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Old 27th May 2005, 10:20 PM   #11
PRR is offline PRR  United States
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> Ok, it would seem that I need to elaborate our designs here.

No. At least I don't care if you use a battleship or a rubber ducky in an ocean or a bathtub. To repeat: What bothers me most is the utter lack of useful interface info on ITC's site. .... what is the supply voltage, what is the preamp output noise, what is its output impedance, is it unbalanced or balanced?

I found www.dolphinear.com and www.AquarianAudio.com offering hydrophones with generic consumer audio plugs. That isn't what you have, and anyway implies maybe-only-20KHz response.

www.reson.com has hydrophones with connectors very much like your ITCs. I do NOT know if the wiring is universal!!!!

Click the image to open in full size.

Looking at that interfacing (assuming internally preamped hydrophones), I tossed off this ten-cent idea:

Click the image to open in full size.

A pair of 9V batteries will power it for many-many hours; I've lived with HiFi preamps powered this way.

The chip should be TL072, available anywhere in the world for $0.32 (except $1.69 at Radio Shack). The sketch has LF412 because this stupid stimulator did not come with any "common" opamp model, and I have not yet herded my collection into it. LF412 is, for a first impression, "equal" to TL072.

Top-end roll-off varies with gain, but at maximum gain is only -1dB at 112KHz.

Bottom roll-off varies with Cin. Omit that and you are good to DC. I suspect for radar-ranging work, you want as little bass response as possible, which suggests Cin could be 1 or 2 nFd (1,000pFd-2,000pFd) for -3dB cutoff at 15KHz to 180KHz (3.5 octaves).
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Old 27th May 2005, 11:14 PM   #12
moamps is offline moamps  Croatia
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Allow me to but in a bit...

PRR,
If the TL072 is used at, say, 100kHz, a small variation in gain between the four amps is likely to produce a variation in the output signal phase angle. This may jeopardize accuracy of the measurement. INA 217 seems much better suited for the job.

Mike,
It might be a good idea to place one transducer into the tetrahedron for calibration purposes.

Regards,
Milan
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Old 28th May 2005, 01:29 AM   #13
jcx is offline jcx  United States
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a good implementation should have anti-alias low pass filtering, the common approach is to use active filters

since anti-alias filters are often rather high order phase/group-delay stability is an issue - both passive component matching between channels and finite/variable op amp gain-bandwidth can be problematic

I have used tl074 in complex composite phase compensating arrangement for a 20 KHz low phase shift measurement device, only because I didn't have time to get anything faster on the corporate approved component list - use fast op amps, its easy today to get op amps fast enough to put the active pole >> 20x the filter corner frequency
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Old 28th May 2005, 04:08 AM   #14
PRR is offline PRR  United States
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> INA 217 seems much better suited for the job.

Agree that an experienced tinkerer will turn to something snazzier than the '072. However Mike has had trouble getting anything out of the 217, and hasn't said why. I suspect he should start with a KISS design, if only to narrow his misunderstanding. When he gets a signal, he can determine if the marginal speed of a '072 is an issue. If it needs something faster, much faster opamps are easy to find. With a working '072 prototype, spiffier chips are semi-drop-in. (Hmmmm.... I should note that even the mild '072 would like bypass caps on its pins, and faster chips demand good bypassing...)

And the INA217 just has too many pins. The differential input appears to be more complication than feature for this unbalanced pre-amplified application, inputs are liable to oscillate, etc.

> should have anti-alias low pass filtering

Yes. But here the sample rate comes out about twice the nominal low-pass of the mikes and the amps. The roll-off may not be very sharp, aliasing is possible, but probably reduced enough to allow proof-of-concept. Also, for direction location we really want the first large sample (that isn't a noise-fluke), so the sampling theorems we rely on in steady-state and in audio are perhaps not applicable.

> a small variation in gain between the four amps is likely to produce a variation in the output signal phase angle. This may jeopardize accuracy of the measurement.

Quite true. I will offer several points in refutation; any, all, or none may be applicable.

The gross phase error seems to be "small", 18 degree, 1uSec, or 0.0015 meters, sub-millimeter (if my sliderule didn't drop a decimal place). Now the onus is on the PVC pipe and mike-clips to maintain millimeter accuracy. (I didn't get much sleep last night, so if I've spilled some zeros and screwed-up bad, sorry.)

The differential error between two '071s from the same crate is smaller; among the units in a dual '072 or quad '074 even less.

The time response of the hydrophones is not specified except as a typical frequency curve. I don't know how they make these things; I know in air-mikes with a top-ring, control of the resonance frequency is tough, and low tolerance means high dollars. There may well be more phase variability among the four "identical" mikes than across all the '072s ever sold. ('072 GBW can be 10%-30% below "typical", no upper limit, but you won't find any vastly-fast '072s.)

This project is about the computer software, not the hardware. And in fact cheap smart CPUs can reduce the need for expensive precision hardware. Calibrate! In context of a raft-test, probably the mikes will be "loose" at first. Hold all four in your hand, rap their noses with a book to align them, have a source a few yards away. The resulting recording instantly and simply gives you the correction for each mike and amp. As moamps says, hanging a small source in the exact center of the tetrahedron (perhaps on four matched bungee-cords) allows re-calibration any time.

Being a proof-of-concept, modest repeatable errors are not fatal. They give you something to do with any funding you can scare-up, or be the core of a thesis for your next degree. If the prototype came out perfect, you'd be down to the deadly-dull business of Marketing.
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Old 31st May 2005, 05:20 PM   #15
d0s4gw is offline d0s4gw  United States
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ITC sent me data sheet with the pinouts, and they are (seemingly) identical to the pinouts from RESON. The only difference I see is that the DC in is called B+. I have contacted ITC asking "what is the supply voltage, what is the preamp output noise, what is its output impedance, is it unbalanced or balanced". I'll put that information up as soon as I receive it.

Here is the schematic provided by ITC (http://www.simoniac.com/~welsh/img/ITC8084A.jpg)

I think I may have finally gotten some output from the INA217. I have my sound card wired into the Vin (playing a constant 440 Hz tone at max volume) and gain set to 40dB (110 ohms across Rg). I am getting about 0.45 Vrms on the input terminals (pins 2 and 3), and 2.0Vrms on the output (I am measuring the voltage across Vin- and Vout, pins 2 and 6).

I am also going to try to build TRR's circuit. One other thing I would like to know is how will things change when I make the adjustment from differential inputs to single ended inputs? Can the INA217 even work on a single ended input?
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Old 1st June 2005, 06:22 PM   #16
d0s4gw is offline d0s4gw  United States
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As promised, the answers to PRR's questions:

(1) what is the supply voltage?
Ans: 24 Vdc

(2) what is the preamp output noise?
Ans: at 100 pF source on input:
for 1 kHz, -155dBV
for 10 kHz, -165dBV
for 100 kHz, -167dBV

(3) what is its output impedance?
Ans: 30 ohms

(4) is it unbalanced or balanced?
Ans: unbalanced
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