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  Noise Interference on Radio Modulation, SONAR Display Correlated with Engine Speed of Outboard

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Author Topic:   Noise Interference on Radio Modulation, SONAR Display Correlated with Engine Speed of Outboard
littleblue posted 10-03-2014 05:12 PM ET (US)   Profile for littleblue   Send Email to littleblue  
I have experienced noise on a few of my electronics. Some examples:

--VHF Radio; when I have the main motor on and above idle speed, I am not able to transmit well. I've been told the transmission is garbled.

--SONAR; with a Furuno FCV-620 sonar unit, even with noise filters on, when [the engine speed is] above idle, there is a large increase in noise that presents itself on the screen.

--chartplotter and VHF [Marine Band Radio]: when [the chart plotter and VHF Marine Band radio were] linked to each other. [there was an] increased noise on the SONAR unit.

--ControlKing; I have a remove throttle unit hooked up to my Yamaha T8 Kicker motor. The unit controls a small servo that is installed inside the engine cowling and connected to the throttle linkages. When I press the VHF PTT to transmit, the motor slows down, i.e., the servo is moving. I'm not sure if this particular symptom is related to engine and boat ground or if this is just interference from the radio and radio signals.

I remember my 1991 18 Outrage had a grounding tab screwed into the transom of the boat. Should there be something similar on a 1993 Outrage 22? I'm think there may be a wiring or grounding [problem] on my 1993 Outrage 22.

acseatsri posted 10-03-2014 05:45 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
If this just started happening, I would definitely check battery connections and wires.
Jerry Townsend posted 10-03-2014 06:15 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
The battery cannot be causing the problem; rather, it is probably from the alternater. To check that out, disconnect the charging lead from the positive battery post and then try your electronics.

If the problem is the alternator, have it checked and repaired. I am not an electronics engineer, but a filter or perhaps even just a capacitor will reduce or eliminate the problem. ----- Jerry/Idaho

jimh posted 10-03-2014 07:40 PM ET (US)     Profile for jimh  Send Email to jimh     
The report of garbled modulation on your transmitter signal from your VHF Marine Band radio that occurs when the engine speed is above idle is very likely due to alternator whine.

The appearance of noise on the raster display of your SONAR when the engine is above idle speed is very likely due to alternator whine.

I have no explanation why connecting the VHF Marine Band radio and a chart plotter together--here I assume via NMEA-0183--would affect the SONAR.

The source of the noise is most likely from alternator whine. The most likely cause of alternator whine is a battery in very poor condition.

In any system where there is a power related problem and the power is provided by a battery, the first component to suspect as the cause is the battery.

littleblue posted 10-04-2014 04:00 AM ET (US)     Profile for littleblue  Send Email to littleblue     
Well, to be clear, this is not a new [problem]. It's been going on since I've had the boat, and in that time I've had the same batteries (going on five years I believe).
jimh posted 10-04-2014 07:57 AM ET (US)     Profile for jimh  Send Email to jimh     
To be clear, the battery acts as a smoothing or filter element for the charging current. If the charging current has such a high alternating frequency content that the battery terminal voltage is being significantly modulated by the charging current, the most likely cause is:

--the battery impedance is unusually high;

--the AC voltage on the charging current is unusually high;

--some combination of the above.

Measure the AC voltage at the battery terminals when the normal charging current is being applied and tell us the result.

What is your "main motor"? I assume you mean your outboard engine that is connected to the battery and is charging it. Give more details.

You have provided no information on the power distribution wiring for your boat. Describe the power distribution wiring in detail. If the alternator whine is being imposed on the battery, it is unlikely that problems in power distribution wiring are responsible.

jimh posted 10-04-2014 09:15 AM ET (US)     Profile for jimh  Send Email to jimh     
Alternator whine or other AC modulation of the DC power that is distributed to electronic devices on a small boat with outboard engine power can be remedied in a brute force manner by installation of added filtering. The DC power distributed to sensitive electronic devices can be separated from the general power distribution, and a power filter can be inserted into those branch circuits.

Before investing in such a filter, the condition of the battery should be carefully checked, as well as the charging circuit.

A test if the interference is being introduced by contaminated DC power distribution can be accomplished by disconnecting the affected electronic devices from their present DC power source and operating them temporarily from a completely isolated and independent battery power source. If the change in DC power to the device eliminates the noise interference during the test, this is a reasonably clear indication that the method of introduction of the noise is via the DC power. The remedy is then to clean up the DC power to the device.

Typical causes of contaminated DC power are:

--a battery in poor condition, whose terminal voltage is easily modulated by an applied charging current with some AC present;

--a charging circuit in which the rectifier or regulator have malfunctioned and the output of the charging circuit contains a much higher than normal amount of AC

fishinchips posted 10-10-2014 01:11 AM ET (US)     Profile for fishinchips  Send Email to fishinchips     
Check battery connections. Check alternator wiring. You might have to separate power wires from ground wires laying next to.
jimh posted 10-11-2014 01:13 PM ET (US)     Profile for jimh  Send Email to jimh     
In my experience with SONAR on small boats, I have seen that interference to the SONAR can be easily generated from operation of the outboard engine. The coupling of this interference is not through the power conductors carrying power to the SONAR. The coupling of interference into the SONAR from the outboard engine in my experience occurs when the SONAR transducer cable is position adjacent to and runs parallel to and for a long distance with the outboard engine wiring harness that runs to the helm from the outboard engine.

If interference is being experienced on a SONAR and the source appears to be from operation of an outboard engine, the first remedy I recommend is to separate and isolate the cable connecting the SONAR transducer to the SONAR head unit from all other electrical cables on the boat, particularly to separate from the engine wiring harness that connects the engine helm controls (such as ignition key switch and the safety lanyard) to the outboard engine. It is very common that all cables between the transom area and helm area are run in a confined pathway, and electrostatic or inductive coupling between the SONAR transducer cable and other electrical cables can be occurring. Because the SONAR transducer signal is amplified in the SONAR head unit at very high-gain, it takes only a very small intrusion of signal into the SONAR transducer cable from other cables to cause significant interference.

A good method to test for this path is to temporarily re-route the SONAR transducer cable to a new path, even if the path is just to lay the cable on the cockpit deck, in order to get a physical separation from other electrical cables. If a re-routing of the SONAR transducer cable reduces interference on the SONAR display from the outboard engine, then a permanent new path for the SONAR cable should be found.

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