EMI-free test chamber?

[bklein]

The area in and around my house is full of EMI - mainly due to the underground utilities. All my detectors have trouble with it.
I want to work on some PI designs and am wondering if anyone has come up with a small test chamber concept for bench testing coils/circuits.

[6666]

Build yourself a large square cage from fine chicken wire and ground it, make sure the wire is all around and connected together this is not a joke it works.
You can test it by putting an AM radio in it and hear the attenuation in signal.

[waltr]

That should work and is called a Faraday cage.
The AM radio is a good test.

[Koala]

will not all that metal have an effect on coil testing ?

[waltr]

The coil needs to be at least several meters from the cage walls.
This does mean the 'cage' must be fairly large like a large room size.

So to the OP question- a "small test chamber" would not work and may make the EMI worse since if the coil is close to the cage since any EM flowing through the cage could be picked up.

[bklein]

I have seen temporary chambers offered using a metallized fabric. Anyone try that?

[Hyena]

I have seen temporary chambers offered using a metallized fabric. Anyone try that?

Basicaly everything will work if its conductive. But I guess that metalized fabric is much more expensive compared to 6666s proposal with quite dirt cheap and effective chicken fence...

[Polymer]

I have seen temporary chambers offered using a metallized fabric. Anyone try that?

I have used silver metallised polyester fabric 0.1mm for experiments 15 years ago. I still have a few square meters and will be
using it for PI coil EF shielding for a land based detector. The water coils didn't need it. My PI cannot detect this fabric, except when
tightly scrunched into a fist sized ball and held against the coil.

This fabric will basically eliminate all electrical fields.
The attenuation of electromagnetic waves (EMI) is very good from like 200 MHz upward to say 10 GHz (99% or so).

Shielding, say, 50/60 Hz EMI will not work with this.

Hope this helps.

[bklein]

My major issue likely is the 60hz. I’ve got a steel roof, steel mesh upper deck surface (concrete over it), but no complete box/cage.

[waltr]

My major issue likely is the 60hz. I’ve got a steel roof, steel mesh upper deck surface (concrete over it), but no complete box/cage.

A good way to eliminate 60Hz interference in a PI detector is having the Period an exact multiple of 60Hz period. 666.66... us etc.Then the period rate of the PI is coherent to 60 Hz and doesn't inerfer.
On my mod'ed HH2 running a PIC processor for timing I have a pot to adjust the pulse rate and this removes 60Hz issues when testing.
This is what the frequency control adjustment does on various commercial PI detector like the TDI.

[green]

A good way to eliminate 60Hz interference in a PI detector is having the Period an exact multiple of 60Hz period. 666.66... us etc.Then the period rate of the PI is coherent to 60 Hz and doesn't inerfer.
On my mod'ed HH2 running a PIC processor for timing I have a pot to adjust the pulse rate and this removes 60Hz issues when testing.
This is what the frequency control adjustment does on various commercial PI detector like the TDI.

Why would having a Period an exact multiple of 60Hz period. 666.66... us etc. matter?

[waltr]

Why would having a Period an exact multiple of 60Hz period. 666.66... us etc. matter?

When the two periods are not multiples then the interfering signal "walks" through the detection window and this is when you will hear the interference as a "beat".
When multiples then NO beat and no hearing the interfering signal.

Look at the integrator output, typically there is a low frequency signal. This is the Beat. Then adjust the PI pulse period, this 'beat will change frequency until it goes away.
When this beat exists threshold must be higher which looses sensitivity
I can run my PI near AC mains with a fairly low threshold. Before I added code to fine adjust the pulse period in house testing required a high threshold.
The only interference I get now is the furnace ignition and hot rod car ignition, both are wide band high tension noise.

[Carl-NC]

A good way to eliminate 60Hz interference in a PI detector is having the Period an exact multiple of 60Hz period. 666.66... us etc.Then the period rate of the PI is coherent to 60 Hz and doesn't inerfer.

True. The walk-thru designs I've done plug into the wall, and create a sync signal off the AC mains. Then everything runs at multiples of 50/60Hz.

[green]

Still trying to understand why. Thinking sampling theory. Wondering what causes beat frequency. Two frequency's summed causes beat frequency, something else? Wondered if aliasing could be causing it. Excel program calculates frequency out with 60Hz and odd harmonics. Don't see that being the reason. What is causing the beat frequency? Does the sample frequency need to be an exact multiple or is .1% close enough?

Does the sample frequency being in sync with the 60Hz reduce he noise, cancel the beat or both?

[waltr]

Similar to sum/difference of two frequencies.

A PI detector pulses coil on then does Sampling (analog switch) into detection circuits (integrator).

Let say this rate is 1500kHz, a 666.666... sec period.

60Hz AC's period is 16.666... msec
Therefore the PI circuits samples the 60Hz AC 25 times (16.666ms/666.666us) in one 60Hz cycle then repeats again and again.
There is no beat since both repeat exactly: 1 60Hz cycle for 25 PI cycles.

Now if the PI frequency is off a bit, say it has a period of 665usec, frequency is 1503.459398...Hz.
Then there are 25.06265664 PI cycles per 60Hz cycle. This creates a beat of:
1503.06265 - (25 * 60) = 3.759 Hz.
If PI period is 660us, frequency is 1515.151515... Hz & 25.2525... times 60Hz so beat is:
1515.151515 - (25*60) = 15.15Hz.

These Beats will be heard from the SAT stage if threshold is too low and causes the threshold to be raised to
not hear this but this looses sensitivity and depth. This can be seen on an O'scope at the integrator and SAT outputs.

There will also be some addition mixing products if the primary 60Hz is not a clean sine (motor spikes, etc) that
create noise if the primary AC is allowed to create beats. so the answer to your question is it does cancel the beats and
help reduce noise that is coherent with the 60HZ AC mains.

It will not reduce random or broad band noise or interference at some other frequency like being close to an AM band
broadcast station.