Detection distance for a US nickel and quarter

[green]

Also, the Pulse/sample rate.

Why does Pulse/sample rate effect optimum integrator time constant?

[waltr]

Why does Pulse/sample rate effect optimum integrator time constant?

It is the Rate at which a new Voltage is applied to the integrator and the time for the integrator is decay.

Carl wrote a good description a while back on how an integrator in a PI detector works. Don't remember the thread but I copied the text. I then did a spreadsheet to do setup the parameters and do the calculations.
Here is Carl's explanation:

[green]

It is the Rate at which a new Voltage is applied to the integrator and the time for the integrator is decay.

Carl wrote a good description a while back on how an integrator in a PI detector works. Don't remember the thread but I copied the text. I then did a spreadsheet to do setup the parameters and do the calculations.
Here is Carl's explanation:

Let's say we use a single-ended integrator shown below; R1=1k, R2=100k, C1=0.1uF. 1000 samples/sec. Carl's example

Integrator time constant=.01 seconds. If .01 is correct should we change the time constant if the sample rate changes to 3000 samples/second? Maybe I'm looking at it wrong?

[green]

110us sample gives 1.33 times the signal as a 10us sample for the nickel. 110us sample gives 4 times the signal as a 10us sample for the quarter. A longer sample might help for the quarter not the nickel. Probably can't do ground balance with a 110us target sample.

Depends on integrator if 4 times the signal. 1C integrator gain=R fdbk/R in*sample rate*sample time. Gain is 11 times higher with 110us sample vs 10us sample. 1C integrator out=average sample*gain?
Still trying to figure MPP integrator.

Some spice sims for MPP and 1C integrator, think they make sense. Sample time for 1C integrator changes gain not response. Sample time for MPP integrator effects response, not much gain effect. Changing R5 MPP integrator effects response, not much gain effect.

Thinking 2C integrator is same as 1C integrator?

[green]

110us sample gives 1.33 times the signal as a 10us sample for the nickel. 110us sample gives 4 times the signal as a 10us sample for the quarter. A longer sample might help for the quarter not the nickel. Probably can't do ground balance with a 110us target sample.

4 times the signal was based on calculations reply #38. Original bench tester had target sample and ground sample capability. Failed awhile back. Made target response tester, record linear and or log amplitude decay. No integrator. Not sure calculation using target sample and ground sample is same as sampling 10us target sample and 110us sample. Thinking making another bench circuit including integrator. Maybe just sample delay and target sample with EF samples. Adjustable: transmit rate(200 to 2000pps), target delay(5 to 30us), target sample(10 to 500us).

Reply in another forum stated a GPX4500 and GPZ7000 could detect a quarter at 24inches. Wondering what it would take. Dave J stated in another thread a 24inch coil might be a place to start for a target at 24inches. Just a learning exercise to see if I could do it on the bench. Hoping the above bench circuit would be usable. Any thoughts what might need changing or added?

[green]

Used Excel to integrate a signal similar to a quarter. Quarter decays straight line log-log to about 100us. Generated a decay with -.68 slope and calculated the average for a 10us sample(6 to 16us) and a 110us sample(6 to 110us). Average for 10us sample=2.159. Average for 110us sample=.834. With a 1C integrator the gain for the 110us sample is 11 times the 10us sample, .834*11=9.17 . The longer sample is 9.17/2.159=4.25 the shorter sample, close to the 4 times I got with my real data calculation.

[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.

[6666]

Do you think that 30uS is the limit ?

[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.

[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.

[waltr]

Interesting thread. On my Hammer Head II I do sample timing as per this post:
https://www.geotech1.com/forums/show...ake-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/show...561#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.

[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.

[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.

[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.

[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