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#111
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Wind/Solar Electrics ???
In article .com,
"philkryder" wrote: "MSW is a shysters sales pitch which misrepresents the product. " Are there deterministic tests that tell when a device has a "good enough" sine wave? Or is there some sort of accepted "spec"? I saw in another post where one of the EU2000 hondas had a beautiful "looking" wave form, but failed to run a furnace. What can we use to "know for sure" that the wave form of a device is adequate BEFORE buying it? Thanks Phil You can use a college education in Electrical Engineering, and $40K worth of test equipment, to "KNOW FOR SURE"...... or you can fool around and see what works........ or you can ask one who already did the previous, and figured it out, and then actually believe what they tell you........ other than that your on your own...... Me |
#112
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Wind/Solar Electrics ???
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. http://www.cs.cf.ac.uk/Dave/Multimedia/node149.html http://www.efunda.com/designstandard...sp_nyquist.cfm A lot of folks mistake it to think you need to sample at least twice as fast as the 'signal of interest' also, and try to ignore high frequency components of the input because they're 'not interested in that noise'. But what Nyquist proved was that any frequency in the sampled signal that is more than 1/2 the sample frequency will 'alias' and 'wrap around' and be *indistinguisable* from a frequency component that is less than 1/2 the sample frequency. For example, if sampling at 1000 hz, and the sampled signal is a 900 hz 'pure sine wave', the sampled data would look *exactly* the same as if you had sampled a 100 hz 'pure sine wave'. They would be 'indistinguisable'. So if/when you try to convert the sampled data back to analog, how do you know whether it should reconstruct a 100 hz wave, or 900 hz? You don't, so you have a conundrum. So, to avoid losing this 'information' of being able to tell if you had a 100 hz or 900 hz input, the standard thing to do is filter the input so that there is *no* 900 hz input. Then, the resulting sample data must have come from the 100 hz component. And if/when you want to reconstruct it, you *know* it should form a 100 hz signal because no 900 hz signal could possibly been present (you eliminated it before sampling). And as Joel mentioned earlier, since most low-pass filters do not have perfect 'cutoff' (IIRC, simple first-orders 'roll off' at something like 3 db/decade), it is more common to eliminate any frequency component that is more than about 40% of the sampling frequency. daestrom |
#113
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Wind/Solar Electrics ???
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. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |
#114
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Wind/Solar Electrics ???
With risers it is the same confusion.
"Rich Grise" wrote in message news On Thu, 22 Dec 2005 22:01:02 -0500, SolarFlare top-posted: Three risers though. Yabbut, that misses the point of the gag. It's easy to point at the three risers: . ------------ . | -- 1 , ----- . | --2 . ----- . | -- 3 . ------------- And, obviously, the middle one is #2. But, while stepping up or down the stairs, the way most people count steps, if you're going down, (to the right) you'd go: . ------------ . | 1 , ----- . | 2 . ----- . | 3 . ------------- And count 3 steps. But if you're going up, which is right-to-left in this exsample, you'd go: 3 . ------------ . | 2 , ----- . | 1 . ----- . | . ------------- because where you started from is zero in either case, but step 2 is different if you're going up or down. Hope This Hemps!^H^H^H^Hlps! %-} Rich "Roger" wrote in message ... Or this one: Imagine a short staircase, say to a "sunken living room" or some such, of 3 steps: ------------ | ----- | ----- | --------------------------- There are only two steps. on the stairway. The others are landings Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com |
#115
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Wind/Solar Electrics ???
Joel Kolstad wrote:
1) It's the bandwidth of the signal that matters, not the highest frequency present... One might say "the highest frequency present" is the highest frequency non-zero component of the power spectrum. Nick |
#116
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Wind/Solar Electrics ???
wrote in message
... Joel Kolstad wrote: 1) It's the bandwidth of the signal that matters, not the highest frequency present... One might say "the highest frequency present" is the highest frequency non-zero component of the power spectrum. Sure, but the point is that you can sample a signal that's has all (of a good approximation thereof, e.g., 99%) of its energy between 144-148MHz (this is the 2m amateur radio band) at 10MSps and recover everything. I.e., the bandwidth of the signal is only 4MHz, so you only have to sample at something 8MSps. |
#117
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Wind/Solar Electrics ???
Joel Kolstad wrote:
Hey Joel, what are you doing over here. Are you a pilot too? I've used this for subsampling, although you have to be very careful of clock jitter when you sub-sample. a couple picoseconds of jitter on the sampling of a 100 MHz signal is going to add substantial noise to the signal. (subsampling, for those here who haven't a clue what we are talking about...this is an airplane owner's forum after all...is taking advantage of the nyquist theorum to sample at less than the frequencyt of the signal when the bandwidth of the signal is narrow. For example, if you have a signal centered at 100 Mhz that only has a 10 MHz bandwidth, you can sample it at something less than 100 MHz and still recover all of the information. The more generally held belief is that you would need to sample it at greater than 200 MHz in order to not lose information). |
#118
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Wind/Solar Electrics ???
"Ray Andraka" wrote in message
news:deZqf.31975$Mi5.3388@dukeread07... Hey Joel, what are you doing over here. Are you a pilot too? Hi Ray! Hmmm... no, I'm not a pilot, I just got sucked in by the cross-posting (starting from sci.electronics.design) and the topic has drifted considerably since it started. I've used this for subsampling, although you have to be very careful of clock jitter when you sub-sample. a couple picoseconds of jitter on the sampling of a 100 MHz signal is going to add substantial noise to the signal. Yes it is... I suspect that's why that projects such as GNURadio (which sub-samples using something like 80 or 100MSps ADCs) tend not to be as sensitive as more traditional analog receivers. (Someone made the comment that the FM decoder in GNURadio doesn't really even work as well as a $5 transistor radio, which is true enough albeit perhaps missing the point of how cool/fun it is to be able to write any modulator/demodulator you like if you're not looking for the ultimate sensitivty.) (For example, if you have a signal centered at 100 Mhz that only has a 10 MHz bandwidth, you can sample it at something less than 100 MHz and still recover all of the information. The more generally held belief is that you would need to sample it at greater than 200 MHz in order to not lose information). I believe that folks who think you need to sample at 200MHz (the intuitively reaosnable answer) are those who were never made to /had the opportunity to open up an undergraduate signals & systems book. :-) Thinking about things like modulation are so much cleaner in the frequency domain once one gets the whole "multiplication in one domain is convolution in the other" bit down. ---Joel |
#119
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Wind/Solar Electrics ???
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. It is hard, what with a flood of imports at bargain basement prices. Still, as long as people are willing to believe that a $59 3000W "modified sine wave" inverter from Walmart, Cost Co, etc, etc has the same specs as a $900 3000W sine inverter is, at best, fooling themselves. I buy inverters from known manufacturers who are willing to provide spec sheets that out line the full parameters of the inverter. You know things like: Efficiency curves Max continuous output 1/2 hour rating Surge Standby Max DC in Well everything really. I did build a kit inverter, once, years ago. It had a max rating of 150W, Which it met. It had a half hour rating of 0W And a surge of about 300W Still it did the job it was built to do for many years. Put your supplier on the spot. Tell them your load and buy on the condition that what they are selling you will do what they claim or you get a full refund, no questions asked. philkryder wrote: "MSW is a shysters sales pitch which misrepresents the product. " Are there deterministic tests that tell when a device has a "good enough" sine wave? Or is there some sort of accepted "spec"? I saw in another post where one of the EU2000 hondas had a beautiful "looking" wave form, but failed to run a furnace. What can we use to "know for sure" that the wave form of a device is adequate BEFORE buying it? Thanks Phil |
#120
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Wind/Solar Electrics ???
A fine example of the correct approach.
wmbjk 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? 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 |
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