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#1
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Bandwidth vs. Noise???
Hey all,
My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? TNX 73 |
#2
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Is this an exam question?
If "tweaking" the slugs for a more than six times reduction in bandwidth doesn't break anything your signal/noise should go up by around 8dB -- you'll lose than 8dB of noise, but you'll also lose a bit of the signal's sidebands. Your pulses will be way distorted, though. Did I pass? "gudmundur" wrote in message ... Hey all, My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? TNX 73 |
#3
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Is this an exam question?
If "tweaking" the slugs for a more than six times reduction in bandwidth doesn't break anything your signal/noise should go up by around 8dB -- you'll lose than 8dB of noise, but you'll also lose a bit of the signal's sidebands. Your pulses will be way distorted, though. Did I pass? "gudmundur" wrote in message ... Hey all, My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? TNX 73 |
#4
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#6
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In article , (Tom
Bruhns) writes: (Avery Fineman) wrote in message ... In article , (gudmundur) writes: My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? Signal to noise ratio changes as the _square_root_ of bandwidth change. Wouldn't be much of an effect going from 1.25 to 1.5 MHz. Um, he was starting with an 8MHz BW... Ooops, mea culpa! My fault. Please see reply to Paul in other post. With 0.8 uSec pulses and a 1.25 to 1.5 MHz bandwidth (I presume Mega Hertz, not milli Hertz), the output envelope will be very rounded, almost Guassian or "cosine-quared" in shape. Rounding happens because of the limitation of passing the harmonics of the pulsed RF; all you have left is the carrier frequency. Yes, the pulses will certainly be rounded when they come out of the filter (though they may have started that way anyway). But depending on the filter type, they may also incur lots of ringing, and if the pulses follow one after another at the right spacing, the phase of the energy in the pulse relative to the phase of the energy left in the filter will matter a whole lot in what you see coming out. The trailing edge of a rectangular pulse fed through a Chebychev filter isn't very Gaussian looking! True. "Gaussian" shape is so often used incorrectly when so many equate that to the statistical distribution curve shape also referred to as "Gaussian." Chebs and Cauers (elliptical) all exhibit ringing in an L-C component application...but it isn't quite the same kind of shape resulting with equal group delays of SAW or digital filters. In terms of RF Envelope shape, the envelope has little hangovers at the trailing edges (I call them "burbles" in my mind, heh heh). Now, some of that comes from energy stored-and-released-at-a-later-time (commonly called "ringing") but I think (from analysis and simulation) that it is the result of pulse sideband energy content summation that includes the relative sideband phases. When working with SAW filters at the 3rd generation of RCA's SECANT (previously mentioned in reply to Paul), there was almost NO ringing possible (or observed) and the "matched-filter" effect was absolutely as predicted and observed in the earlier generations using L-C filters. As for the original question, the answer depends on the spectral distribution of the noise...if it happens to be strongly peaked at the carrier frequency of the pulses, the narrowing won't make much difference; if it happens to be peaked at some other frequency, it may help a lot. If it's uniformly distributed, AND you keep the filter shape the same and narrow the bandwidth by 1.5:8, then you will have 1.5/8 as much noise _power_. You'll also have somewhat less signal power, depending on the shape of the pulses. And of course, the filter won't get rid of noise that's introduced after the filter--fairly obvious but sometimes overlooked. In looking at the basic system, one has to assume that the random noise IS uniform in energy distribution. If the real world has a different set, such as peaking at certain parts of the spectrum, that can be calculated later and overall S:N modified...while keeping all the other factors the same. Any other method of looking at too many variables at once results in long hours and a marked increase in aspirin intake. :-) I've got a copy of Claude Elwood's original "Shannon's Law" 1948 paper in the BSTJ and still need to keep the Tylenol bottle handy when I study that again. It makes sense, but getting to the "sense" part isn't intuitive. Neither is time-domain response of filters. If it weren't for the computer simulation programs, I'd still have to rely on old aphorisms of a very general nature. :-( I once spent a fruitless night trying to figure out the spectral content of an RF pulse that was on for only one RF cycle. The next day I brown-bagged it and hooked up some test equipment during lunch hour to get the results. Rather remarkable spectral content shape and the RF waveshape was exactly one RF cycle as observed on a wideband scope. Never did finish the analysis. Sometimes ya hafta get down and dirty on the bench to prove a point and get results. Len Anderson retired (from regular hours) electronic engineer person |
#7
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In article , (Tom
Bruhns) writes: (Avery Fineman) wrote in message ... In article , (gudmundur) writes: My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? Signal to noise ratio changes as the _square_root_ of bandwidth change. Wouldn't be much of an effect going from 1.25 to 1.5 MHz. Um, he was starting with an 8MHz BW... Ooops, mea culpa! My fault. Please see reply to Paul in other post. With 0.8 uSec pulses and a 1.25 to 1.5 MHz bandwidth (I presume Mega Hertz, not milli Hertz), the output envelope will be very rounded, almost Guassian or "cosine-quared" in shape. Rounding happens because of the limitation of passing the harmonics of the pulsed RF; all you have left is the carrier frequency. Yes, the pulses will certainly be rounded when they come out of the filter (though they may have started that way anyway). But depending on the filter type, they may also incur lots of ringing, and if the pulses follow one after another at the right spacing, the phase of the energy in the pulse relative to the phase of the energy left in the filter will matter a whole lot in what you see coming out. The trailing edge of a rectangular pulse fed through a Chebychev filter isn't very Gaussian looking! True. "Gaussian" shape is so often used incorrectly when so many equate that to the statistical distribution curve shape also referred to as "Gaussian." Chebs and Cauers (elliptical) all exhibit ringing in an L-C component application...but it isn't quite the same kind of shape resulting with equal group delays of SAW or digital filters. In terms of RF Envelope shape, the envelope has little hangovers at the trailing edges (I call them "burbles" in my mind, heh heh). Now, some of that comes from energy stored-and-released-at-a-later-time (commonly called "ringing") but I think (from analysis and simulation) that it is the result of pulse sideband energy content summation that includes the relative sideband phases. When working with SAW filters at the 3rd generation of RCA's SECANT (previously mentioned in reply to Paul), there was almost NO ringing possible (or observed) and the "matched-filter" effect was absolutely as predicted and observed in the earlier generations using L-C filters. As for the original question, the answer depends on the spectral distribution of the noise...if it happens to be strongly peaked at the carrier frequency of the pulses, the narrowing won't make much difference; if it happens to be peaked at some other frequency, it may help a lot. If it's uniformly distributed, AND you keep the filter shape the same and narrow the bandwidth by 1.5:8, then you will have 1.5/8 as much noise _power_. You'll also have somewhat less signal power, depending on the shape of the pulses. And of course, the filter won't get rid of noise that's introduced after the filter--fairly obvious but sometimes overlooked. In looking at the basic system, one has to assume that the random noise IS uniform in energy distribution. If the real world has a different set, such as peaking at certain parts of the spectrum, that can be calculated later and overall S:N modified...while keeping all the other factors the same. Any other method of looking at too many variables at once results in long hours and a marked increase in aspirin intake. :-) I've got a copy of Claude Elwood's original "Shannon's Law" 1948 paper in the BSTJ and still need to keep the Tylenol bottle handy when I study that again. It makes sense, but getting to the "sense" part isn't intuitive. Neither is time-domain response of filters. If it weren't for the computer simulation programs, I'd still have to rely on old aphorisms of a very general nature. :-( I once spent a fruitless night trying to figure out the spectral content of an RF pulse that was on for only one RF cycle. The next day I brown-bagged it and hooked up some test equipment during lunch hour to get the results. Rather remarkable spectral content shape and the RF waveshape was exactly one RF cycle as observed on a wideband scope. Never did finish the analysis. Sometimes ya hafta get down and dirty on the bench to prove a point and get results. Len Anderson retired (from regular hours) electronic engineer person |
#8
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I went from 8 mhz to 1.5 mhz,,,,
As for breaking things, the wide i.f. was stagger tuned to achieve bandwidth and swamped with resistors, I could chop the resistors, and retune, and get 25khz bandwidth if I wanted to. It is a short pulse radar i.f., and it will be used in a long pulse application,,, Therefore going from 8mhz at 6db edges to 1.5mhz at 6db edges is a must. Any narrower, and I lose object resolution, any wider, and I amplify unwanted and detrimental noise. In article , says... In article , (gudmundur) writes: My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? Signal to noise ratio changes as the _square_root_ of bandwidth change. Wouldn't be much of an effect going from 1.25 to 1.5 MHz. With 0.8 uSec pulses and a 1.25 to 1.5 MHz bandwidth (I presume Mega Hertz, not milli Hertz), the output envelope will be very rounded, almost Guassian or "cosine-quared" in shape. Rounding happens because of the limitation of passing the harmonics of the pulsed RF; all you have left is the carrier frequency. The relatively narrow bandpass and pulse rounding MAY be okay in your application. It wasn't stated. I spent some years on a program that deliberately used 1 MHz bandwidth filters for 1 uSec wide pulses on carriers 1 MHz apart. Interesting to see the effect of "matched filters" on adjacent frequencies...those immediately on each side came through with a reduced amplitude "bow tie" envelope shape, the "knot" in the middle. Len Anderson retired (from regular hours) electronic engineer person |
#9
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(Avery Fineman) wrote in message ...
In article , (gudmundur) writes: My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? Signal to noise ratio changes as the _square_root_ of bandwidth change. Wouldn't be much of an effect going from 1.25 to 1.5 MHz. Um, he was starting with an 8MHz BW... With 0.8 uSec pulses and a 1.25 to 1.5 MHz bandwidth (I presume Mega Hertz, not milli Hertz), the output envelope will be very rounded, almost Guassian or "cosine-quared" in shape. Rounding happens because of the limitation of passing the harmonics of the pulsed RF; all you have left is the carrier frequency. Yes, the pulses will certainly be rounded when they come out of the filter (though they may have started that way anyway). But depending on the filter type, they may also incur lots of ringing, and if the pulses follow one after another at the right spacing, the phase of the energy in the pulse relative to the phase of the energy left in the filter will matter a whole lot in what you see coming out. The trailing edge of a rectangular pulse fed through a Chebychev filter isn't very Gaussian looking! As for the original question, the answer depends on the spectral distribution of the noise...if it happens to be strongly peaked at the carrier frequency of the pulses, the narrowing won't make much difference; if it happens to be peaked at some other frequency, it may help a lot. If it's uniformly distributed, AND you keep the filter shape the same and narrow the bandwidth by 1.5:8, then you will have 1.5/8 as much noise _power_. You'll also have somewhat less signal power, depending on the shape of the pulses. And of course, the filter won't get rid of noise that's introduced after the filter--fairly obvious but sometimes overlooked. Cheers, Tom |
#10
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I went from 8 mhz to 1.5 mhz,,,,
As for breaking things, the wide i.f. was stagger tuned to achieve bandwidth and swamped with resistors, I could chop the resistors, and retune, and get 25khz bandwidth if I wanted to. It is a short pulse radar i.f., and it will be used in a long pulse application,,, Therefore going from 8mhz at 6db edges to 1.5mhz at 6db edges is a must. Any narrower, and I lose object resolution, any wider, and I amplify unwanted and detrimental noise. In article , says... In article , (gudmundur) writes: My current I.F. bandwidth is 8mhz at the 6db points. I am looking at pulses of .8microseconds length, or about 1.25mhz. If all else remains the same, and I change the swamping resistors, and tweak the slugs for a 1.5mhz I.F. bandwidth at the 6db points, what increase in signal to noise ratio should I see? Signal to noise ratio changes as the _square_root_ of bandwidth change. Wouldn't be much of an effect going from 1.25 to 1.5 MHz. With 0.8 uSec pulses and a 1.25 to 1.5 MHz bandwidth (I presume Mega Hertz, not milli Hertz), the output envelope will be very rounded, almost Guassian or "cosine-quared" in shape. Rounding happens because of the limitation of passing the harmonics of the pulsed RF; all you have left is the carrier frequency. The relatively narrow bandpass and pulse rounding MAY be okay in your application. It wasn't stated. I spent some years on a program that deliberately used 1 MHz bandwidth filters for 1 uSec wide pulses on carriers 1 MHz apart. Interesting to see the effect of "matched filters" on adjacent frequencies...those immediately on each side came through with a reduced amplitude "bow tie" envelope shape, the "knot" in the middle. Len Anderson retired (from regular hours) electronic engineer person |
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