correctly. One hand motion. No mental gymnastics that exceed what you have to do (we both need to look at where we're operating, for instance... you can't get out of that.) > I understand why you do that. We have been doing that in our heads for > probably 75 years when operating RTTY. But see, you didn't understand at all, because I don't even do it. > But that doesn't mean that it is what is really > happening. It is what we do by convention to ensure that we put the > signal where it should be in the RF spectrum. > We are generally using a VOICE mode (USB, LSB) with a 3 KHz bandwidth to work these modes. We should remain sensible of that at all times. If for no other reason than the transmitter is capable of sending significant RF anywhere from 300 Hz or so above the (suppressed) carrier frequency to 3 KHz, and so we should NEVER tune the transmitter further than 3 KHz of the edge of the band (upper for USB, lower for LSB.) You may have an audio jack plugged in that you think will never carry a signal other than what you mean to send, and then your cat or dog or your own tripping foot pulls the jack, and bingo, you're transmitting out of band. So the right thing, in the very most technical sense, is to control the frequency of the transmitter. Not operate by some sum that is made up of audio and RF frequencies and that -- at the moment, because the cat is asleep -- doesn't exceed the band edge. > 73 de Brian, WB6RQN/J79BPL >
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