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#131
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Wind/Solar Electrics ???
You can represent the bandwidth with double the
sampling rate as the bandwidth frequency but there is a component missing from the sample information that has to be known and is not part of the samples. Namely the base frequency has to be added back into the formula. "daestrom" wrote in message ... So, if I have a signal with a 1000 hz carrier, with a bandwidth of 50 hz, you think I can sample it at just 150 hz and get accurate reproduction? That's just wrong. It is the maximum frequency component in the signal that is important. The bandwidth is not related unless the lower edge of the band is at 0 hz (whereupon the upper side of the band is equal to the max frequency). daestrom |
#132
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Wind/Solar Electrics ???
In article ,
"daestrom" wrote: "Spehro Pefhany" wrote in message ... On Fri, 23 Dec 2005 18:46:49 GMT, the renowned "daestrom" wrote: wrote in message ... Joel Kolstad wrote: (I can't tell you how many times I've seen people stating something like, 'The Nyquist theorem requires sampling at at least twice the highest frequency present in the signal," when of course it says no such thing.) What do you think it means? Nyquist figured out that higher frequency components of the input signal will 'alias' and you will lose the ability to tell them from lower frequency components. In order to avoid 'losing information' and not being able to tell whether a particular sample stream was from a low or high frequency component, Nyquist's theorem states you must sample at least twice as fast as the highest component present. snip More than twice the bandwidth. So, if I have a signal with a 1000 hz carrier, with a bandwidth of 50 hz, you think I can sample it at just 150 hz and get accurate reproduction? That's just wrong. It is the maximum frequency component in the signal that is important. The bandwidth is not related unless the lower edge of the band is at 0 hz (whereupon the upper side of the band is equal to the max frequency). daestrom You are getting your terms confused here guys. Nyquist requires that you input both the Center Frequency, and Bandwidth when determining the Sampling Rate. If the sampling is done at BaseBand then only the Bandwidth is relevent. If the sampling is not done at baseband, then the Center Frequency, and Bandwidth are required to determine samling rate. Example, if the Bandwith of the signal is 3Kc and the sampling is done at BaseBand then sample rate needed would 6Kc. If the sampling is done at 100 Mhz with the same 3Kc bandwidth, then a 200.006 Mhz sampling rate would be required. It is much easyier to do DSP at baseBand, than at IF Frequencies, and if you do DSP at IF Frequencies, the lower the IF Frequency, the easyier it is to do, and the slower the DSP has to run. Me |
#133
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Wind/Solar Electrics ???
If only the baseband frequency is sampled at 6kHz then
information is missing to recreate the original 100kHz and the sampling information is insufficient to recreate the original signal. This is analogous to saying the number 1234 can be represented by (1234-234) / 1000 = 1 If I supply the number 1.0 you can regenerate the number 1234 from it? Not true, without the rest of the sampling information. The sample is incomplete. Bandwidth sampling only cannot recreate the original signal. "Me" wrote in message ... In article , "daestrom" wrote: "Spehro Pefhany" wrote in message ... On Fri, 23 Dec 2005 18:46:49 GMT, the renowned "daestrom" wrote: wrote in message ... Joel Kolstad wrote: (I can't tell you how many times I've seen people stating something like, 'The Nyquist theorem requires sampling at at least twice the highest frequency present in the signal," when of course it says no such thing.) What do you think it means? Nyquist figured out that higher frequency components of the input signal will 'alias' and you will lose the ability to tell them from lower frequency components. In order to avoid 'losing information' and not being able to tell whether a particular sample stream was from a low or high frequency component, Nyquist's theorem states you must sample at least twice as fast as the highest component present. snip More than twice the bandwidth. So, if I have a signal with a 1000 hz carrier, with a bandwidth of 50 hz, you think I can sample it at just 150 hz and get accurate reproduction? That's just wrong. It is the maximum frequency component in the signal that is important. The bandwidth is not related unless the lower edge of the band is at 0 hz (whereupon the upper side of the band is equal to the max frequency). daestrom You are getting your terms confused here guys. Nyquist requires that you input both the Center Frequency, and Bandwidth when determining the Sampling Rate. If the sampling is done at BaseBand then only the Bandwidth is relevent. If the sampling is not done at baseband, then the Center Frequency, and Bandwidth are required to determine samling rate. Example, if the Bandwith of the signal is 3Kc and the sampling is done at BaseBand then sample rate needed would 6Kc. If the sampling is done at 100 Mhz with the same 3Kc bandwidth, then a 200.006 Mhz sampling rate would be required. It is much easyier to do DSP at baseBand, than at IF Frequencies, and if you do DSP at IF Frequencies, the lower the IF Frequency, the easyier it is to do, and the slower the DSP has to run. Me |
#134
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Wind/Solar Electrics ???
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#135
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Wind/Solar Electrics ???
Take the credit when you can as it does not happen that often.
wmbjk wrote: On Sat, 24 Dec 2005 09:33:02 +1100, George Ghio wrote: On 22 Dec 2005 19:49:37 -0800, "philkryder" wrote: What can we use to "know for sure" that the wave form of a device is adequate BEFORE buying it? wmbjk wrote: I've purchased a couple of ~$1500 machines from a local welding supplier on condition that if there were any problems running them off my SW inverters then the machines could be returned in as-new condition the following day and I'd buy a different model instead. That flexibility, and being able to see the machines in person, made the extra cost of buying locally worthwhile. Wayne A fine example of the correct approach. Oh crap. Since your agreement has to be counted as a negative, now anyone reading will have to wait for someone credible to concur. Wayne |
#136
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Wind/Solar Electrics ???
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#137
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Wind/Solar Electrics ???
I'll pay that. Thank you for the correction. Spent too many hours under
the car, I guess. daestrom wrote: "George Ghio" wrote in message ... Yes this is the problem. While there are people who will tell you anything to make a sale, how do you know what you are really getting. One test is the "Modified Square Wave" test. When you hear these words you know you are dealing either with a shyster or an ignorant person who should not be selling things he does not understand. Judging from your previous posts, I think you mean when you hear "Modified *sine* wave", then you know you are dealing with shyster or an ignorant person...." A salesperson that says their unit puts out a modified *square* wave would be a sign[sic] of a knowledgable salesperson. daestrom |
#138
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Wind/Solar Electrics ???
On Thu, 22 Dec 2005 20:36:17 GMT, "daestrom"
wrote: "Roger" wrote in message .. . On Wed, 21 Dec 2005 14:29:00 GMT, Matt Whiting wrote: George Ghio wrote: Tell us why anyone would modify a sine wave. To vary the power delivered to a load. Chopping off part of a sine wave cycle is a standard means of power control. That makes three phase SCR (Silicon controlled rectifiers and not saturable core reactors) interesting as chopping off part of the wave form develops spikes and harmonics that tend to make the control of one phase interact with the others. I've built a lot of them for single phase control, but I never once was able to build one for three phase that didn't interact. Turn one up and maybe another would go up, Turn the second down and the other two might go up or down. Twas interesting:-)) which is probably why Saturable core reactors are so popular in industry. Now there is a controller that is a tad on the weighty side. Also, some old systems used self-saturating reactors (magnetic amplifiers, 'magamps') for instrumentation. Things could take some severe environments, but calibration tended to drift a lot. Required fairly frequent 'trip & cals' to keep them in spec. For the small and large stuff we used solid state SCRs while the intermediate still used saturable core reactors. 10 years ago I think they still had some mag amps, but the ones we had were pretty stable. They use larger SCRs now, but I have the silicon wafer out of one that is over 1 1/2" in diameter. They operated up to 480 and 1000 Amps and ran near maximum for many hours. The SCRs themselves were about the size of a hockey puck or slightly larger. Now there was some power and I'd guess they use much larger systems now. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com daestrom |
#139
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Wind/Solar Electrics ???
So, if I have a signal with a 1000 hz carrier, with a bandwidth of 50 hz,
you think I can sample it at just 150 hz and get accurate reproduction? That's just wrong. No, that's the whole point of this discussion. You have to understand aliasing. The signal you want aliases down into the baseband. Your anti-aliaising filter has to get rid of all the junk you don't want. In this case it includes the baseband. Since there is no baseband signal (or other out-of-band junk) you can reconstruct the original signal. It's a common trick with software radios. You do need some extra information that doesn't go in through the A/D channel. That's the design of the system, in particular what the anti-aliasing filter lets through. Maybe the reason that this is so confusing is that you also need that info the the normal/baseband case. But since that's the normal case we don't bother mentioning it. -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. |
#140
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Wind/Solar Electrics ???
On Sun, 25 Dec 2005 17:50:50 +1100, George Ghio
wrote: "George Ghio" wrote in message ... Yes this is the problem. While there are people who will tell you anything to make a sale, how do you know what you are really getting. One test is the "Modified Square Wave" test. When you hear these words you know you are dealing either with a shyster or an ignorant person who should not be selling things he does not understand. daestrom wrote: Judging from your previous posts, I think you mean when you hear "Modified *sine* wave", then you know you are dealing with shyster or an ignorant person...." A salesperson that says their unit puts out a modified *square* wave would be a sign[sic] of a knowledgable salesperson. daestrom I'll pay that. Thank you for the correction. Spent too many hours under the car, I guess. OHMYGOD! A blunder admitted! Who the hell are you and what have you done with George Ghio? Wayne PS It's in everyone's best interest that you not be found out, so don't forget to hide your pod. |
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