Possible Fix for EMI

[laidback4sho]

Hey guys,

I was on another forum a second ago, reading up on the Fisher Gold Bug 2, and I came across someone who was experiencing the effects of EMI on his machine.

Well, another fella chimed in and claimed that he had basically EMI-proofed his machine by attaching one of these little ferrite chokes to the cable, just before it enters the control unit. The photo below shows a ferrite choke. It's a little snap-on device that kills electromagnetic interference and can be purchased on Amazon very inexpensively.

Well this got me to thinking... if it would work on the Gold Bug 2, then there's no reason why it shouldn't work on the Equinox. So to test my theory, I'm about to buy a pack of these ferrite chokes from Amazon and test one out on my 800.

So far, the only fix I've seen for this machine's erratic behavior has been to put your cell phone in airplane mode. But what if it's not your cell phone? What if it's power lines or some other source that's causing your machine to act up? That's why I want to test this out for myself.

Anyone want to chime in with any questions or comments, please feel free. Maybe one of you have already done this with great success. Maybe one of you can tell me with certainty why this will or will not work.

Anyway, I'm out for now. I'll post my findings here once I've reached a conclusion as to whether this will work or not.

 

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[digger27]

These have been talked about for years on the forums.
The consensus is they won't really hurt anything but they also don't actually work when it comes to detectors.
They are designed to filter out higher frequencies than our normal enemy, EMI that is emitted by high tension electric wires.
Then again who knows, stranger things have happened.
Report back for sure.

 

[DDancer]

As digger27 says, cant hurt. I also agree that ferrite beads are intended for radio frequencies having used them in electronics myself. I suspect the emi interference from cell phones on the NOX is not due to the coil lead but internal to the control head. But I to will be interested to see what you come up with.

 

[DizzyDigger]

Ferrite beads, or "chokes", as you call them, are designed to
absorb the conducted emissions traveling up the exterior of the
cable. If the cable itself is acting as an antenna, then the ferrite
bead can attenuate those signals before they enter the device.

If the input port where the cable connects to the device is
not properly grounded, the conducted emissions can cause
all manner of issues.

https://en.wikipedia.org/wiki/Ferrite_bead

 

[fuceye]

could wrap the control box in tinfoil and have a chain draging on the ground....

 

[vferrari]

I have not run into EMI that hasn't been addressed by:

Keeping my phone away from the vicinity of the control head (I don't put it in airplane mode because I use it for site coverage tracking).

Doing an auto, and if necessary, manual noise cancel. Must be done individually for each mode. Same for Ground Balance.

Lowering sensitivity, as necessary. The machine is super hot, so lowering sensitivity 5 or so points does not kill depth that much, but there is a point where it starts to rapidly drop off. There may be a sweet spot between 15 to 20 on sense where you are actually increasing signal to noise ratio and relative depth performance, so don't be afraid to do this to combat EMI.

If running in All Metal, ground feedback noise may appear to mimic EMI. If you are getting a lot of variable negative numbers then do a ground balance.

Agree the ferrite beads will not hurt, but depending on the source of the EMI, they may not be effective either. Worth a shot.

 

[dirtlooter]

it would be cool if it does work. one of the things that I have learned in life, sometimes things work and work very well that weren't supposed to work and some didn't work at all that were supposed to work the best. kind of like an unrated team stomping a number 1 team, only one way to find out. sometimes the simplest fix works the best. Good luck

 

[Quisp]

You will need to identify the offending frequencies and obtain the correct ferrite bead to suppress it. Best case it helps, worse-case, you've suppressed the signals going to and from your coil, which would not be good.

 

[Rawhide]

EMI is the machines way of saying it is working. I would just hunt with it as long as there is no danger to the machine.

 

[vferrari]

You will need to identify the offending frequencies and obtain the correct ferrite bead to suppress it. Best case it helps, worse-case, you've suppressed the signals going to and from your coil, which would not be good.

The ferrite bead/choke should only suppress radio frequency level signals (above the ferrite bead cutoff frequency) that are trying to penetrate the coil cable shield from an external source or exit the coil cable through the shield, it should not affect signals travelling within the cable from the coil, therefore, it should not affect desirable signals to/from the coil to the control box and should not degrade detector performance. Again, the ferrite bead will only help if the source of the external interference is entering the control box via the cable acting as antenna and only interference that is above the cutoff frequency of the choke.

 

[Hawks88]

Thanks for all that info.

 

[WaterWalker]

My camera will not download its pictures without the Ferrite Choke on the cable, as supplied by the manufacturer.

The cause for the EMI on one PI detector I have was an extra ground lead on only one end of the cable. The fix was to un-solder the the ground lead, what a difference that made. The PI ran almost as smooth as my Excaliburs.

 

[Quisp]

The ferrite bead/choke should only suppress radio frequency level signals (above the ferrite bead cutoff frequency) that are trying to penetrate the coil cable shield from an external source or exit the coil cable through the shield, it should not affect signals travelling within the cable from the coil, therefore, it should not affect desirable signals to/from the coil to the control box and should not degrade detector performance. Again, the ferrite bead will only help if the source of the external interference is entering the control box via the cable acting as antenna and only interference that is above the cutoff frequency of the choke.

Ferrite beads are like a resistive load to high frequencies. If the cable shielding is allowing the unwanted frequencies to enter the cable, then the wanted "good" signals inside the cable could see a slight voltage drop if you choose the wrong ferrite value.

 

[vferrari]

Ferrite beads are like a resistive load to high frequencies. If the cable shielding is allowing the unwanted frequencies to enter the cable, then the wanted "good" signals inside the cable could see a slight voltage drop if you choose the wrong ferrite value.

True regarding the ferrite beading actually "impeding" high frequency AC signals. But these are AC signals EXTERNAL to the cable that are coupled onto the shield acting like an antenna and feeding that RF energy into the control box via the shield pin connection. There is coupling between the shield and the signal on the inner conductor but there the shield acts as an attenuator to the conductor on any external voltage that coupled onto the shield. So say 5 volts AC of common mode rf noise is coupled onto the shield as external common mode EMI, the effect will be small (typically 1/100 or greater depending on the impedance of the coax and its own cutoff frequency). The ferrite bead merely lowers this external coupled voltage further if the source is high frequencies above the ferrite bead cutoff freqency. The cutoff frequency should only impact the effectiveness of the bead (depending on the frequency of the noise), but it still not have a significant if any effect on the conducted signal, but should only have no effect on the noise or make the situation better, but should not lower the conducted signal level between the coil and the control box as far as I can tell based on the fundamental circuit theory and some of the articles I have read.

BTW - I did check out your website and it is fantastic. You have built some cool replicas and gadgets from some of my favorite classic films. Thanks for the link.

 

[Rookster]

Let's say it did help(some). Where on the Deus where you attach it? As of now, I just reduce the Sens. or move away from power lines or whatever is causing the problem.

 

[vferrari]

Since the Deus is completely wireless, you can only use internal ferrite beads and I am sure XP makes use of them. The Deus uses multiple radios (proprietary wireless to get the signal from the coil to the control box/headphones/pinpointer and even a bluetooth radio to connect to your phone for the phone app). It is an RF EMI potential nightmare. It is AMAZING they have gotten it to work so well. But the advantages of the DEUS wireless tech are countered by some of the disadvantages such as EMI (already mentioned) plus the antenna claptrap to have to add to be able to use it with the coil submerged in water as the RF energy does not travel so well in water.

For the Deus and EMI - first do the auto noise cancel initialization which is simply keeping the coil in the air after you first turn it on at the site until the coil connects. You can try switching frequencies if you are having EMI issues. The higher frequencies are less susceptible. The EMI shielding on the white HF coils is pretty good compared to the LF coils and you have a wider selection of high frequencies so you are more likely to find a non-interfering frequency using them. HTH (Probably TMI Deus information for this forum, lol).

 

[Quisp]

True regarding the ferrite beading actually "impeding" high frequency AC signals. But these are AC signals EXTERNAL to the cable that are coupled onto the shield acting like an antenna and feeding that RF energy into the control box via the shield pin connection. There is coupling between the shield and the signal on the inner conductor but there the shield acts as an attenuator to the conductor on any external voltage that coupled onto the shield. So say 5 volts AC of common mode rf noise is coupled onto the shield as external common mode EMI, the effect will be small (typically 1/100 or greater depending on the impedance of the coax and its own cutoff frequency). The ferrite bead merely lowers this external coupled voltage further if the source is high frequencies above the ferrite bead cutoff freqency. The cutoff frequency should only impact the effectiveness of the bead (depending on the frequency of the noise), but it still not have a significant if any effect on the conducted signal, but should only have no effect on the noise or make the situation better, but should not lower the conducted signal level between the coil and the control box as far as I can tell based on the fundamental circuit theory and some of the articles I have read.

Uncle, uncle. You win! I didn't know there was going to be math involved. Ha!

BTW - I did check out your website and it is fantastic. You have built some cool replicas and gadgets from some of my favorite classic films. Thanks for the link.

Thanks! I try to have fun and enjoy challenging projects.

Later,
Dan

 

[vferrari]

Dan,

Don't sweat it. EMI makes even hard core electrical engineers' heads explode and a lot of times its just a guessing game as to what will work, what won't, and what will make matters worse. Especially if the source of the noise is not well understood. Suffice to say the snap on ferrite bead as a passive element is one of those "do no harm" fixes. That is, it will either fix the problem or it won't, but rarely if ever will it make the situation worse.

 

[Quisp]

Dan,

Don't sweat it. EMI makes even hard core electrical engineers' heads explode and a lot of times its just a guessing game as to what will work, what won't, and what will make matters worse. Especially if the source of the noise is not well understood. Suffice to say the snap on ferrite bead as a passive element is one of those "do no harm" fixes. That is, it will either fix the problem or it won't, but rarely if ever will it make the situation worse.

I've always said I know just enough about EMC to be dangerous. Spent half a day on Thursday in a 3M chamber fixing a client's problem. That's enough for me for a while. Maybe I'l try throwing a ferrite bead on my Teknetics 8500 to see if I can quiet it down some.

Thanks,
Dan

 

[laidback4sho]

Ok, I bought a ten pack of these things for around seven dollars on Amazon. Ten was the smallest quantity I could find. But for seven dollars, I ain't gonna whine about it.

Anyway, this is what it looks like. The ID of this thing is 5mm and it fits perfectly. It's real easy to snap on/off and doesn't seem like it will come off accidentally.

Might get out tomorrow and do a little experimenting.