High pass filter to block out 60Hz signal

[davids85]

your entire circuiit needs to be inside a steel case

Yes, we have enclosed the circuit boards in aluminum boxes with small holes cut for wires to enter them. I'm sort of thinking the noise is a grounding problem (which I can't find for the life of me) or just the little length of exposed wires?

 

[Thunderchild]

I beleive and others should correct me if I'm wrong that aluminium will not shield against EMF ?

have you created a ground loop by mistake ? all ground wires must ground at the same place or you will create small voltage differences in the grould voltage and in ampliers this can cause a lot of trouble

 

[davids85]

A highpass filter passes high frequencies and blocks low frequencies. It has no effect on high frequency noise.


Three RC sections connected together have a loss of -29dB at the calculated cutoff frequency. It is droopy so if the calculated frequency is 480Hz then 60Hz will not be reduced much but 4.8kHz and even 9.6kHz will also be reduced a little.


Maybe hum comes from your power supply.

The power supply was originally a PSU from a computer, but I have since moved to 2 12 batteries in a + - + - configuration to provide a +12, -12 and a ground.

Just to be sure I'm clear, you're suggesting using two LT1115s, one as an amp and one as a filter?

If a high pass filter were calculated for 480Hz (I'm not sure where that number came from), it should still affect all frequencies below it as well right? **broken link removed** I'm sorry the quality is poor, I just did a quick search to find something. Also, wouldn't the phase of the higher frequency signals be affected? or would only the phase of the lower frequency signals be changed? I did some calculations off some stuff from wikipedia and it seemed like there'd be a phase change of about 90 degrees. If they all have similar phase changes it's ok...

Sorry I didn't understand the Sallen and Key Butterworth filter, I'm still a little new to this stuff and I didn't see anything about a higher cut off. If it turns out it's necessary I'll likely be ordering more ICs and parts this week. Right now I'm looking to do something quick and dirty because I'm trying to get some data for an extended abstract due next week.

 

[davids85]

I beleive and others should correct me if I'm wrong that aluminium will not shield against EMF ?

have you created a ground loop by mistake ? all ground wires must ground at the same place or you will create small voltage differences in the grould voltage and in ampliers this can cause a lot of trouble

I have checked the continuity of the ground from start to finish and it seems to be continuous everywhere. I'm not sure if that's an accurate check for ground loops though I believe we were careful not to create any loops.

 

[Thunderchild]

no your multimeter will not very very good for thisbecause you are talking about the resistance of a short length of wire but that is enough to upset your sensitive amplifier that will turn the u or mV into much larger signals, if you have any wires carrying ground then make sure they all connect to the same point in the unit, either a point on the board or a single earthing point on the case

 

[audioguru]

Just to be sure I'm clear, you're suggesting using two LT1115s, one as an amp and one as a filter?

Yes of course, two opamps. I was thinking about somebody else's opamp that had no gain.

If a high pass filter were calculated for 480Hz (I'm not sure where that number came from), it should still affect all frequencies below it as well right?

Each RC cuts the response 6dB/octave and 480Hz is 3 octaves above 60Hz. So 3 RC sections with an opamp create a slope of 18dB/octave with lower frequencies cut the most.

wouldn't the phase of the higher frequency signals be affected?

If the cutoff frequency is 480Hz then frequencies above about 1kHz will not have phase shift.

would only the phase of the lower frequency signals be changed?

Yes. The cutoff frequency will also have its phase changed.

I did some calculations off some stuff from wikipedia and it seemed like there'd be a phase change of about 90 degrees.

The phase change is 90 degrees for each section.

 

[davids85]

Thanks for the clarifications. I appreciate it.

 

[Mikebits]

Dave, take a look at this appnote which may give you some insight and some ideas on how to solve this 60 hz hum dinger of a problem that you are having.
Field Wiring and Noise Considerations for Analog Signals - Developer Zone - National Instruments

 

[Mikebits]

One more note. The following quote from the above mentioned note gives some good advice in localizing your noise issue. Take care to follow it.

Side note: at this point, assuming you read the appnote Field Wiring and Noise Considerations for Analog Signals - Developer Zone - National Instruments Then you should be able to identify how your sensor is referenced with respect to the measurement portion of your system, from what I gather, you have floating sensor side to a ground earthed measurement side. Have a look at fig 18 of appnote if this is the case. Perhaps you can share your grounding configuration to us Well that is all I have.

Solving noise problems in a measurement setup must first begin with locating the cause of the interference problem. Referring back to the block diagram in Figure 1, noise problems could be anything from the transducer to the data acquisition device itself. A process of trial and elimination could be used to identify the culprit.

The data acquisition device itself must first be verified by presenting it with a low-impedance source with no cabling and observing the measurement noise level. This can be done easily by short circuiting the high and low signals to the analog input ground with as short a wire as possible, preferably at the I/O connector of the data acquisition device. The noise levels observed in this trial will give you an idea of the best case that is possible with the given data acquisition device. If the noise levels measured are not reduced from those observed in the full setup (data acquisition device plus cabling plus signal sources), then the measurement system itself is responsible for the observed noise in the measurements. If the observed noise in the data acquisition device is not meeting its specifications, one of the other devices in the computer system may be responsible.

Try removing other boards from the system to see if the observed noise levels are reduced. Changing board location, that is, the slot into which the data acquisition board is plugged, is another alternative.

The placement of computer monitors could be suspect. For low-level signal measurements, it is best to keep the monitor as far from the signal cabling and the computer as possible. Setting the monitor on top of the computer is not desirable when acquiring or generating low-level signals.

Cabling from the signal conditioning and the environment under which the cabling is run to the acquisition device can be checked next if the acquisition device has been dismissed as the culprit. The signal conditioning unit or the signal source should be replaced by a low-impedance source, and the noise levels in the digitized data observed. The low-impedance source can be a direct short of the high and low signals to the analog input ground. This time, however, the short is located at the far end of the cable. If the observed noise levels are roughly the same as those with the actual signal source instead of the short in place, the cabling and/or the environment in which the cabling is run is the culprit. Cabling reorientation and increasing distance from the noise sources are possible solutions. If the noise source is not known, spectral analysis of the noise can identify the interference frequencies, which in turn can help locate the noise source. If the observed noise levels are smaller than those with the actual signal source in place, however, a resistor approximately equal to the output resistance of the source should be tried next in place of the short at the far end of the cable. This setup will show whether capacitive coupling in the cable due to high source impedance is the problem. If the observed noise levels from this last setup are smaller than those with the actual signal in place, cabling and the environment can be dismissed as the problem. In this case, the culprit is either the signal source itself or improper configuration of the data acquisition device for the source type.

 

[MrAl]

Hi there,


Passive twin T notch filters can be used (as well as active) but they are a little hard to tune as a slight change in value of one of the components causes the valley cut to rise quite a bit sometimes.
A high pass filter can be made with three RC sections, each successive section increasing in impedance by a factor of say 10, and that provides quite a bit of 'cut' for the 60Hz noise frequency while allowing 1kHz to pass with only a cut factor of about 0.9 or so.
Another place this comes up is when using an IR transmitter/receiver where the receiver picks up ambient light driven by a 60Hz source so it has a frequency of 120Hz. The signal carrier can be much higher like 10kHz but still a 6th order high pass filter works quite nicely.

What we dont know (unless i missed it) is what kind of 'cut' at 1kHz would be acceptable. Is 0.8 ok or 0.9 or does the application need 0.98 or something like that?
Also, is an in band phase shift a problem or not?
Also, is the output of the analog section going to feed an AD converter for analysis where program code is also under the control of the designer of the analog front end?

 

[davids85]

how can I effectively shield and insulate my exposed section of wires?

Ok, so this is a little embarrassing, but I think the exposed 6-7 inches of wire were the main cause of the noise. My question is now how can I effectively shield and insulate my exposed section of wires?

I have attached a picture of the wires so it's clearer what I need to do.

I found that when I put my hand around the unshielded wires, I got a huge spike in the 60Hz signal, and when I took my hand away the signal went down considerably.

I tried wrapping the exposed portion in copper foil tape and then in electrical tape and the noise went to near 0.

 

[crutschow]

You can enclose the wires in shrink wrap tubing and then run that through a hollow wire braid which is then grounded..

 

[davids85]

*EDIT* It would appear that the ceramic on the bottom of the accelerometers forms some sort of ground so I have a ground loop... covering it in foil and then insulating it removes the 60 Hz noise (like magic). I'm a little concerned about the accelerometers response (to accelerations) now. Can anyone recommend a method protecting the accelerometer from capacitive coupling without significantly altering the size/ response of the accelerometer? I'm sort of thinking along the lines of RF spray, but I'm not sure that would help against capacitive coupling.

You can enclose the wires in shrink wrap tubing and then run that through a hollow wire braid which is then grounded..

Actually upon closer inspection, shielding was not really the solution. I think there may be some sort of capacitive coupling between the ceramic on the bottom of the accelerometers and the sand... and the inductive coupling is some sort of second order, not so important feature.

Every time I place the accelerometers my sandbox (for testing, of course, not for castles and the like), the noise increases from about half a volt to about 2 to 2.5 volts. At first I though that there may just be some hum from my vibrator causing the 60Hz signal, but with further testing (burying/not burying the accelerometers, having the vibrator in the sand/out of the sand). I found the noise only increased if the ceramic part of the accelerometer touched the sand.

Does this make sense to anyone? It only sort of barely makes sense to me...

**** Ignore this for now****
//What I'm looking at in the interests of time if no one outright says it's a bad filter - is a two stage high pass filter where I'm trying to figure out what my resistor values (the ???) need to be in the "2 //stage filter sc" where I've got 1 220k ohm in the "current amp schem" and I think my input bias current (if that's what you're supposed to use to determine the resistor value in the op amp) is 50 uA, //so V=IR, V=11. Which actually doesn't sound right, but it seems to work... anyway, how do I make sure the same voltage goes through with two resistors and two capacitors?
************************

 

[Mikebits]

I really would like to assist you but up to this point you have failed to help us, help you. At this point we can only speculate on what you have going on, as you have not provided any sketches, diagrams of your system configuration. At this point I think you may be chasing your tail and building filters may not be the solution as the real problem has not been identified. As of yet, you have not mentioned how your cable shield is grounded, is it like in fig 18 of app note I provided? Too many unknowns so I am at a loss. Is your high gain amp at the far end of the sensor or at the sensor side of things? Maybe you might consider using a differential driver and receiver for the sensor data. The differential method is good for noise immunity. Anyways good luck.

 

[davids85]

I really would like to assist you but up to this point you have failed to help us, help you. At this point we can only speculate on what you have going on, as you have not provided any sketches, diagrams of your system configuration. At this point I think you may be chasing your tail and building filters may not be the solution as the real problem has not been identified. As of yet, you have not mentioned how your cable shield is grounded, is it like in fig 18 of app note I provided? Too many unknowns so I am at a loss. Is your high gain amp at the far end of the sensor or at the sensor side of things? Maybe you might consider using a differential driver and receiver for the sensor data. The differential method is good for noise immunity. Anyways good luck.

Mike I apologize for failing to answer any question as I have tried to address them all, but I believe I have addressed or attempted to address the above questions and was not clear enough or I didn't understand the question well enough to answer. This is a long thread and there are lots of questions and lots of answers.

A sketch is attached.
All wires are twisted, shielded (shield to ground).

Aside from that, I have stated in my last post the source of the noise and requested advice regarding the problem. I'm not sure you read the last post, as I identified, at the least, the source of the problem, but not a complete physical reason why it's happening.

Anyway, I appreciate your other help. The notes on NI were especially interesting because I'm using an NI card. It's certainly too bad you can't edit the first post of a thread to organize all the important information.

Continuing on my post before this one:
I tried wrapping my accelerometer in copper foil and connecting that to ground and bam! the bulk of the noise disappeared. I'd appreciate any suggestions on insulating the accelerometer, as the ceramic on the bottom appears to be causing a ground loop (through capacitive coupling?) and wrapping the accelerometer in copper foil is probably the least elegant solution.

 

[Speakerguy]

nevermind, your most recent post made mine moot

 

[davids85]

No, the proper solution here if you have a long cable run is a battery powered single ended to differential amplifier with some gain directly at the sensor, driving shielded twisted pair, to a differential receiver on the far end. High pass it then if you want to. But just making a high pass filter to get rid of the 60Hz is a poor solution.

I realize some of the post was mooted, but feel free to respond to this one if you like.

I'm not sure where the talk of a 4.6kHz signal came in, but through theory (and practice) you can strongly dampen certain frequencies with computer processing.

I may not understand this properly, but we're using a JFET ontop of the amplifier. It's how it's built, so I'm not sure we have much leeway there especially since the sensor needs to be rather small and mobile. If it turns out we still really need to watch for the noise, we may shorten the wires substantially, but at the moment they are only about a meter of twisted shielded pair.

I agree about just making a high pass filter being a poor solution. I was proposing it because I needed to save time. I planned on implementing a proper solution after I submitted an abstract next week.

To be clear, the system you were proposing was to try to eliminate potential noise to eliminate the need for a high pass filter? Because otherwise it's very similar to what I was proposing.

 

[Sethian]

You guys aren't paranoid enough. 60Hz correlates to a wavelength with the diameter of the Earth. Consider that many "frequencies" are operating with a 60 Hz "fundamental". Pulse a *LED**Display** at 60 Hz by means of diode-AC-rectification (also standard refresh rate for monitors)....now add bone oscillations to your skull, if done correctly the "pulsed display" will appear to ~w~a~v~e~. Audio devices playback "sample rates" are also designed uniform, 432.1, 440, 44100. Get a tall insulated pipe, attach a sheet of foil to the top with an insulated wire coming down, measure the voltage to ground (at 15' I measured 25v). Now connect electrodes to yourself, and using your oscilloscope measure between yourself and ground. You will notice if you duck down low, your voltage gets smaller, but if you stand up tall and lift your arm your voltage becomes greater. I have done this in remote locations using a battery powered o-scope, the results are the same. Bear in mind our usage of mains-grounding strongly resembles a "phantom-circuit", meaning power could be ran from a remote location through Ground to Both-Split-Phase-Lines-Equally, or a Ground to Air path, without being normally detectable by devices powered inside the building. At 60Hz Audio Induction Loops could cover a large area.

Now consider alternative physics for everything you ever learned. Earth's magnet field is caused by "???", not hot lava. RF work keeps components close to ground plane to reduce Ground-Ionosphere 60 Hz interference. By grounding, we assign things a common reference point, this makes signal outside of the loop hidden, the Ground may in reality have a voltage, hence Ground-Loops form between 2 communication devices powered at different phases if they share a "ground" in their data wire. Aluminum Foil hats will not help. Spark-Gaps don't radiate anything, they form a type of short between Ground-Ionosphere, which disrupts normal oscillations that are actually being used to carry higher frequency signals. DOD sealed research of a 2007 discovery where RF was being used to burn salt-water. 2 ultra-sound sources at opposite phase, one of which is modulated AF, can cancel out at a target location leaving only the original AF. The Frey Effect was discovered during WWII by soldiers standing in front of radar dishes. They built a time-machine, doesn't matter when. De-javu is a natural result of consciousness not being composed of matter, thus not a part of The Space-Time Continuum. Basically everything you see on-line, in the news, or are taught in school, is a giant lie being fed to you.

So what I'm trying to say is that you should see 60Hz everywhere, not a big deal. Just a gvrnment cover-up to hide a massive conspiracy, situation normal, nothing to worry about. Also, you shouldn't trust clouds...seriously they already condensed and need to fall, they just sit up there watching you...only thing worse is fog. I bet you do a lot of measurements with Ground for your Ground. According to garbage I was taught, at 60Hz you can't radiate without an antenna the size of the Earth, or else you get only short 30' range, house wiring uses 2-Gang or 3-Gang Romex meaning 'area' for magnetic loop is canceled by Line - Neutral wires being ran as a balanced pair. 4-Wire Delta configurations have a coil-segment resembling your split-phase home wiring, this implies I should hit a Max of 208v should I ever connect a sheet of foil to a helium balloon with long-thin wire, to get high enough altitude to measure 60Hz hum.....I say that expecting lightning to be Static DC, not some form of AC Equilibrium. Which still leaves the question of mechanisms behind "known" thought/vision transmission. Did you know ferric compounds have been discovered within human pineal glands, meaning magnetic senses.

 

[crutschow]

Another newbie thread-necropheliac.
This thread is over 6 years old.