field test unit no 001 "model T"

[Aziz]

Hi all,

I'm publishing the final result only. So you have to work further for the "free lunch" (either making the proof or proving me wrong).

Some comments on the results can be seen on AEGP Forum.
The dB's refer to the attenuation of the source EMI.



Now good luck in trying something.

Aziz,
gone mad.

[Carl-NC]

Aziz, please don't post links to Doug's forum. Most people can't read anything there anyways. Just re-post here.

[dougAEGPF]

Aziz, please don't post links to Doug's forum. Most people can't read anything there anyways.

Not true!!!!!!!
dougAEGPF

[Qiaozhi]

Not true!!!!!!!
dougAEGPF

When I previously clicked on the link, supplied by Aziz, I was not allowed access. Now it seems to be working.
Presumably you've changed some of the permissions?

[PATCHES JUNIOR]

Not true!!!!!!!
dougAEGPF

I know many, many folks who can not get access to your forum. So it is true. And only folks who follow your every word can get access to all of your forum. So stop trying to be so innocent.

[dougAEGPF]

I know many, many folks who can not get access to your forum. So it is true. And only folks who follow your every word can get access to all of your forum. So stop trying to be so innocent.

How many boards on your "forum" are open for guests?
dougAEGPF

[Carl-NC]

Hi all,

I'm publishing the final result only. So you have to work further for the "free lunch" (either making the proof or proving me wrong).

Some comments on the results can be seen on AEGP Forum.
The dB's refer to the attenuation of the source EMI.

[ATTACH]22173[/ATTACH]

Now good luck in trying something.

Aziz,
gone mad.

As Eric suggested, the sample-subtraction method is not the same as a HPF. I think your 2-point subtraction didn't include the necessary gain differential used in subtractive GB. Probably more of a sin^2 response, from a sinusoidal view.

[Davor]

The gain difference is a mere means of applying a weighting function to linearise an exponential curve. If weighting is pre-applied there would not be any difference.

[Aziz]

As Eric suggested, the sample-subtraction method is not the same as a HPF. I think your 2-point subtraction didn't include the necessary gain differential used in subtractive GB. Probably more of a sin^2 response, from a sinusoidal view.

Hi Carl,

it is a HPF of 1. order.

The "filter" is a phase sensitive filter and its characteristics can get phase dependent (phase of source EMI). So one have to find the worst conditions and take this into account (as I did).

There is no difference between the subtraction of integrals (integrated windows in a PI) and sampled voltages. Both end up at the same filter characteristics.

Aziz

[Qiaozhi]

I think there may be some confusion creeping into this discussion. Originally Moodz was talking about his detector being able to pick up a moving magnet from 12 feet away, and the discussion then moved on to Earth field elimination. Somewhere along the line, magnetic ground was mentioned and then ground balancing was being discussed. For EFE, the two sample pulse widths need to be the same, but for ground balancing the second sample needs some additional gain in order to be able to cancel the first sample. This was mentioned by Carl in post #637.

However, in both cases above, a simple HPF does not produce the same result as sampling.
Let us consider the case of EFE, and ignore the more complex issue of ground balancing and the inevitable "hole". When the coil is moved through a magnetic field, current is generated in the coil. When a metal target enters the magnetic field of the coil, eddy currents are generated in the target and these in turn create a magnetic field that slows the decay curve at the receiver. Since these eddy currents die away over a short period of time (generally in the region of a few microseconds) a second much later sample can be used to determine if the received signal is due to a metal target or some externally induced signal. The assumption being that an externally induced signal will not decay appreciably over the time period during which the two samples occur. A simple subtraction process involving the two samples can be used to totally eliminate the signal from even a strong magnetic souce such as a white board magnet. In this case we are only allowing through signals that decay over a given time period, and blocking signals that do not decay but remain at a constant amplitude.
If you add a high-pass filter at the input as a replacement for this method, you will allow through any signal with a frequency about the cut-off frequency of the filter, and block signals with a frequency below cut-off. This is obviously not the same thing as subtractve sampling. Although you may see some reduction in EFE when the coil is waved around, or a magnet is moved in front of the coil; the amount of cancellation will vary with the speed of the coil (or magnet) movement. The same argument applies to PI ground balancing. The two techniques are simply not the same.

[Aziz]

Hi all,

I suggest, you are focussing to the following simpliciations in the first instance:
- No target signal response taken into account (simply EMI)
- Simple sample voltage (no integration window)
- Doing this all for a single EMI frequency and subtraction period time t
- Looking at the worst case (phase lag dependency of the EMI)

Once it is solved, one can look further at the integrals (sampling windows).
The table you haven seen is made for simple sample voltages.
Aziz

[ZED]

Aziz is correct in saying that the 2 sample GB method has high pass filtering characteristics,it also produces low frequency signals that it normally blocks on the inputs.

[Qiaozhi]

Hi all,

I suggest, you are focussing to the following simpliciations in the first instance:
- No target signal response taken into account (simply EMI)
- Simple sample voltage (no integration window)
- Doing this all for a single EMI frequency and subtraction period time t
- Looking at the worst case (phase lag dependency of the EMI)

Once it is solved, one can look further at the integrals (sampling windows).
The table you haven seen is made for simple sample voltages.
Aziz

Let's make it clear, so that we can all understand what you're claiming:

Are you suggesting that a simple PI detector (that has only a single sample) will have the capability of eliminating the Earth field effect (EFE) by simply adding a high-pass filter at the input?

Yes or no?

[Ferric Toes]

A PI with early sample and late sample which is then subtractively integrated is really quite complex. As has been said, if the two samples are equal i.e closely spaced in comparison with the variation of the interfering field, be it earth's field or powerline, then good attenuation is achieved. The longer the integration time the better, as more samples are processed. However, there is a limit depending on the response speed required of the detector in passing over an object. Also a factor is the number of pulses per second that the whole TX and sampling cycle operates at. However, the rejection is badly compromised when interfering signals are sychronous, or nearly so, with the TX/sampling rate, or the intersampling pulse spacing. Filter response is more like a comb with little attenuation for synchronism and high attenuation for asychronous. That is why PI detectors benefit from a frequency control to shift the TX/sampling cycle out of synchronism. Serious synchronous interference can occur with a low frequency radio signal that is within the bandwidth of the detector i.e. 200kHz. A small adjustment of the frequency control and it is gone.

Ground balance is different, and one way of doing it is to have two pairs of sample pulses and two receiver channels. The second pair is time shifted from the first but each channel gives earth's field and low frequency rejection. The integrated results are then gain balanced to be equal and then subtracted. Hence earth's field rejection is not compromised.

Eric.

Eric.

[Davor]

Too much ado... wouldn't the whole shebang perform even better with a separate Rx and bipolar pulsing?