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ContinuousWave: Small Boat Electrical
Modern SONAR Systems
|Author||Topic: Modern SONAR Systems|
posted 08-13-2008 10:14 AM ET (US)
I just came across this Lowrance "enhancement:"
It looks like it turns a off the shelf unit into a HiDef one. I wonder how it would work in resolving the detail of a wreck?
posted 08-13-2008 11:54 AM ET (US)
The notion of "digital" SONAR for me is somewhat indistinct. I had a very inexpensive SONAR device made by LOWRANCE which has a raster display of the sonar-grams (or sonagrams). Because this device was not very expensive, the raster only had two levels of presentation--black or white pixels. This is certainly a "digital" SONAR.
posted 08-13-2008 02:13 PM ET (US)
You are somewhat more advanced then me in your understanding of anything electronic. I am just an ols alt, captain, and fisherman. I believe although your unit is digital the signal is analog. (I may be wrong) The new Lowrance Broadband Sounder Module, takes the signal that is returned from the tranducer, and digitally enhances it and cleans the image. That is the difference with these units. Similar to the RayMarine Module for their displays. If I add a second unit to my boat later I may consider the module, but keep in in they cost an additional $585.00 on top of what the unit cost. So in my case I was looking at first adding my NMEA network this year. Then maybe next year I may add the LCX-38C HD to my dash. That unt cureently cost around $1,600. Add the Broadband module and you are aver $2,100.
I do not think you will gain the kind of detail that looks like an image of the wreck, but I think it will help draw fish and other targets more clearly in a cluttered enviroment. Lake Lanier would be a good eniroment for such a unit for several reasons. First the lake is very deep. Secondly it has heavy cover which confuses all but the best sonar units. And lastly it has a thick thermalcline that is present almost constantly throughout the year. That is why so many fisherman on Lanier use high-end Lowrance units. They often push 4000 watts or more and can distinquish the fish among all the water logged structure on the bottom. Laner is just covered with trees that rise 30-60 feet off the bottom. These waterlogged trees are over 50 years old. All but the best units will not even show a fish if they are close to all the structure.
Maybe by the next year the prices will go down enough for me to justify the additional cost. These are my thooughts.
posted 08-13-2008 02:43 PM ET (US)
I haven't found much detail on the Lowrance Broadband unit and how it works. I have deduced (speculation) from their website that it replaces all the sonar functionality in the host display. It sounds like this is a transition product that will become the embedded functionality for all Lowrance mid to high end sonars. Right now they have it as an add-on until the next product refresh cycle.
posted 08-13-2008 04:38 PM ET (US)
My guess is that the software update told the display's firmware to take the data from ethernet and write it to the display and to shut down the embedded sonar. This idea raises an interesting possibility. If the Broadband Sounder has all the electronics and software needed to drive the transducer and present data to the host that's ready for display, then (maybe) one could get a transducer and the Sounder box and add sonar to a chartplotter. Now that would make the price of admission worth it, especially if I had one of the really big screen (expensive) units.
posted 08-13-2008 05:49 PM ET (US)
The device being discussed here is the
LOWRANCE BROADBAND SOUNDER-1
which is a new product for Lowrance that sells for about $600. It is a black box SONAR which needs a display device to show its sonargrams. The BROADBAND SOUNDER-1 connects to an "ethernet-capable Lowrance head unit." I don't know if the connection is truly an Ethernet data network, but the speed and bandwidth of the connection is apparently enough to be able to transport real-time video from the black box sonar to the display unit.
Lowrance does not have a lot of literature available about the BROADBAND SOUNDER-1. The word "digital" appears only in the following context:
"[The BROADBAND SOUNDER-1] delivers digitally-purified marking/separation of fish, structure, thermoclines and bottom, while virtually eliminating surface and turbulent water clutter."
From this I make the inference that there must be some sort of digital signal processing involved in the processing of the echo signals. These days the art of analog-to-digital signal processing has advanced to the point where you can use an analog-to-digital converter (ADC) at very high frequencies, even into high-frequency radio frequencies of 20-MHz or more. This means that at SONAR frequencies, which are in comparison much lower than radio frequencies and well below 0.5-MHz, there is no longer a big cost to use digital technology.
My guess is that the BROADBAND SOUNDER-1 probably receives echo signals from the transducer, amplifies and buffers them to a stronger signal level, and then applies them to an ADC to convert the entire signal to the digital realm. Once the echo signal is in digital form it can be manipulated by a digital signal processor (DSP). Again, the state of the art is so advanced that a DSP is not a big expense. In fact, it might very well be that using an ADC and a DSP might be less expensive than building an analogue signal processing chain.
With the signal and its processing in the digital realm, the processing is accomplished by software algorithms. All sorts of signal enhancement techniques should be possible, such as reducing surface clutter, adjusting gain with increasing depth, enhancing contrast with filters and thresholds, and so on.
The other claim for the Lowrance BROADBAND SOUNDER-1 is that it can deliver superior performance with lower power and with smaller transducers. On outboard boats the size of the transducer and its mounting on the transom are considerations. Previously to get high definition and deep water performance it was necessary to install rather large through-hull transducers. Lowrance says that their smaller SKIMMER transducer will deliver amazing results.
Also, Lowrance says that the BROADBAND SOUNDER-1 can turn a single 200-kHz transducer into a broadband transducer, using frequencies as low as 83-kHz.
posted 08-13-2008 10:00 PM ET (US)
OK. We're getting into the area of advanced signal processing. A field that was pioneered in the sonar world in the '50'-60s (1950-1960). From what little is available on the net about the Lowrance unit it seems to me that it is using some (rather simple) waveforms to get through the clutter. Most consumer sonars use a simple "ping" that's a CW pulse that does the job, most of the time. I suspect that the new Lowrance unit is using an FM pulse that lends itself to some modern signal processing to resolve targets from background clutter. I base this on the Lowrance Users Manual and their website that states that the Broadband Sounder send out a 2000 W pulse but only uses 250 W p-p. To me that means an FM pulse that is spread spectrum within the limits of the transducer. This is a good thing, By taking advantage of the low cost of DSP chips they can bring, what used to be only military grade, sonar to us simple mortals who are looking for some fish.....
posted 08-14-2008 01:35 PM ET (US)
David--If you will, please explain more about the difference in using a simple pulse to excite the transducer and using a frequency-modulated pulse. I am familiar with ultrasonic transducers where there is some piezo-electric resonance. My experience is that these devices are driven (or excited) by application of a rather high-voltage DC pulse. The pulse causes the piezo-electric material to vibrate at its resonant frequency, producing a powerful ultrasonic acoustic output. For example, a 300-Vdc pulse is applied to the transducer, typically from an SCR and capacitor discharge circuit, and this excits the transducer to emit acoustic energy. The resonance of the piezo-electric material shaped the frequency of the output.
My understanding of this technique, which may be flawed or inaccurate, is that the spectrum of the acoustic output from the transducer is not just a single frequency, but instead it is a spectrum of frequencies which are centered on the primary resonance of the transducer. The Q of the transducer would affect the width of the spectrum.
What technique is used to create a frequency modulation of the acoustic output? And how does that enhance the detail of the echoes?
ASIDE: I used to work for Branson Instruments Company, who made ultrasonic devices quite similar to SONAR, which were used as non-destructive test equipment to inspect weldments and for ultrasonic thickness gauging. This was in c.1973 - c.1976.
posted 08-14-2008 04:57 PM ET (US)
Oh, boy. You're asking a lot Jim. To put it simply the transducer can put out frequencies around its resonant frequency, 200 KHZ +/- ~2 KHZ. An FW pulse would go from 198 to 202 KHZ in a long "slide". This long pulse contains more effective power, than a short CW pulse. The whole received pulse is integrated in the receiver circuitry. The software and a hardware DSP can then manipulate the received echo and extract target characteristics like speed (from doppler), reflectivity, and size. Software filters can then be applied to discard undesireable target echos, like from weeds. The big benefit in using more sophisticated waveforms is that by using long pulses the effective power to the "ping" is greatly increased without having to have a monster big power amp driving the transducer.
posted 08-14-2008 07:45 PM ET (US)
David--I think I understand. Instead of a "main bang" with a big pulse of DC, which, if you did a transform from the time domain to the frequency domain would probably be very wide spectrum, you excite the transducer with a tuned burst of energy, right at its resonant frequency. So you get a lot more acoustic output energy per watt of electrical input energy. Is that right?
posted 08-14-2008 09:10 PM ET (US)
But what you also get is the information available in the bandwidth of the signal. This lets one to process the signal with, what are now really cheap, digital signal processors (DSP) that can really do magic with the signals. I still haven't "pinged" Lowrance about any releasable technical details of what they are really doing but I still think that they are doing some more advanced stuff. Hey, they call it the Broadband Sounder.
posted 08-20-2008 09:34 AM ET (US)
Lowrance also mentions in some of their literature about their Broadband Sounder-1 product that it works with their existing transducer, which they previously considered to be a "200-kHz" transducer. With the Broadband Sounder-1 product this same transducer is now made to work at frequencies as low as 83-kHz.
In resonant circuits there is a notion of the sharpness of the resonance, and this is expressed technically as the circuit's Q-factor or just "Q." A high-Q circuit will have a narrow and sharp resonance. A low-Q circuit will have a broader resonance.
In the literature of some other SONAR manufacturers, I am now seeing references to "low-Q" transducers. For example, this publication from AIRMAR mentions SONAR transducers and their Q-factor:
The data presented in the above shows that the resolution of the SONAR is enhanced by using low-Q (or broadband) transducers. Also, from frequent mention of impedance, I also infer that in the newer techniques used there is an attempt at impedance matching in the excitation of the transducer. Or, perhaps a dampening shunt resistance is used to broaden the frequency response of the transducer and lower its Q-factor.
posted 08-26-2008 07:29 PM ET (US)
In military SONARs a GREAT DEAL of attention is placed on impedance matching between the transducer and the water. Most SONARs are in oil filler rubber domes, the physical characteristics are matched to sea water so that there is maximum efficiency in getting electrical power converted into acoustic power in the water. One thing that was discovered as power levels went up is that the transducer could exceed the cavitation limit of the SONAR installation. Boiling water is not good for transmission efficiency. Lower peak power FM pulses were a solution to this problem, by staying below the cavitation threshold more power was put into the water. Digital technology was able to integrate the long pulse into a short high power echo, which is good for range resolution.
posted 08-27-2008 01:35 AM ET (US)
If Lowrance was using a broader (FM) pulse 198kHz - 202kHz); what would you do with the equally smeared spectrum echo.
As Hoosier stated theoretically you are putting more energy into the water; and at the same time reducing the interface impedance.
The only immediate advantage is that you would hopefully get a stronger return signal. But I can't figure out how it would make the returned echo (analyzed spectrum) any easier to resolve. Instead of having a crisp return echo at a defined frequency; the returned echo gets smeared because the initial input was smeared.
Modulated signals in the frequency domain (spectrum) usually result in "sidebands" centered around the carrier frequency. This makes the spectrum harder to assess, not easier.
And I assume that +/- 2 kHz modulation (at 200 kHz) is not enough separation to allow for drastically different returned echoes from objects of slightly different density. I would expect at least a 10% difference in signal frequency to be needed. (But this is out of my area of expertise, just speculating on what level of separation would be needed.)
So then maybe the advantage really is matching impedance, and preserving the transducer to water interface.
posted 09-15-2008 11:15 PM ET (US)
There is a good article in POWER AND MOTORBOAT magazine this month about SONAR transducers, particularly transducers from industry leader AIRMAR. The article explains the Q-factor of a transducer and how it affects performance.
A SONAR transducer that has a high-Q will respond to the transmit pulse by producing ultrasonic output, but it also rings for sometime afterward. The ringing interferes with reception of return echoes. A low-Q or dampened transducer does not ring (as much). As soon as the exciting pulse is removed, the transducer is ready to receive return echoes without interference. The article mentions a bell as an analogy. When struck, a bell rings for a long time. A low-Q transducer does not ring like a bell after being struck by the exciting pulse.
posted 09-16-2008 08:13 AM ET (US)
Most of the time you guys just get way to technical for me but I love reading it anyway. As I mentioned in another post, I replaced my defective dual power Lowrance Skimmer transducer several weeks ago. I would love to know what actually goes bad in a transducer. I have heard of others who had transducers go bad but this was a first for me. Just more of a curiosity thing for me.
posted 09-16-2008 08:55 PM ET (US)
A SONAR transducer is like a drum head. You "bang" it with an electrical pulse and it produces a sound output, although the sound is at ultrasonic frequencies and cannot be heard. If you hit a drum head hard enough and long enough, it eventually wears out.
Also, transducers run submerged or partially submerged. There is a lot of hydrodynamic pressure on the transducer. This pressure could damage the wiring connection or force water into the interior of the transducer.
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