Signal generator for my 1MHz underwater range finding project - Page 1
Forum section: electronics / testgear

author: Nevereven
author_rank: Contributor
timestamp: April 05, 2015, 03:49:24 am
content: So I'm about to get into an area I don't have much experience with and rather than buy the wrong thing I thought I'd ask what features I should look for.I hope to develop a system for short range (< 5m ) underwater range finding. The transducer I want to use is rated at 1MHz which is higher than your typical underwater transducer. I figure I need something with an external trigger so the function generator can generate a short (20 u s) 1MHz pulse and my oscilloscope will start capturing at the same time. I don't think fidelity or wave form accuracy will be very important in my application as I just need to amplify the signal then detect the reflected signal and measure the time difference. I'm not looking for multiple targets (fish), just the largest (bottom).I haven't done any analog electronics in several decades so I'm not at all familiar with analog signal generators, much less a DDS. I'm not opposed to used gear from eBay or even some cheap chinese device if it will get the job done. Portable or semi-portable is a plus as I expect to have to do some field testing once I'm done with a barrel of water. Mains powered is fine as I'll have my oscilloscope out on the dock with me anyways.Thanks in advance for any advice and suggestions on what might suit my needs.

author: kaz911
author_rank: Super Contributor
timestamp: April 05, 2015, 04:09:10 am
content: Why use 1 MHz? for the higher resolution? Or the number of fast returns? Usually with in water transducers you need to beef the voltage up quite high - many transducers in the 200-400 KHz area are triggered with up to 700 or more volt. But for short range you can probably do with less.There are a couple of great but expensive books likehttp://www.amazon.com/Transducers-Arrays-Underwater-Sound-Acoustics/dp/0387329404/ref=sr_1_5?ie=UTF8&qid=1428206320&sr=8-5&keywords=transducer+book- andhttp://www.amazon.com/dp/3540876618/ref=wl_it_dp_o_pC_S_ttl?_encoding=UTF8&colid=HQ18XYPMTAPO&coliid=IQHG4QSYVG6R2And then of course there is "Dave" the local oracle in underwater technology who runs the little site called EEVBlog - he has EXTENSIVE experience in the field (from a previous part of his life)

author: Nevereven
author_rank: Contributor
timestamp: April 05, 2015, 04:46:18 am
content: The primary driving factor is size. I need it to be small cheap and low power. This transducer has:Diameter of only 20mm!Maximum voltage is 300v0.1m - 10m range-26dB min sensitivity6 degree beam angleminimum shunt impedance of 110ohmsstatic capacitance of 1300 pfTo reduce power, I don't plan on sampling very often.I understand underwater acoustics is a big field. Lots of big high power deep water stuff, but no cheap short range low power systems that I have found. Probably some expensive ROV altimeters, but I need cheap. I hope my application is simple enough that I can manage it.I do recall that Dave has mentioned some past experience in this field. Oil exploration related I think? Perhaps he might have some tips on my cheap low power system.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: April 05, 2015, 05:23:57 am
content: cheap proof of concept? or cheap system price and what volume?if its just cheap proof of concept then the Analog Discovery USB DAQ may be able to do the jobhttp://www.eevblog.com/2014/12/13/eevblog-692-digilent-analog-discovery-review/still lots of sensor interface questions to answer between the sig gen, receiver and the transducer as others have mentionedyou may be able to use a transformer tuned to resonance with the transducer C with leakage or real inductance to get the V step up for driving

author: Nevereven
author_rank: Contributor
timestamp: April 05, 2015, 05:39:22 am
content: Thanks,I think you're right about the voltage being substantially less than 300v. I've got a linear power supply which will do 30v which should work for the rain barrel test. Eventually it will be part of a device I've already developed which samples a bunch of water quality parameters. I'd just like to add water depth. It's powered by eight AA batteries and runs for several days ... but first things first. I really don't need much more thanone of these.which are only $30something.I will be using the same transducer for transmit and receive. I'm hoping that it will settle quickly since it's so small and the frequency is high. Another reason to reduce power perhaps.I think I need a function generator with a burst mode, but how to search on this feature. I did find theProtek B8010FDseems to have this feature. As does theSiglent SDG805I can get the transducer for US$100. I'd like to be able to put the whole package together for less than $350. My volumes will be < 100/year, so I'll probably be assembling the PCBs.

author: zapta
author_rank: Super Contributor
timestamp: April 05, 2015, 06:31:37 am
content: Quote from: evb149 on April 05, 2015, 04:46:59 amYou also need to decouple the transmitter such that it doesn't load the transducer during receiving. IIRC, it can be done with series diodes that do not conduct with the small receiving signal.Quote from: Nevereven on April 05, 2015, 04:46:18 amIt was a military application, sink triggers for sonobuoys or something of that kind, not actually underwater acoustics, but I may be wrong.