Hi all, This is my first post. Hope I will be handled softly. I have a transmitter which transmits LFM and PCW signals. Both have same PWs around 30-50ms. Frequency of PCW and center frequency for LFM are the same. LFM sweep is small, around 3kHz to 4kHz. My question is that when these are transmitted into a "tank" (The "tank" is big enough to allow these pulse widths), I see that the LFM signal is very badly modulated when compared to the PCW signal. More so if the SNR is less. Some amount of modulation is expected since I am transmitting from the edge of the tank and the boundaries of the tank reflect. I am receiving from the middle of the tank. So there is no reflection at the receiver side. Why is that the LFM signal suffers more? _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Behavior of chirp signal in water
Started by ●April 22, 2007
Reply by ●April 22, 20072007-04-22
Sai2403 wrote:> Hi all, > This is my first post. Hope I will be handled softly. > > I have a transmitter which transmits LFM and PCW signals. Both have same > PWs around 30-50ms. Frequency of PCW and center frequency for LFM are the > same. LFM sweep is small, around 3kHz to 4kHz. > My question is that when these are transmitted into a "tank" (The "tank" > is big enough to allow these pulse widths), I see that the LFM signal is > very badly modulated when compared to the PCW signal. More so if the SNR > is less. Some amount of modulation is expected since I am transmitting > from the edge of the tank and the boundaries of the tank reflect. I am > receiving from the middle of the tank. So there is no reflection at the > receiver side. > Why is that the LFM signal suffers more?I don't understand what you have. Some of what you write makes the tank appear to be a container of some sort: "I am transmitting from the edge of the tank and the boundaries of the tank reflect. I am receiving from the middle of the tank." Some of what you write seems to contradict that: "The 'tank' is big enough to allow these pulse widths", which I interpret to mean that the resonant tank circuit has broad enough bandwidth to allow adequate reproduction of the pulse shapes. Please explain more clearly if you can. jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●April 22, 20072007-04-22
Jerry Avins wrote:> Sai2403 wrote: >> Hi all, >> This is my first post. Hope I will be handled softly. >> >> I have a transmitter which transmits LFM and PCW signals. Both have same >> PWs around 30-50ms. Frequency of PCW and center frequency for LFM are the >> same. LFM sweep is small, around 3kHz to 4kHz. My question is that >> when these are transmitted into a "tank" (The "tank" >> is big enough to allow these pulse widths), I see that the LFM signal is >> very badly modulated when compared to the PCW signal. More so if the SNR >> is less. Some amount of modulation is expected since I am transmitting >> from the edge of the tank and the boundaries of the tank reflect. I am >> receiving from the middle of the tank. So there is no reflection at the >> receiver side. >> Why is that the LFM signal suffers more? > > I don't understand what you have. Some of what you write makes the tank > appear to be a container of some sort: "I am transmitting from the edge > of the tank and the boundaries of the tank reflect. I am receiving from > the middle of the tank." Some of what you write seems to contradict > that: "The 'tank' is big enough to allow these pulse widths", which I > interpret to mean that the resonant tank circuit has broad enough > bandwidth to allow adequate reproduction of the pulse shapes.Further thoughts: the tank is physical. "Big enough to contain ..." has to do with the size of the tank divided by the speed of the signal exceeding the duration of the signal. Do you transmit RF or sound? Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●April 22, 20072007-04-22
Good practice in these situations is to have the projector and the receiver situated as close to the middle of the tank as possible while as far apart as necessary. The measurement must be made while there is temporal separation between boundary reverberation (don't forget the surface and the bottom boundaries) and the direct path. Your description regarding this criterion is so vague as to be highly suspect. Never place a sensor at a boundary unless you really know why you're doing that and understand quite well why that would be OK. You surely have boundary reflections that are contributing to what you see at the receiver. Just for reference, at 5000fps rough speed of sound in water, a 50ms pulse is 100 feet long. Let's assume that separation of TX to RX of 20 feet is OK for your purposes. So, each of them might be10 feet from the center. In order for boundary reverberation to not interfere, the boundaries must be over 50 feet away from the TX/RX pair - so the travel distance, out and back, is at least 100 feet.. So, the tank has to be 100 feet in diameter and 100 feet deep with the sensors at 50 feet - minimum dimensions! Is that what you have? If the transmitter is at an inwardly curved boundary then it's pretty certain that *any* tank size won't work if you expect to get clean pulses at the receiver (which is the normal objective). The type of pulse has very little to do with this. The pulse length obviously has a *lot* to do with it! Fred
Reply by ●April 22, 20072007-04-22
Fred Marshall wrote:> Good practice in these situations is to have the projector and the receiver > situated as close to the middle of the tank as possible while as far apart > as necessary. > > The measurement must be made while there is temporal separation between > boundary reverberation (don't forget the surface and the bottom boundaries) > and the direct path. Your description regarding this criterion is so vague > as to be highly suspect. > > Never place a sensor at a boundary unless you really know why you're doing > that and understand quite well why that would be OK. > > You surely have boundary reflections that are contributing to what you see > at the receiver. > > Just for reference, at 5000fps rough speed of sound in water, a 50ms pulse > is 100 feet long. Let's assume that separation of TX to RX of 20 feet is > OK for your purposes. So, each of them might be10 feet from the center. In > order for boundary reverberation to not interfere, the boundaries must be > over 50 feet away from the TX/RX pair - so the travel distance, out and > back, is at least 100 feet.. So, the tank has to be 100 feet in diameter > and 100 feet deep with the sensors at 50 feet - minimum dimensions! Is that > what you have? > > If the transmitter is at an inwardly curved boundary then it's pretty > certain that *any* tank size won't work if you expect to get clean pulses at > the receiver (which is the normal objective). > > The type of pulse has very little to do with this. The pulse length > obviously has a *lot* to do with it!Reflection from the water-air interface at a shallow angle will produce Lloyd's mirror fringes. Reflection from a solid bottom may not (I need to think about longitudinal waves a bit more.) If not, reflection from a rigid flat wall may also be acceptable if the transmitter and receiver are close to it. Taking advantage of constructive interference might reduce the needed tank volume by a factor of four. Does that sound reasonable? I haven't tried it. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●April 22, 20072007-04-22
On Apr 22, 5:33 am, "Sai2403" <vijayamalap...@yahoo.co.in> wrote:> Hi all, > This is my first post. Hope I will be handled softly. > > I have a transmitter which transmits LFM and PCW signals. Both have same > PWs around 30-50ms. Frequency of PCW and center frequency for LFM are the > same. LFM sweep is small, around 3kHz to 4kHz. > My question is that when these are transmitted into a "tank" (The "tank" > is big enough to allow these pulse widths), I see that the LFM signal is > very badly modulated when compared to the PCW signal. More so if the SNR > is less. Some amount of modulation is expected since I am transmitting > from the edge of the tank and the boundaries of the tank reflect. I am > receiving from the middle of the tank. So there is no reflection at the > receiver side. > Why is that the LFM signal suffers more? > >First, transmitting from the edge of the tank is suspect unless you have prior knowledge of the directionality of the transmitter and keep it isolated from the tank boundary. Hopefully you are not also transmitting from near the surface. Second, how did you decide that the tank is large enough for the pulse width? To prevent Lloyd's mirror effects, the direct and first reflection (from any interface) arrivals must differ by more than the pulse width. Third, any variations in transmission response as a function of frequency will appear as modulation on the LFM, but as a single gain at the fixed frequency of your PCW. Try some tests. If you change the LFM sweep range, what does the modulation look like? Does the modulation remain constant as a function of frequency? If you change the PCW frequency, how does the gain change? Does the PCW gain match the modulation vs frequency of the LFM? Dale B. Dalrymple http://dbdimages.com
Reply by ●April 22, 20072007-04-22
>Hi all, >This is my first post. Hope I will be handled softly. > >I have a transmitter which transmits LFM and PCW signals. Both have same >PWs around 30-50ms. Frequency of PCW and center frequency for LFM arethe>same. LFM sweep is small, around 3kHz to 4kHz. >My question is that when these are transmitted into a "tank" (The "tank" >is big enough to allow these pulse widths), I see that the LFM signal is >very badly modulated when compared to the PCW signal. More so if the SNR >is less. Some amount of modulation is expected since I am transmitting >from the edge of the tank and the boundaries of the tank reflect. I am >receiving from the middle of the tank. So there is no reflection at the >receiver side. >Why is that the LFM signal suffers more? > > > > >_____________________________________ >Do you know a company who employs DSP engineers? >Is it already listed at http://dsprelated.com/employers.php ? >Thanks for the comments. I am sorry for the lack of clarity in my description. My "tank" is actually an area of sea bounded on all sides except for a small gate-way. It could be a 1km by 1km dimension. And both the transmitter and receiver are at a depth of 5 feet and separted by about 200m. The only reason the transmitter is at the shore is operational difficulty. We will need two boats to get both of them into the middle of the tank. Though the edges will reflect and all that, my concern is why is the LFM signal more modulated (there are total breaks in the signal for about 1ms or less). The PCW signal seemed to be less disturbed. The transmitter response in the small sweep of LFM is almost flat. I have tried other frequencies also (not sweeps) but the transmitting response itself was not that good at these frequencies and so it was all the more modulated. Thanks. _____________________________________ Do you know a company who employs DSP engineers? Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●April 22, 20072007-04-22
I think you are seeing two effect. You are too close the surface and with a broadband signal, you are likely seeing the effects of the transmitter power variation as a function of frequency. Variation as a function of frequency may also include strange phase effects. In article <ULadne9awtDYjbHbnZ2dnUVZ_hOdnZ2d@giganews.com>, "Sai2403" <vijayamalapaka@yahoo.co.in> wrote:>>Hi all, >>This is my first post. Hope I will be handled softly. >> >>I have a transmitter which transmits LFM and PCW signals. Both have same >>PWs around 30-50ms. Frequency of PCW and center frequency for LFM are >the >>same. LFM sweep is small, around 3kHz to 4kHz. >>My question is that when these are transmitted into a "tank" (The "tank" >>is big enough to allow these pulse widths), I see that the LFM signal is >>very badly modulated when compared to the PCW signal. More so if the SNR >>is less. Some amount of modulation is expected since I am transmitting >>from the edge of the tank and the boundaries of the tank reflect. I am >>receiving from the middle of the tank. So there is no reflection at the >>receiver side. >>Why is that the LFM signal suffers more? >> >> >> >> >>_____________________________________ >>Do you know a company who employs DSP engineers? >>Is it already listed at http://dsprelated.com/employers.php ? >> > >Thanks for the comments. >I am sorry for the lack of clarity in my description. My "tank" is >actually an area of sea bounded on all sides except for a small gate-way. >It could be a 1km by 1km dimension. And both the transmitter and receiver >are at a depth of 5 feet and separted by about 200m. The only reason the >transmitter is at the shore is operational difficulty. We will need two >boats to get both of them into the middle of the tank. Though the edges >will reflect and all that, my concern is why is the LFM signal more >modulated (there are total breaks in the signal for about 1ms or less). >The PCW signal seemed to be less disturbed. The transmitter response in >the small sweep of LFM is almost flat. >I have tried other frequencies also (not sweeps) but the transmitting >response itself was not that good at these frequencies and so it was all >the more modulated. >Thanks. > > >_____________________________________ >Do you know a company who employs DSP engineers? >Is it already listed at http://dsprelated.com/employers.php ?
Reply by ●April 23, 20072007-04-23
"Jerry Avins" <jya@ieee.org> wrote in message news:KLmdnfXglp7TF7bbnZ2dnUVZ_vCknZ2d@rcn.net...> > Reflection from the water-air interface at a shallow angle will produce > Lloyd's mirror fringes. Reflection from a solid bottom may not (I need to > think about longitudinal waves a bit more.) If not, reflection from a > rigid flat wall may also be acceptable if the transmitter and receiver are > close to it. Taking advantage of constructive interference might reduce > the needed tank volume by a factor of four. Does that sound reasonable? I > haven't tried it. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > �Jerry, Lloyd's mirror does indeed work that way. Think of it as a 2-tap FIR filter. It has a sinusoidal magnitude response depending on the temporal separation of the coefficients (path length difference). As long as the paths are relatively equal magnitude there are zeros as well as peaks. If the paths aren't of equal magnitude then there's a constant term and the troughs aren't zero any more. It remains sinusoidal though - if the model is kept this simple. You don't want any kind of interference, constructive or not. Way too hard to figure out, manage, calibrate, etc. Been there. Methinks you would have to be very hard up to make the measurement and very skilled to attempt this. And then, would probably be very unlucky, frustrated and just plain tired out! Fred ����������������������������������������������������������������������
Reply by ●April 23, 20072007-04-23
On Apr 23, 12:29 am, "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote:> "Jerry Avins" <j...@ieee.org> wrote in message > > news:KLmdnfXglp7TF7bbnZ2dnUVZ_vCknZ2d@rcn.net... > > > > > Reflection from the water-air interface at a shallow angle will produce > > Lloyd's mirror fringes. Reflection from a solid bottom may not (I need =to> > think about longitudinal waves a bit more.) If not, reflection from a > > rigid flat wall may also be acceptable if the transmitter and receiver =are> > close to it. Taking advantage of constructive interference might reduce > > the needed tank volume by a factor of four. Does that sound reasonable?=I> > haven't tried it. > > > Jerry > > -- > > Engineering is the art of making what you want from things you can get. > > =AF > > Jerry, > > Lloyd's mirror does indeed work that way. Think of it as a 2-tap FIR > filter. It has a sinusoidal magnitude response depending on the temporal > separation of the coefficients (path length difference). As long as the > paths are relatively equal magnitude there are zeros as well as peaks. If > the paths aren't of equal magnitude then there's a constant term and the > troughs aren't zero any more. It remains sinusoidal though - if the model > is kept this simple. > > You don't want any kind of interference, constructive or not. Way too ha=rd> to figure out, manage, calibrate, etc. Been there. > Methinks you would have to be very hard up to make the measurement and ve=ry> skilled to attempt this. And then, would probably be very unlucky, > frustrated and just plain tired out! > > Fred > > =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF==AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF= =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF If the tank/setup is an engineering-grade environment, it is possible to remove some of the boundary effects by measuring the impulse response of the tank without the target present. This impulse response can then be used to deconvolve the tank response from the echo achieved from the target. All the typical deconvolution caveats apply, however. I (and other engineers) have done this successfully in characterizing low Ka responses from torpedo and submarine -like structures. Oh, if possible, try to characterize the tank with a PRN waveform for better delay-doppler resolution. Cheers, John
