[PSUBS-MAILIST] OTS and motor controller noise

Mon Apr 28 14:06:13 EDT 2025

 On the R300, the OTS SSB-2010 is mounted inside the boat with the transponder out.  I have had similar noise issues using Minn-Kota 36V PWM motor controllers.  The noise is greater with power being supplied by 24V bus, which is sourced from 36V main battery bank via a DC-DC converter. PWM is putting a lot of noise into the voltage supply to the OTS unit, which can be seen on an oscilloscope.
When I went back to using the 6 AA batteries for the SSB-2010, the noise was dramatically reduced in shop test but not eliminated. My tranponder does not have any acoustic dampening at its support.
Might be time to think about doing what Hank and David are doing, which is mounting the OTS SSB-2010 externally and only running the headset wiring through the hull.
The issue for the R400 is the depth rating.  I am not sure what depth the OTS SSB-2010 with its plastic housing can withstand.  For the R400, the plan is to test to 500 ft.
Cliff

    On Monday, April 28, 2025 at 11:17:07 AM CDT, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

  Not a lot of room to work with of course inside the cabin.  My controllers are mounted up near the top of the hull, the coax comes in from the bottom and is probably only about 18 inches laterally from the controllers.  The coax cable thru-hull is only 5/8 inch diameter (15.8 mm) so PL259's won't fit through it to allow me removal for maintenance which is why I opted for SMA.  I suppose I could try it since I won't be removing the cable regularly but it's definitely going to be a PIA change.
No I'm not sure whether the noise is electrical or acoustic in source but now that you've asked I can tell you that the controllers make an audible tone when the PWM is active, even with the OTS off.  The tone changes of course depending upon the PWM frequency I select, but it is always there.  The same tone comes out the OTS, but amplified.
Jon

    On Monday, April 28, 2025 at 11:51:58 AM EDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

Two things I would look at:

1) Relative location and shielding between the motor controller and the transducer cable.  Is additional shielding (conduit?) possible?

2) I have encountered problems in the past with noise on coax due to impedance mismatch / partial signal reflection at any connectors. In my experience, this is solved by moving from RG58 up to RG8, but you don't have that option because the transducer is a potted assembly. If you have enough cable tail available to experiment, I wonder about swapping the SMA connectors for PL259 to see if that makes a difference.

Are you certain that the noise you are hearing is being introduced electrically, and not acoustically in the ultrasonic range of the transducer?

Sean

-------- Original Message --------
On 2025-04-28 06:47, Jon Wallace via Personal_Submersibles wrote:

Hey folks,
My OTS STX-101 receiver is picking up some fairly significant and annoying electronic noise from my Roboteq PWM motor controllers.  I tried changing channels on the OTS hoping a different frequency might help but it didn't.  I also tried changing the controller PWM frequency between a range of 10khz and 24khz but nothing between those frequencies made a significant improvement.  The noise is enough to override the normal squelch setting but at the same time a strong communication signal through the OTS transducer is enough to override the noise.
My configuration has the transducer outside the hull and receiver inside the hull which required cutting the cable.  I potted the cable in a thru-hull and reconnected the cable inside the cabin with RG-58 SMA connectors (soldered, not crimped).
In talking with a couple other folks it seems I am not alone in this experience.  Some others have found that using local batteries instead of tying into the same batteries used by the motors has significantly reduced the noise, however in my case I am already using a separate battery isolated from the motor batteries.  Another choice that has seemed to make a difference is mounting an SB-1001 outside the hull and running the headset wires through the hull instead.
Anyone else have similar experiences with OTS?
Jon
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