PI noise

[green]

Still a very good signal S/N, at 14inches, specially when considering an 8 inch coil. 1.75 times diameter of the TX coil is excellent.
The glitches seem to be inherent to our quality level Digital Storage scopes. I have also observed various aliasing errors on my scope.

There is a tiny noise sinewave of about 2K frequency visible on the 14 inch picture. Is that your PP rate?

The analog switches add some noise. I add a discrete LP filter at the output of the switch that seems to help.

Limiting the bandwidth to the desired one seems to be a good idea for every opamp circuit, but maybe it would even be a good idea to reduce the pass-band frequency for the initial stages to the absolute minimum in the intention of reducing as much noise as possible before further amplification. In the case of your last stage preamp, I would consider 5Hz as low, but I have not measured the actual target response time of an 8 inch coil. At 1m/s, the time of the target above the 8 inch coil is 200ms = 5Hz, or should we call it 10Hz, as the response is more like a half sine wave? However, at a distance, the detection field is narrower than the coil, so the frequency increases. A common frequency used is about 15Hz.

I found that for the pendulum, filling a glass bottle with water, so that the pendulum is really heavy, helps keeping the pendulum swinging for quite a while, giving me time to look at the scope and saving traces.

Not sure what the 2Hz oscillation is. It's not always there, comes and goes. The butterworth filter overshoots at about that frequency with a step input. Next order I'll add a Bessel filter and try it. Sampling at 1kHz, about 10usec target and EF sample.

The noise is a lot lower with the integrator input connected to common, so I'm thinking the switches aren't causing the problem.

More scope traces comparing the different output stages in(Amplifier frequency response)thread. Look at the enclosed schematic. Integrator cutoff 16Hz low pass, two more stages with a 16Hz low pass cutoff. More on the added filtering in(Amplifier frequency response). I don't know what the best frequency cutoff is. A scope trace of target signal and noise should give a good indication.

I use a bottle of water for the pendulum. I caught it on the return swing so at least half of the trace was noise.

When I first joined this site I read a reply in a thread that stated if you can't detect a US nickel at 15 inches keep looking. From my testing a mono coil gives the best signal but so far I haven't been able to get the best signal to noise using a mono coil. Could probably get there using what I have by increasing coil diameter and peak current. Or by lowering the noise.

Stated the Rx coils were 8 inch. They are spider web(flat basket)wound, 8 inch I.D. so the mean diameter is about 8.3 inches

[greylourie]

green, I recall seeing "1 metre/second" mentioned on other forums..... and what Monolith says about limiting bandwidth ties in with that maybe. Possibly half or even a little more than less than half of what you are using currently...

[Monolith]

Monolith suggested using a pendulum to swing the target in another thread(Amplifier frequency response). A test swinging a US nickel 8, 10, 12 and 14 inches above the coil on a pendulum(about 76 inches in length, .36Hz). The target was pulled back 17 to 18 inches and let go and caught on the return. Should be about 1 meter/second peak velocity. Coil, Rx(two eight inch round coils, figure eight) Tx(oval coil surrounding Rx). Nickel centered over point where two coils touch, swings end to end. Loose about 2/3 of the signal for each two inch increase in distance. Eventually you run into noise level. The reason I wonder what is causing the noise increase when sampling. I would be interested if some one would do a test. Look for noise at post amplifier out on a scope, normal and with a added capacitor across the fdbk resistor of the last stage preamp (fdbk RC=200usec to 500usec). I have been getting about 1/4 the peak noise with the added capacitor. The short glitches happen when the control one shots switch and are there even if the scope ground and probe are connected to common. Thought I had solved the glitch problem, evidently not.

Still a very good signal S/N, at 14inches, specially when considering an 8 inch coil. 1.75 times diameter of the TX coil is excellent.
The glitches seem to be inherent to our quality level Digital Storage scopes. I have also observed various aliasing errors on my scope.

There is a tiny noise sinewave of about 2K frequency visible on the 14 inch picture. Is that your PP rate?

The analog switches add some noise. I add a discrete LP filter at the output of the switch that seems to help.

Limiting the bandwidth to the desired one seems to be a good idea for every opamp circuit, but maybe it would even be a good idea to reduce the pass-band frequency for the initial stages to the absolute minimum in the intention of reducing as much noise as possible before further amplification. In the case of your last stage preamp, I would consider 5Hz as low, but I have not measured the actual target response time of an 8 inch coil. At 1m/s, the time of the target above the 8 inch coil is 200ms = 5Hz, or should we call it 10Hz, as the response is more like a half sine wave? However, at a distance, the detection field is narrower than the coil, so the frequency increases. A common frequency used is about 15Hz.

I found that for the pendulum, filling a glass bottle with water, so that the pendulum is really heavy, helps keeping the pendulum swinging for quite a while, giving me time to look at the scope and saving traces.

[green]

Monolith suggested using a pendulum to swing the target in another thread(Amplifier frequency response). A test swinging a US nickel 8, 10, 12 and 14 inches above the coil on a pendulum(about 76 inches in length, .36Hz). The target was pulled back 17 to 18 inches and let go and caught on the return. Should be about 1 meter/second peak velocity. Coil, Rx(two eight inch round coils, figure eight) Tx(oval coil surrounding Rx). Nickel centered over point where two coils touch, swings end to end. Loose about 2/3 of the signal for each two inch increase in distance. Eventually you run into noise level. The reason I wonder what is causing the noise increase when sampling. I would be interested if some one would do a test. Look for noise at post amplifier out on a scope, normal and with a added capacitor across the fdbk resistor of the last stage preamp (fdbk RC=200usec to 500usec). I have been getting about 1/4 the peak noise with the added capacitor. The short glitches happen when the control one shots switch and are there even if the scope ground and probe are connected to common. Thought I had solved the glitch problem, evidently not.

[moodz]

moodz, have you looked at the discrete input of/on your rigol oscilloscope. Also appears to use a discrete differential arrangement following. Though seems like a lot of effort compared to just using factory made opamps.

http://www.eevblog.com/files/Rigol-DS1054Z-Schematic-FrontEnd.pdf

http://www.eevblog.com/files/Rigol-D...ic-DiffAmp.pdf

Yes I did see those on eevblog .. they may have use discretes to save every last dollar as labour inputs are probably much cheaper than inventory inputs in their cost model for those boxes. There is also the possibility that the frontends are more robust using discrete transistors as the rigol scopes are targeted heavily at the education and repair bench market where all sorts of abuse occurs :-)

[greylourie]

Thank you Eric ,its good to learn something.

I echo your sentiments, 6666.

Now I have learnt about a new beast (cross coupled stage/differential), and the potential benefits.

[greylourie]

moodz, have you looked at the discrete input of/on your rigol oscilloscope. Also appears to use a discrete differential arrangement following. Though seems like a lot of effort compared to just using factory made opamps.

http://www.eevblog.com/files/Rigol-DS1054Z-Schematic-FrontEnd.pdf

http://www.eevblog.com/files/Rigol-DS1054Z-Schematic-DiffAmp.pdf

[moodz]

Same as what I use in my detector. I also use the OPA1642 for the rest of the stages. No problems with noise out in the field.

Ti apparently acquired National Semis line of business in opamps and has been shutting down the main Fabs that make them ... hmm ... I have even been looking at "discrete opamps" like this open source Jensen design.

http://mnats.net/JE-990.html

Theres a commercial version of it here ..http://www.johnhardyco.com/pdf/990.pdf by JohnHardy

[mickstv]

OPA1611 OPA1612

Same as what I use in my detector. I also use the OPA1642 for the rest of the stages. No problems with noise out in the field.

[6666]

The cross coupled stage after the preamp gives two outputs, one normal and the other inverted. That means that after gating, you can sum them into a single ended integrator and common mode signals, such as EF, cancel. You can use just a simple inverter stage but I believe the cross coupled system gives better accuracy.
Using the NE5532, I made all resistors 1K, except for the 75R which I believe was 180R (not critical). If you want some gain make Rf variable or different value. Exact symmetry is maintained as gain is varied. You could make the whole preamp like this but I preferred an independent front end. My present arrangement is gain 10x for front end and 47x for differential. You don't need the 49.9R resistors of course and would go straight to your integrator input gates with resistors summed on the inverting input of the integrator, which has a single R and C in the feedback.
[ATTACH]36575[/ATTACH]

Eric.

Thank you Eric ,its good to learn something.

[green]

Thanks for the replies. Lowering the noise before sampling makes sense. My question is should sampling increase low frequency noise(less than 15Hz). At work we filtered the signal before sampling to prevent aliasing. I haven't found any thing on what to expect if I sample resistor noise with out pre filtering. Maybe there is another cause I'm missing, sampling frequency jitter or some thing else? If I knew sampling resistor noise without pre filtering caused a four times increase in low frequency noise I could quite thinking about it.

[Monolith]

NOISE SEEN DYNAMICALLY

I like to look at the noise dynamically. Here is the output of the integrator, for a Nickel target pendulum swinging at 13cm above the coil.
The coil is 245mm diameter, but I find it better to define the distance as just over 1r, or half the diameter.

4 different sample windows are taken, the red and the yellow traces are window 2 and 4. The blue trace is the mathematical function of the scope, channel 1 minus channel 2.

The noise:
noise level on the board GND, about 10mV PP at frequencies of 2.5MHz, 2MHz, 4.1MHz. I don't know what I can do about that.
The most worrysome noises were at 50mV, PP, 0.22 Hz and 20mV, PP, 6.3Hz, so I increased the frequency of the high pass filter to 3.2Hz. Now I will look again in detail at the output traces to see if there is any improvement.

The preamp is the LME4999. I am now looking for another opamp with good 1f noise figure.

[Ferric Toes]

This is interesting, is a 1C integrator also known as a single ended integrator ?
and how does it cancel EF ?

The cross coupled stage after the preamp gives two outputs, one normal and the other inverted. That means that after gating, you can sum them into a single ended integrator and common mode signals, such as EF, cancel. You can use just a simple inverter stage but I believe the cross coupled system gives better accuracy.
Using the NE5532, I made all resistors 1K, except for the 75R which I believe was 180R (not critical). If you want some gain make Rf variable or different value. Exact symmetry is maintained as gain is varied. You could make the whole preamp like this but I preferred an independent front end. My present arrangement is gain 10x for front end and 47x for differential. You don't need the 49.9R resistors of course and would go straight to your integrator input gates with resistors summed on the inverting input of the integrator, which has a single R and C in the feedback.


Eric.

[moodz]

yup .. interesting about the 1C integrator .. I would have thought you could compensated for 'unbalance' by varying the sample duty cycles +/- x nanoseconds. The LME49990 is nice but apparently going end of life / production.

OPA1611 OPA1612

[green]

This is interesting, is a 1C integrator also known as a single ended integrator ?
and how does it cancel EF ?

Don't know what all it's called. I removed the inverter and the inverted input from the schematic since I wasn't taking an EF sample for the noise test. A schematic I posted awhile back including the inverter.