Detection distance for a US nickel and quarter

[Carl-NC]

I think the 1k resistor from U10a's output should go to U10a's inverting input not the non-inverting.

My conclusion as well.

[waltr]

Eric,
Are you sure that schematic is correct?
I tried it in LTSpice and the outputs of both op-amps went to their rails.

I think the 1k resistor from U10a's output should go to U10a's inverting input not the non-inverting.

Making this change in LTSpice gave 50x gain and 180? phase between the two outputs.

[Ferric Toes]

Thanks for the circuit Eric, trying to learn what I can about nfet switch's as used in PI. and getting them biased correctly.

For many years now I generally use Cmos bilateral switches and not nfets. However for the posted designs in this thread, I have been using an old board for experimenting, which had nfets mounted. The pulse generator driving the nfets swings from -5V to 0V. With -5V on the gate with respect to source holds the nfet off, and 0V with respect to source turns it on. J112 and J113 work fine. It is a good idea to limit the negative swing on the preamp output with four diodes in series/parallel, type 1N4148, otherwise if the preamp swings to -5V then the nfet will turn on when it is not wanted. In the circuit I posted, the diodes are connected across the 50K resistor as per the attached picture

Eric

[6666]

Thanks for the circuit Eric, trying to learn what I can about nfet switch's as used in PI. and getting them biased correctly.

[Ferric Toes]

Hello Eric what components are between the pre amp out and the fets , is it just a resistor, and what dc voltage did you set for the pre amp out ? thanks

Here is the preamp stage that drives the gates. The first stage is a 5534 with a gain of 10, although other devices can be as good or better. Cross coupled inverter has gain of 50 (the two gates are shown).
Offset adjustment on first stage sets the the inverter stage outputs to zero. 5532 IC is used for this stage, although other duals can be used.



Eric.

[green]

Hi Green, I went back to the instrument I made to do soil measurements, and which I used to measure the decay for nickels, quarters etc in this thread. The sample and integrator is the same arrangement as in my post 55 except that the 560k resistor is replaced by a push switch to manually zero the circuit between each reading. This removes any non-linear offset in the baseline. The output is displayed on a LCD voltmeter module scaled such that 1999mV is the max reading. I can take readings at suitable intervals down the decay curve and then plot it in linear or log. With this unit I can increase the sample pulse width to a maximum of 400uS and the noise level does not change. The sensor used is a small rectangular fig.8 coil inside a box with an internal and grounded graphite shield, so the dominant noise is circuit noise rather than pickup on the sensor. Noise level on the display is + or - 1mV. The reason the noise level does not change is that there is a constant ratio between sample width (on time) and off time before the next TX pulse. e.g. if the sample width is 10uS and the time to the start of the next TX pulse is 100uS, then for 100uS sample width the time to the next TX would be 1000uS. Whatever the sample width, this ratio results in the same integrator time constant and response speed. Of course it doesn't have to be 10:1, could be 15 or 20:1.

The best TX width from my tests is 350uS for a quarter and sampling early gives extra signal from skin depth currents added to the final single exponential. 150 - 200uS sample width is recommended for best range on a quarter. The range on a nickel should still be at least as good as with a short sample pulse. If two samples are taken for EF cancellation the width should also be the same as the first sample. The inter-sample pulse time should also follow the same ratio. All of this will most likely result in the pulse repetition rate having to be reduced to provide enough time for one complete TX and RX pulse train.

More later,

Eric.

Hi Eric, Makes sense with noise. Integrator gain stays the same, noise should stay the same or maybe be less noise with longer sample. Not clear with the quarter. https://www.geotech1.com/forums/attachment.phpattachmentid=50311&d=1590168202 the chart I did with Excel shows the average for the quarter is less with longer sample time if gain stays the same. Maybe I'm misunderstanding what your saying or I'm doing something wrong.

[6666]

Hello Eric what components are between the pre amp out and the fets , is it just a resistor, and what dc voltage did you set for the pre amp out ? thanks

[Ferric Toes]

Hi Green, I went back to the instrument I made to do soil measurements, and which I used to measure the decay for nickels, quarters etc in this thread. The sample and integrator is the same arrangement as in my post 55 except that the 560k resistor is replaced by a push switch to manually zero the circuit between each reading. This removes any non-linear offset in the baseline. The output is displayed on a LCD voltmeter module scaled such that 1999mV is the max reading. I can take readings at suitable intervals down the decay curve and then plot it in linear or log. With this unit I can increase the sample pulse width to a maximum of 400uS and the noise level does not change. The sensor used is a small rectangular fig.8 coil inside a box with an internal and grounded graphite shield, so the dominant noise is circuit noise rather than pickup on the sensor. Noise level on the display is + or - 1mV. The reason the noise level does not change is that there is a constant ratio between sample width (on time) and off time before the next TX pulse. e.g. if the sample width is 10uS and the time to the start of the next TX pulse is 100uS, then for 100uS sample width the time to the next TX would be 1000uS. Whatever the sample width, this ratio results in the same integrator time constant and response speed. Of course it doesn't have to be 10:1, could be 15 or 20:1.

The best TX width from my tests is 350uS for a quarter and sampling early gives extra signal from skin depth currents added to the final single exponential. 150 - 200uS sample width is recommended for best range on a quarter. The range on a nickel should still be at least as good as with a short sample pulse. If two samples are taken for EF cancellation the width should also be the same as the first sample. The inter-sample pulse time should also follow the same ratio. All of this will most likely result in the pulse repetition rate having to be reduced to provide enough time for one complete TX and RX pulse train.

More later,

Eric.

[Ferric Toes]

I found a page in a 1970's report which has some math on the operation of an integrator of the type I have used. It also looks at the effect of a wide sample pulse. The report was part of a commentry by John Alldred on Johnson's theoretical 1956 paper, 'A Pulsed Bomb Locator'.



Eric.

[green]

Circuit noise does not appear to change when sample pulses are widened, reply 55

Wondering why noise doesn't change when sample pulse width is widened(gain increases). Need to get my bench circuit with integrator working again. If I remember correctly, noise looked like it increased as the square root of the sample time increase. Tried to find a scope trace to show increase in noise but couldn't.

Found an example that shows an increase in noise. Integrator out absolute valued.

[waltr]

Interesting thread. On my Hammer Head II I do sample timing as per this post:

https://www.geotech1.com/forums/showthread.php?24251-My-take-on-the-HH2

In use I found that a GEB sample time of 90-120usec works best to remove ground response and maximize response to high conductors (silver Quarter, etc). Also have increased the TX pulse time to 180usec to further increase the response of high conductors.

The first sample and second sample (GEB) are into opposite inputs of the integrator stage. In this post I measured the integrator output Voltage for various targets.

https://www.geotech1.com/forums/showthread.php?24251-My-take-on-the-HH2&p=239561#post239561

Note is the 2nd graph that high conductors give a negative Voltage and low conductors a positive Voltage.
This was done with a 100usec TX pulse and a 50us second (GEB) sample time.
Increasing both the TX pulse time and second sample time increases (in a negative direction) the response to high conductors.

This correlates well with what is being discussed here about longer TX pulse time and longer sample times for high conductors (Quarter). Is the field I use a second sample time of 90-120usec and Quarters respond as well as Nickles do

There is 'hole' for targets with a TC between Dimes and Nickles but not many good target have a TC in that range. This method also give a High verse low conductor identification.

Circuit and code details are in my HH2 thread.

[Ferric Toes]

I have been doing some further testing on nickels, shillings and quarters but still find that I get more range on a quarter when using a sample pulse width of 150 - 200uS. Delay between the sample pulses is 500uS. My TX width is fixed at 350uS and the pulse rate is nominally 500/sec. which can be varied by about 10% either way. I have had to use a circular coil of 3 inches diameter to cut down on noise pickup. I have rectangular fig.8 coils but not of the correct inductance or resistance. My pickup noise is regular pulsing and I have not yet found the source, although I expect it may come from the alarm system, even when deactivated. With this small coil the external noise is not troublesome. I am limited to a first delay of 15uS however as the wire is enamelled solid with a tight bundle which increases self capacitance.

The integrator I use is this- . The two stage preamp has a gain of x500 and has both normal and inverted outputs which connect to the two gates on the schematic. The final amplifier is capacitively coupled to give a simple motion type auto-zero. Circuit noise does not appear to change when sample pulses are widened, but repetitive pickup noise changes depending on TX pulse rate, sample pulse width and spacing, all of which can be adjusted so that any residual noise is minimised. Usually all that is needed is a small adjustment to the overall repetition rate. Earth's field adjustment is by means of the 200 ohm trimmer. The dual opamp is TLC2262. The EF adjustment does seem a bit more critical with long samples and needs examining further.

With 150uS sample pulses I can detect a nickel at 8 inches with the 3 inch coil and the similar size cupro-nickel shilling about 1/2 inch less. If I drop to 30uS samples I lose about an inch on the quarter, but a similar range on the shilling. It remains to be seen what range a quarter gives with a larger coil and no external noise. I will have to box up the birds nest and with a suitable 12 inch coil, take it to the field out back and maybe in a week or two hence, I will have some results to post.

Eric.

[green]

Do you think that 30uS is the limit ?

Just a guess. Integrator gain increases with an increase in sample time, would expect noise to increase. Not much increase in signal.

[6666]

Do you think that 30uS is the limit ?

[green]

Charted the quarter and a nickel(TC=10us) with Excel(decay signals generated in Excel). 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 and 110us sample time starting after 6us delay. Wondering if there is an increase in sample time with the quarter where S/N decreases. Thinking S/N decreases after 30us sample with the nickel. My bench circuit isn't working, wondering if anyone has tried increasing sample time and seen S/N decreasing.