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Radio frequency noise question?

R/C Submarine modelers

Re: Radio frequency noise question?

Postby Ramius-II » Thu Jan 14, 2010 10:57 am

Hi Pirate and All!
Thanks RickNelson I forgot to include the type and "how to install" part! I will check with one of our RF/RFI/EMI "experts for the grounding of the gear box you have me curious about this. Typically there are the "most likely" causes of things going "spritual" on you. Probably the most common cause of problems is the 5-6 volts supply for the receiver and servo connection box. This is especially true if you are using a BEC to provide the lower voltage say in a 12 volt setup. BEC's are made to be cheap and often a very necessary 1 uf tantilum capacitor has not been installed. BEC by their nature "oscillate" at frequencies upto 400 Mhz and can have modulation on this oscillation! So you have a transmitter you do not know about! :shock: It would not hurt to add some .1 mfd caps on the servo power leads or you can remove the servo connector and run the wires through a ferrite donut. Mc Master-Carr has copper foil tape with conductive adhesive. http://www.mcmaster.com/#copper-foil-tape/=5ddn98 As for ground wires, the most direct path to the main battery negative terminal is best. What to avoid is creating a "ground loop" by grounding the motor case, mechanically connecting the shaft of the motor to the gear box using only metal couplings and then having a ground wire on the gear box! Then you would have a ground wire that starts from the battery negative, to the motor case, with the motor shaft connected to the gear box and a wire back to the battery negative or one big loop which should be avoided at all cost. For your servo, probably take a make a hole in the copper tape for the servo shaft, wrap the tape vertically around the servo, then wrap another piece of copper tape horizontally. Since the copper tape has conductive adhesive everything will be good. Then yes, solder a ground wire to the copper tape and run it to the battery negative. A note about ground wires. They need to be as heavy a gauge as possible. If you are concerned with a large wire bundle, then consider using teflon coated wire as the wires insulation is thinner. Yes, it is a much more expensive wire to use. Why? Because it's worth it! :wink:

Best, Ed
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Re: Radio frequency noise question?

Postby Sub culture » Thu Jan 14, 2010 3:44 pm

Pirate wrote:
Thanks for the gear lead Subculture. Are you Andrew?

Pete


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Re: Radio frequency noise question?

Postby Sub culture » Thu Jan 14, 2010 4:05 pm

Before you start going mad with the suppression techniques, just try a bit of deduction.

Unplug everything from your system, try your radio. Assuming it's glitch free, add things back in one at a time, until the problem reoccurs, that way you can isolate what is causing the grief.

Ed's suggestion of BEC problems could be on the money. Linear BEC's can cause all sorts of grief if you're running at 12 volts or more and have a lot of kit hanging off it. It might be worth trying your system with a separate RX battery pack if problems persist. Switchmode BEC's are very worthwhile for higher voltage boats. John Slater in Austrailia was having similar problems, installed a switchmode BEC, end of problem. But try the simple and cheap things first.
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Re: Radio frequency noise question?

Postby raalst » Thu Jan 14, 2010 5:39 pm

this article might help.
at least it confirms that metal rubbing metal (or carbon fiber) can generate rfi

http://www.charlesriverrc.org/articles/radio/joelfoner_rcrfi_scanningfaq.htm
Regards,

Ronald van Aalst

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Re: Radio frequency noise question?

Postby RickNelson » Thu Jan 14, 2010 6:00 pm

Gear RFI is static electricity. Simular effect as running across a rug and creating sparks or rubbing a ballon on your shirt and then placing it on the wall.
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Re: Radio frequency noise question?

Postby roedj » Thu Jan 14, 2010 7:05 pm

Rick and Ronald,

I see your answer on RFI generation. I've asked some experts I know what they think about this issue. I'll be sure to share their answers. I just want to make sure that we're not confusing an RFI emission point with the actual RFI source point. As Ronald says, "I'm only here to learn".

Dan
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Re: Radio frequency noise question?

Postby RickNelson » Thu Jan 14, 2010 7:12 pm

roedj wrote:Rick and Ronald,

I see your answer on RFI generation. I've asked some experts I know what they think about this issue. I'll be sure to share their answers. I just want to make sure that we're not confusing an RFI emission point with the actual RFI source point. As Ronald says, "I'm only here to learn".

Dan



Would you please explain what you think the difference is between: "generation", "emission point", and "source point".

TIA
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Re: Radio frequency noise question?

Postby Pirate » Thu Jan 14, 2010 8:38 pm

Doesn't it make sense that if you get rid of the generation, or source point, the rest will go away too? What difference does it make where the emission is?

Like if I wrapped my servos in shielding material, and the problem went away, why would I care where it was coming from or what was generating it? And if I shielded the motor and the problem went away, again, why would the others matter?
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Re: Radio frequency noise question?

Postby roedj » Thu Jan 14, 2010 9:18 pm

To All,

All I'm trying to do, in my mind at least, is to truly understand the problem.

The "generation" point is where the RFI is created. It could be a spark somewhere or an actual signal generated from a square wave which are notorious broadband RFI generators. The point is, as the other gentleman observed, is who cares where the emission point is - kill the RFI at its source.

The "emission" point may be somewhere else entirely. I can make a case for the RFI being "generated" somewhere inside the motor where there can be a spark. The RF energy is then carries along a motor shaft outside the motor case where it's radiated by the motor shaft and any attached gears acting as the "antenna". This situation is sometimes enhanced by having servo wires too close to a RFI emission point especially if the wires are parallel to the RFI "antenna".

I'm also having trouble with the static electricity theory of balloon rubbing. That works because both surfaces are essentially non-conductors and it's possible to build up an unbalanced quantity of electrons between the two surfaces which is the same as a voltage which can lead to a spark which leads to RFI - maybe. But to say that two conductors, in this case gears, rubbing against one another especially in a humid environment of a boat especially one that designed to go under water - that I'm having trouble with. Somebody suggested using plastic gears. When I read that I thought to myself that that may exactly the wrong thing to do as plastic gears are non-conductors.

Of course, I could be totally wrong.

I see someone else in this discussion gives his amateur radio (Ham) call. I, too, am a Ham - K8XW. I have submitted a synopsis of this thread and the referenced article to the ARRL's (Google ARRL for more info) Technical Information Service. These people are very good at such issues as we (hams) are constantly fighting the issue of RFI as in, "Damn it, you're screwing up my TV again." I'll let y'all know what they say.

Dan
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Re: Radio frequency noise question?

Postby Pirate » Thu Jan 14, 2010 9:55 pm

Dan,
You could be right. There are visible sparks inside the motor case. I can see them through the ventilation hole. But much of this may be mute, or at least minor.

I started the simple process of elimination as suggested. When I disconnect the ESC with its built-in BEC, and use the little NiCd batteries that came with the receiver, no more twitching. But there's another problem. The motor will not run in reverse. Off of the ESC it will.

So I tested the ESC on a volt meter and get 12V on forwards, 1V on reverse. So that is bad, and if that part is bad perhaps so also is the BEC built into it, as you guys mentioned could be a cause. So I'm going to try to get a replacement ESC and try again.

Pete
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Re: Radio frequency noise question?

Postby Sub culture » Fri Jan 15, 2010 1:20 am

Onboard BEC's cause more grief than they justify. The problem is that the majority of BEC's are only really designed to supply enough current for a couple of servos, go beyond that and they start to struggle. They are also usually based on linear regulators. The current these supply reduces as the voltage is increased. This is because the excess voltage, in this case anything over 5 volts is brickwalled by the regulator and shunted to ground and burnt off as heat.

Switchmode BEC's on the other hand don't suffer from this malady, and work efficiently over a wide voltage range. As they only cost a couple of pounds, it makes sense to fit them in a sub. They weren't available up until a couple of years ago.

Unfortunately, few manufacturers are upfront about the limitations of onboard BEC's.
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Re: Radio frequency noise question?

Postby roedj » Fri Jan 15, 2010 6:29 pm

Gentlemen, I've received an answer from the ARRL TIS:

Zack Lau W1VT wrote:
1)I suppose it a possibility, if the gears are used as the electrical path for the radio signal—one can get a rusty joint effect that will rectify a signal passing though the gear train. The obvious solution is to remember to use real wires for your signal paths.
2)Another possibility is that if the gears make intermittent contact, and they connect pieces of metal together, the metal involving the gears can detune the antenna system intermittently.
-----------------------

1) What he's talking about is not quite on point but close. It's a known fact that rust acting as a diode rectifier can cause RFI. I doubt if rust is the issue here. Please correct me if I'm wrong. Also we're not using, intentionally anyway, the drive shaft as the antenna or as an RF conductor.

2) The drive shaft can in theory detune the receiver antenna thus opening up the receiver to more RFI. I have my doubts about that being the case here as well.

As has already been suggested, kill the RFI at it's source with bypass capacitors on the motor, good grounding everywhere and systematically eliminating or adding ONE variable at a time from/to the system until you find the culprit. From what you've said and discovered, it sounds like you're well on your way. Good Luck!!

Quick story-- Some years ago the hams in my neighborhood and I were being driven out of our minds by a rasping white noise signal all over the 80 meter band (3.5MHz). It made using that band impossible. It was the dead of winter, in Michigan, and we were not too eager to go out looking for the source but finally it just got be too much to stand. We suspected ordinary power poles in our area but which one? One of us, not me, stood by a receiver in a nice warm house while the rest of us went out in freezing conditions RFI hunting. I used a very sophisticate tool to find the culprit - the biggest, heaviest shovel I could find. As we talked to our home base we went around to one pole at a time whacking the pole as hard as we could with the shovel and asking the base station if there was any change in the RFI noise. When we whacked the culprit pole the static noise at the base went off the scale. We called the power company to report that they had a bad pole; it turned out be a defective transformer. They asked how we knew which pole it was. I told them we could see sparks at night - they bought that story. Problem solved.

The moral - don't be afraid to take the direct approach when hunting RFI.

Dan
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Re: Radio frequency noise question?

Postby Pirate » Fri Jan 15, 2010 10:21 pm

Dan,
I once caught a snipe the same way.
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