Using an AWG to simulate an Ultrasonic Depth Sounder Transducer Pulse Return - Page 1
Forum section: electronics / beginners

author: MegaHurtz
author_rank: Contributor
timestamp: August 29, 2021, 02:46:24 am
content: Good day. I'm interested in using aUTG962E FGto simulate the ultrasonic depth sounder xdcr return.The depth sounder is an ancient Standard Horizon DS10 (manual) that is functional. I am doing a small bit of an overhaul on the thing (replaced the festoon LCD backlight bulb, replacing the LCD polarizer (on order), replacing MIA knobs, polishing out scratches from the enclosure lens, repainting, and replacing aged labels, etc.).Since I have it apart, I figured that I would try to validate that the adjustable aspects are in spec pursuant to the guidance in the manual. I've the circuit that is depicted in the DS10 manual cobbled together to enable my viewing the transducer signal (images attached hereto).What I desire to do, if possible, is to have the FG be triggered by the tx pulse (or maybe a trigger output from the scope (DS1054 manual)) and generate a mock return to stimulate the depth display - it currently just displays a couple of dashes with no xdcr return. The xdcr is installed in the boat in a slip and can't be removed from the hull sans sinking the boat (or having it yanked).Per the manual, it presents that a >= 10mV signal on the xdcr signal line, between 1.11mS (2.6' rtt) and 170.2mS (400' rtt) after tx gate starts (a 736uS wide 200KHz pulse (@ 2.28Hz PRF)) would cause the unit to display the depth. I've attached some salient imagery.The round trip times (rtt) are presumed, based on a 4700FT/S Ultrasonic Pulse Velocity (wikipedia).What I struggle with is the fact that the xdcr excitation pulse is 300Vpp. The scope handles that input just fine (x10 probe). I am not entirely sure that I could just put the func generator on the transducer line w/o smoking the finals on either the Depth Sounder's or the FG's outputs. If so, it'd be like chopping wood to get things working like I want, I perceive (famous last words...). I looked at the FG manual and am not able to determine if it can handle a 300Vpp signal (in High-Z, as the Depth Sounder docs indicate the xdcr needs to be a 350Ω 1300pF unit).Can some of you learned bench techs advise some means in which I might go about this, in a manner in which I don't destroy the UUT or the Test Equipment? If so, it would be very much appreciated if you could mentor me a bit. Thank you, in advance.-MHz

author: Terry Bites
author_rank: Super Contributor
timestamp: August 29, 2021, 08:56:06 pm
content: I admit I am not sure what your measurement set up looks like. As your pinger it uses a single transducer, there must be a T/R switch in the circuit, find what drives this switch and get your trigger from there.

author: CaptDon
author_rank: Super Contributor
timestamp: August 30, 2021, 01:23:38 am
content: You can run the Depth Sounder output into a load resistor (I think you indicated 350 ohms)and with your 10X probe connected to your scope you can get a reliable trigger. (I willassume one side of the transducer is actually at boat battery negative or 'ground'. Thismay not however be accurate, some transducers are fed 'balanced' above ground) I amalso guessing your scope has an 'external trigger output' or may be marked 'trigger out'.Use that trigger out to gate your AWG and take the output of the AWG through a sensibleresistor which will be connected to the hot side of the load. All grounds will be commonedtogether. I tried some simple math, if your AWG can do 10vpp into 50 ohms it will probablydo nearly 20vpp open circuit. It looks like 10mvrms is 28mvpp so what resistor will take20vpp and push 28mvpp at 350 ohms, apparent answer seems to be a stock value of around240,000 ohms for your resistor in series with the AWG output. This should surely preventany damage occurring to the AWG from the 300 volt output pulse of the Depth Sounder.Check my math, not guaranteed accurate?Those sounders do indeed have a high voltage output pulse, I have seen them arc acrosscorroded connectors!!!

author: MegaHurtz
author_rank: Contributor
timestamp: August 31, 2021, 09:37:41 pm
content: Thanks, folks! It is very much appreciated.Re picking up the internal trigger that is modulating the tx/rx switching circuit therein is not practical. I lack prints on the device, and am afraid that if I go trying to reverse engineer the darn thing, it'll possibly end up broke. I am guessing that this is a circa mid-/late-70's design, if not older, being manufactured and installed on a 1981 vessel. The fact it still works is a testament to the robustness of the design. Being vintage, it is nearly all discrete (excluding the 4-bit mpu) and shite be packed pretty tightly on the boards that look to be conformal coated with hand drawn traces - I'm pretty sure that Kicad and autorouters didn't exist back then, except in engineer's dreams.Ok, so, after sleeping on it a couple of nights, my noggin is hollering the old adage at me: "ifin it taint broke, don't go fixing it". Ima prolly adopt that posture for now, and revisit CaptDon's recommendations should things go South before we replace it with some new-fangled digital all in one for a gazillion dollars system.Again, thank you. I bid you and yours many blessings!~MHz