60Hz noise problem

[Tinkerer]

60Hz noise
Suggestions for reducing 60Hz noise, please
Attached are 2 pictures.
#1 shows the signal, as it exits the cable. The target manifests itself by changing the signal amplitude and a phase shift. The scope is set at 10uS/div and 50mV/div.
As you can see, the signal is very noisy.
#2 shows the same signal, that is, the probes are at the same place and all other things are equal, except the scope settings. This time the scope is set at 5mS /div and 50mV /div. Now you can see mostly the 60Hz noise.
Any suggestions of how to reduce the 60Hz noise are welcome.
I am going to try synchronizing the timing schedule with the 60Hz power.
Another idea that comes to mind is a 60Hz notch filter, but what is the best way to execute that?
I would like to eliminate a good part of the 60Hz noise before any amplification.
Any suggestions would be much appreciated.

Tinkerer

[kt315]

Tinkerer, this link is not related to your problem... but you can find some similar moments.
http://www.findmall.com/read.php?34,...796#msg-170796

good luck

[humhum]

noise filter

60Hz noise
Suggestions for reducing 60Hz noise, please
Attached are 2 pictures.
#1 shows the signal, as it exits the cable. The target manifests itself by changing the signal amplitude and a phase shift. The scope is set at 10uS/div and 50mV/div.
As you can see, the signal is very noisy.
#2 shows the same signal, that is, the probes are at the same place and all other things are equal, except the scope settings. This time the scope is set at 5mS /div and 50mV /div. Now you can see mostly the 60Hz noise.
Any suggestions of how to reduce the 60Hz noise are welcome.
I am going to try synchronizing the timing schedule with the 60Hz power.
Another idea that comes to mind is a 60Hz notch filter, but what is the best way to execute that?
I would like to eliminate a good part of the 60Hz noise before any amplification.
Any suggestions would be much appreciated.

Tinkerer

Noise filter with calculation

[Tinkerer]

Thanks for the link kt315, it makes good reading.

Attached is the same signal when running the generator during general black-out.

the settings are not quite the same, but the reduction in noise is obvious.

Tinkerer

[Tinkerer]

humhum thanks for the notch filter circuit. Could you tell me what the roll-off is? about 6db decade?

The target signal is up to about 20Hz, that does not give much room to play.
But I am not really sure how that works. On the one hand I have a target response of a few uS, but the sweep speed of the coil produces a target signal of 3 to 20 Hz.
I wish somebody would explain how all that fits together. But somehow it does.

Tinkerer

[humhum]

uS Filter for PI

Tinkerer try this filter for few uS noise.

http://ul.to/11xot9

[moodz]

interference rejection.

Because the system knows exactly what the timing of the TX pulse is any noise that is not related to the TX timing can be eliminated very easily. The 50 / 60 hz will not be related to the Tx timing. A synchronous filter is actually simpler than an async filter.

Moodz.

[Tinkerer]

Tinkerer try this filter for few uS noise.

http://ul.to/11xot9

Thanks for the effort. Unfortunately the file does not download.
Could you please attach the file to the Geotech forum directly?

Tinkerer

[humhum]

Thanks for the effort. Unfortunately the file does not download.
Could you please attach the file to the Geotech forum directly?

Tinkerer

directly

[kt315]

Tinkerer, it is real trouble that you do not wish to note what I wrote on Delta Pulse schematic - 120 Hz for U.S.A., 100 Hz - for another countries.
your working freq must be due LINKED to 60 Hz. or 50 Hz if you are located in Ukrainia or Belorusia. it must be even to 60Hz in your case!
it is irreversible law! why guy you are so blind???

[simonbaker]

humhum thanks for the notch filter circuit. Could you tell me what the roll-off is? about 6db decade?

The target signal is up to about 20Hz, that does not give much room to play.
But I am not really sure how that works. On the one hand I have a target response of a few uS, but the sweep speed of the coil produces a target signal of 3 to 20 Hz.
I wish somebody would explain how all that fits together. But somehow it does.

Tinkerer

Hi Tinkerer:

It seems that the real bandwidth of your signal is well above 60 Hz, if you look at one pulse at a time.

The 60 Hz noise would seem to just "float" that little pulse up and down.

If you could find a way to establish the baseline of your pulse in a floating environment, then maybe your absolute level is not important.

The first thing that comes to mind is a high-pass filter -- just knock out everything below 1 kHz or so and take a look at what the received pulse looks like. Maybe good enough to work with? Play with the filter to optimize.

Another trick might be to have another small coil oriented to just pick up the 60 Hz signal, and try to subtract it out of your RX signal. Sounds good on paper, but I think very tricky to make work. The separate coil could actually be tuned to 60 Hz to eliminate your TX signal, and with proper phase adjustment, appropriate to subtract from your RX. Again, probably a hair-puller to make work, just a wild-eyed idea.

Another wild-eyed idea would be a fast-acting "clamp" circuit that adjusted the RX bias so the peak of the RX signal clamps to a fixed voltage level, so any DC float bias is essentially removed. Don't ask me how!

A computerized signal processor could probably easily perform the above clamping -- in other words just compute everything relative to the peak voltage of the RX signal.

Question: is this 60 Hz noise a potential problem in the field, or just an annoyance on the workbench?

Q2 - are you measuring this 60 Hz noise on a disconnected coil, or is the coil connected into a powered-on circuit?

Regards,

-SB

[Tinkerer]

Tinkerer, it is real trouble that you do not wish to note what I wrote on Delta Pulse schematic - 120 Hz for U.S.A., 100 Hz - for another countries.
your working freq must be due LINKED to 60 Hz. or 50 Hz if you are located in Ukrainia or Belorusia. it must be even to 60Hz in your case!
it is irreversible law! why guy you are so blind???

Hi kt315,
thanks for the help.
Yes, my eyesight is bad and my hearing is bad and my mind is slow and my hands are shaky. These are the things that come wit old age.
This is why I much appreciate your help and patience.

I have now changed the timing so that the cycle is 166.5uS, to coincide with 60Hz. The noise is less.
However, the 60Hz of the local power is not so perfect either, so I should think of making small changes possible in the field. Can anybody help me with the PIC code to do that?

Tinkerer

[Tinkerer]

Hi Tinkerer:

It seems that the real bandwidth of your signal is well above 60 Hz, if you look at one pulse at a time.

The 60 Hz noise would seem to just "float" that little pulse up and down.

If you could find a way to establish the baseline of your pulse in a floating environment, then maybe your absolute level is not important.

The first thing that comes to mind is a high-pass filter -- just knock out everything below 1 kHz or so and take a look at what the received pulse looks like. Maybe good enough to work with? Play with the filter to optimize.

Another trick might be to have another small coil oriented to just pick up the 60 Hz signal, and try to subtract it out of your RX signal. Sounds good on paper, but I think very tricky to make work. The separate coil could actually be tuned to 60 Hz to eliminate your TX signal, and with proper phase adjustment, appropriate to subtract from your RX. Again, probably a hair-puller to make work, just a wild-eyed idea.

Another wild-eyed idea would be a fast-acting "clamp" circuit that adjusted the RX bias so the peak of the RX signal clamps to a fixed voltage level, so any DC float bias is essentially removed. Don't ask me how!

A computerized signal processor could probably easily perform the above clamping -- in other words just compute everything relative to the peak voltage of the RX signal.

Question: is this 60 Hz noise a potential problem in the field, or just an annoyance on the workbench?

Q2 - are you measuring this 60 Hz noise on a disconnected coil, or is the coil connected into a powered-on circuit?

Regards,

-SB

Simonbaker, thanks for the feedback and the interesting suggestions.

The all pervading 60 Hz noise is really annoying on the workbench, but is probably omnipresent in the region, since the power is "Single Wire Earth Return" as Moodz kindly explained. Now, if I can totally remove the noise, that means the detector will be immune to it, wherever it is going to be used.

The noise shown is at the input of the preamp, with the circuit functioning. The high frequency that you see above the 60Hz wave, is the actual RX signal shown on the other picture.

Sweeping the coil over a target, produces an output sine wave that can vary between 3 and 20Hz, depending on the target size, coil diameter and sweep speed. Any filtering of this frequency band directly reduces all target response.

On the high end, about 200kHz is the limit for very small targets. Reducing the Bandwidth to below 100kHz eliminates the smallest targets.

Tinkerer

[Tinkerer]

Because the system knows exactly what the timing of the TX pulse is any noise that is not related to the TX timing can be eliminated very easily. The 50 / 60 hz will not be related to the Tx timing. A synchronous filter is actually simpler than an async filter.

Moodz.

Thanks for the feedback.
"not related to the TX timing" got me confused now. When I made the timing to sync with 60 Hz, by using a cycle of 166uS, I established a relationship.
Does this make it more difficult for digital filtering?

What cycle time do you suggest I use?

With 24bit ADC I could sample the signal directly, without amplifying. I have enough target signal and Ground signal amplitude for that. then all other processing could be done digitally, including filtering to eliminate unwanted noise.

Tinkerer

Tinkerer

[simonbaker]

Simonbaker, thanks for the feedback and the interesting suggestions.

The all pervading 60 Hz noise is really annoying on the workbench, but is probably omnipresent in the region, since the power is "Single Wire Earth Return" as Moodz kindly explained. Now, if I can totally remove the noise, that means the detector will be immune to it, wherever it is going to be used.

The noise shown is at the input of the preamp, with the circuit functioning. The high frequency that you see above the 60Hz wave, is the actual RX signal shown on the other picture.

Sweeping the coil over a target, produces an output sine wave that can vary between 3 and 20Hz, depending on the target size, coil diameter and sweep speed. Any filtering of this frequency band directly reduces all target response.

On the high end, about 200kHz is the limit for very small targets. Reducing the Bandwidth to below 100kHz eliminates the smallest targets.

Tinkerer

Hi Tinkerer:

I don't think any filtering of the 3 to 20 Hz band should significantly affect your pulses, because your pulses create an analogy to a "chopper" amplifier, which is able to amplify down to DC with a pass-band amplifier by chopping the input signal to shift the band up into the amplifier pass-band.

But maybe you have observed otherwise. I would like to understand why if that is so.

Regards,

-SB