There has been interest expressed in Acoustic Modems and Location by a number of people in the community so I thought it might be useful to share the design of simple acoustic modem I experimented with a couple of years ago, and some references for more advanced designs. I based my design on the same Steminc transducer used by Bridget Benson for her PhD Thesis at UCSD in her very elegantly designed acoustic modem. I don't think her design has been picked-up up commercially yet and is not available. My project used a much simpler modem based on some obsolete Exar modem chips (I didn't include the modem designs due to upload limits, but will add to another post if anyone is interested). I would think use of software designed modems in microprocessors is certainly the way to go rather than this approach, but perhaps the analog amplifier designs may be of some use to experimenters. The transmitter consists of an Exar 2206 driven by opto-isolated TTL serial data at 110 baud with the resulting FSK modulated signal (43KHz +/- 500Hz) fed into an amplifier driving the transducer through a 2.5 mH inductor. The receiver has a 40 dB gain preamplifier fed into a Sallen-Key High-Pass filter and decoded back into Serial Data using an XR2211 demodulator. The designs are taken directly from manufacturers datasheets and publications and modified as needed to fit what was in my junk box. Since my hand drawn schematics are so bad I can't follow them most of the time I just painted the correct components values in the original drawings for reference. Also included a picture of transmitter and receiver prototype. Both were designed to be powered off of two 9V batteries and packaged into small Otter cases good to around 100 ft. but I never completed the packaging and testing at greater distances than a few inches in my front room. My original application was for use in sending data in diving equipment.
I have been thinking about using the amplifier and receiver for location of a ROV. Since the vehicle is tethered it should be possible to generate an acoustic pulse and send the pulse both through the water acoustically and over the tether electrically along with the current depth. Comparing the arrival times should allow calculation of the distance over ground to the ROV since both the hypotenuse and one leg of a rt. triangle are known. Using a couple of transducers or heading information and a GPS fix on the boat location, Lat-Long coordinates could be assigned to the ROV. Probably do the same thing less elegantly by pulling the tether tight and noting the angle off the stern.
Following are the references I used for the design. They are also noted at the bottom of the pictures.
Bridget Benson's PhD Thesis (Do search for additional publications)
Nathanial Garcia's Shark Tag Locator Thesis
Transmit and Receive Transducer Used by Bridget Benson in her UCSD PhD Thesis
New Part Number:
Transmitter: XR2206 FSK Modulator Datasheet
Transmitter: Intersil CA3140 Datasheet with Transmitter Amplifier Circuit
I used a NE 5532 in my modification of the design.
Receiver: Transducer Preamplifier Modified from this Analog Devices Presentation See Page 5.28
I used a TL084 amp from the junk box but using the recommended AD745 and the 100 Meg input
resistors would be likely be an improvement.
Reciever: I used a HPF instead of the MFB filters used by Garcia and Benson.
This is a good site for designing these filters and I'm sure their designs would be superior
to the simple one I used.
Receiver: FSK Decoder using XR2211 FSK Demodulator Datasheet as follows: