Preamp

[6666]

pendulum techniques

They work ok, but you can also use a computer fan, mount the coil horizontal , useing double sided foam tape mount a wooden arm to the blade part of fan, use a few d cells to power fan and adjust speed of rotation, , mount coin or whatever on end of arm and arm just rotates over coil.

[green]

Many people swing the coil in a 3 dimension arc. Left to right but also up at the ends of sweep and down at the center. It is not the best way.
Swinging the pendulum above the coil kind of imitates that motion.
If we fix the coil in the vertical position instead of the horizontal position, the pendulum swing passes the coil in a parallel motion, maintaining an even distance.

I seem to remember that you posted charts of various surface area size aluminium can pieces, could you please point me to the thread?

The surface area of the target, versus the coil surface area is a very important relationship. Also important is to have an estimate of the actual wire coil diameter/radius. With a 200mm coil, it makes quite a difference in surface area, if the actual wire is 170mm or 180mm while the outer coil housing diameter is 200mm.

http://www.geotech1.com/forums/showt...935#post208935 (time constant chart)
I like coil vertical. Ran the TC for a 50x50mm al can, 10.7usec (will add to chart when I do some more targets), charted amplitude vs target distance for a US nickel and a 50x50mm al can. Integrator out volts*1million/300, preamp gain=300, integrator gain=1, sample delay 7usec, target sample 6usec.

[Monolith]

http://www.geotech1.com/forums/showt...935#post208935 (time constant chart)
I like coil vertical. Ran the TC for a 50x50mm al can, 10.7usec (will add to chart when I do some more targets), charted amplitude vs target distance for a US nickel and a 50x50mm al can. Integrator out volts*1million/300, preamp gain=300, integrator gain=1, sample delay 7usec, target sample 6usec.

Thanks for the link and info.
Interesting to see the influence of the skin effect on the can sample. The alu is a much better conductor than the nickel, but the TC ends up being nearly the same.
On the pendulum: I used to attach a piece of wood to the string, to give it some weight. Then the target clamped to the wood with a rubber band, in different positions. I could then let the pendulum swing a few times while looking at the signal on the scope.

[Monolith]

Monolith, that is genius. Maybe these pendulum techniques should be reserved for depth testing and response speed testing. I am having more reservations about their validity for noise testing.

For the noise testing, we want to have a small target signal. Then look closely at the noise waveform.
Is it random?
Is it a specific frequency?
Are there several frequencies superimposed?
White noise or Johnson noise or Thermal noise should be totally random. However, when we filter and amplify this noise, we often accidentally create a synthetic frequency.
Once the noise is in the system it is difficult to take it out again. Stacking and integrating helps. Amplifying or multiplying does not reduce the noise.

[Sean_Goddard]

Like Monolith says, we need to standardise the test environment so we are all working to the same rules.

Once the test set up has been defined, we can then try different circuits to see which gives the best performance. I have no doubt that there are those out there who are laughing at what we are doing and thinking, "OMG, if ONLY they knew"! The point being that YES someone else has done this before, probably spent years on it, the difference here is that we intend to SHARE the results of our labour with the rest of the world, not hide it away in some company safe.

If nothing else we will learn more about how hard the design of a truly good machine is. We may also learn that maybe the simplest solution is the best and we have been overthinking the whole thing. Only ONE way we will tell. Theorise, experiment, record, improve and repeat!

What about amplifying the front end amplifier using a high end audio amp (the hard work was been done there) to the point where the noise stands out, then characterise it (if possible). As Monolith says, until we know the exact source of the problem, we can't do anything about it.

Whilst "Amplifying or multiplying does not reduce the noise", averaging does and what we are looking for is the same as a motion detector, a rapid dv/dt of the TARGET signal. if the noise is of a much higher frequency then we can filter it out, maybe some form of switched integrators are needed or a completely new type of circuit which as yet has not been designed (fat chance).

Just thought that a PLL could be used and the dv/dt of the target used to modulate a baseband to give a target that would be relatively noise free. Thoughts on this method? Perhaps the whole approach is wrong and we need to think outside the box a little more.

I'm loathed to put a micro anywhere near this sort of system as we are talking a lot of RFI from clocks etc so unless we use a DSP and then DSP out the noise the DSP introduces we are left with good ole analogue.

[Old cart]

Measuring noise

When measuring noise it is a good idea to amplify the test amp with an external low noise amp. Oscilloscopes are not designed to make measurements on small signals at frequencies this low. Scope external preamps are available, but are rather expensive. For example the Tektronix ADA400 but it only works with modern TEK scopes.
Such a preamp should have very low noise and adjustable gain and bandwidth. Adjustable offset is also nice to have. This group could of course design such a preamp or we could just use the design of the ADA400 as a basis. The schematic is available.
A simpler, cheaper design is available here

http://tangentsoft.net/elec/lnmp/


This design is well documented and even includes gerbers so no real design work would be necessary. It does have one issue though and that is that the input impedance is quite low at 100ohms. To fix this would require a high impedance low noise buffer to be added before the first op amp.
It has several component options explained and should work well with scopes and true RMS multimeters.

Lastly last night I read a post by Eric Foster who said the coil, if used in such a noise test must be horizontal as most noise is polarized. Placing the coil vertical will allow more external noise to enter. If we use a pendulum with a long line, like 2 M, the arc that the target makes wil be very slight and should not affect the results.

[green]

another option

Make a test signal to replace coil. Advantages: known signal amplitude, no EMI from coil pickup. Use a external 9 volt battery, connect preamp in across (R5) 1 ohm resistor, short for 1 micro volt signal, open for 10 micro volt signal. Depending on circuit might have to disconnect fet switch. Monitor post amp out with a scope for peak to peak volts. Connecting coil back up should give an indication of increase in noise from EMI, avalanche, etc. I'll give it a try, thinking about .15 seconds on and 1 to 2 seconds off.

[Old cart]

another option

Make a test signal to replace coil. Advantages: known signal amplitude, no EMI from coil pickup. Use a external 9 volt battery, connect preamp in across (R5) 1 ohm resistor, short for 1 micro volt signal, open for 10 micro volt signal. Depending on circuit might have to disconnect fet switch. Monitor post amp out with a scope for peak to peak volts. Connecting coil back up should give an indication of increase in noise from EMI, avalanche, etc. I'll give it a try, thinking about .15 seconds on and 1 to 2 seconds off.

I don't think that will work. With that small of a signal noise from the 555 will bleed into the input. To measure the preamp noise you could disable the transmitter and remove the coil then short with a resistor equal to the coil resistance. Of course you should also measure system noise with a coil attached and the transmitter running. This could be done with a small coil of the same inductance or a figure 8 coil. I suppose if one is dedicated you could make a small coil with matching LCR characteristics and then mount it inside a shielded enclosure ( faraday cage).

[green]

It didn't work. The target and EF samples are the same so they cancel, no signal. Have to try something different.

[Old cart]

Yep, pulse rate is slow and unsychronized. Maybe better to just use a resistor. If it is preamp noise you are trying to measure you have to measure the noise after the preamp, not the integrator as it will reduce the noise itself.

[green]

Yep, pulse rate is slow and unsychronized. Maybe better to just use a resistor. If it is preamp noise you are trying to measure you have to measure the noise after the preamp, not the integrator as it will reduce the noise itself.

I would like to know the practical peak-peak noise voltage to try for with a shorted coil. Peak-peak noise at post amplifier out/total gain(pre amp gain x integrator gain x post amp gain). A OPA1612 has a .1 to 10Hz peak to peak noise voltage of 60nvolts. I'm guessing a NE5532 or5534 is 5 to 10 times that. Resistor noise, current noise, make it higher. Would 30mv peak to peak with a total gain of 30,000 be good or not? Signal strength is effected by coil size, shape, ampere turns, target, target distance making comparing signal to noise more difficult. I think it's the second time I've tried to generate a target response forgetting the EF sample cancels. The target signal isn't necessary if total pass band gain is known. Maybe I'm looking at noise wrong.

[Monolith]

I would like to know the practical peak-peak noise voltage to try for with a shorted coil. Peak-peak noise at post amplifier out/total gain(pre amp gain x integrator gain x post amp gain). A OPA1612 has a .1 to 10Hz peak to peak noise voltage of 60nvolts. I'm guessing a NE5532 or5534 is 5 to 10 times that. Resistor noise, current noise, make it higher. Would 30mv peak to peak with a total gain of 30,000 be good or not? Signal strength is effected by coil size, shape, ampere turns, target, target distance making comparing signal to noise more difficult. I think it's the second time I've tried to generate a target response forgetting the EF sample cancels. The target signal isn't necessary if total pass band gain is known. Maybe I'm looking at noise wrong.

There is an absolute way and there is a relative way at looking at noise. The way I look at it, is the S/N. There is a point, where the target signal disappears in the noise.

What can we do about that? Reduce the noise or increase the signal amplitude, or both.

Eric foster mentioned that he had a minimum of 3mV noise. This is about +/- 1 lsb on a 10bit ADC.

[Monolith]

Target signal & noise

found a a picture from many years ago. This was made with the target swinging on a pendulum. It shows 3 passes of the target.
The target signal is about 2 times the amplitude of the noise.
In the noise we can easily see a specific frequency, but if we look closely we see many more frequencies.

[green]

There is an absolute way and there is a relative way at looking at noise. The way I look at it, is the S/N. There is a point, where the target signal disappears in the noise.

What can we do about that? Reduce the noise or increase the signal amplitude, or both.

Eric foster mentioned that he had a minimum of 3mV noise. This is about +/- 1 lsb on a 10bit ADC.

My thoughts on signal to noise, hope some are close to correct. First, need to compare peak to peak not RMS. Sweep coil over target and miss the next sweep. Have a pulse .1 to.2 seconds long. RMS is going to approach zero. Peak to peak white noise typically 6 times RMS. When does the target signal disappear. Searched minimum signal to noise, 3 to 1 was a suggested value. Example: scales, lsd 1 gram, noise 0 to 5 grams. Add 5 grams, displays 5 to 10 grams. If the weight was added and removed when the no weight reading was 0 the reading would be in the noise band. Signal needs to be greater than noise to have signal reading greater than noise reading every time. 3mv noise doesn't tell me what I need to know. Eric was probably referencing a test point on a particular detector which would if I could calculate the gain. Noise and signal should be referenced to the input(coil volts). I'm thinking if I could detect a 1 uvolt change in coil volts that would be good. Did another schematic for a 1 and 10 uvolt test signal. Reply #67 didn't work, missed the obvious reason why. Appreciate comment if I missed something this time.

[Old cart]

That is useful info . From a system point of view, this can define system signal to noise ratio as long as you do the testing in an electrical quiet setting. If your scope or measuring device includes an FFT function you can look at the signal in the frequency domain which makes determining the frequency of the noise components trivial. Unfortunately, using the FFT on most scopes is tough for an inexperienced user. If anyone is interested in trying I can give some general tips. Most are not obvious...