PI noise

[Ferric Toes]

My prefered analogue integrator

This is an arrangement that works well. It has the benefit that the capacitors 'hold' until the next sample i.e. there is no leak down through a parallel resistor.

In the differential version the loop gain controls the time constant, so you can have an external pot to vary the 'response speed' or 'noise averaging'.



Eric.

[green]

...not sure why it should increase by 5 times ... dont forget your sampling is synchronous .. the noise is not.

You are suggesting the noise doesn't increase as much as the signal because noise is non synchronous? The noise is non synchronous at 1k and 5k samples/second with the gain 5 times higher at 5k but you still might be right. Don't know how to prove why the noise doesn't increase as much as the gain.

[green]

Several things going on here so let's go back a bit. You appear to be saying that you are increasing the whole cycle of TX pulses and RX sampling from 1000 operations per sec. to 5000. As you say, this should give 5 times the signal at the integrator output. It appears that you are keeping the integrator TC the same. If you are happy with the response speed at 1kHz (in radar we used kpps which is more accurate for pulses), then you can increase the TC by a factor of 5 and you will see some improvement in S/N.

Eric.

Trying to understand how the integrator feedback TC and sample rate correlate. I was thinking sample rate effects gain and feedback TC effects low pass filter response.

[green]

I haven't studied the circuit closely, but the increased noise might be due to the differentiator, which amplifies the input in direct proportion to the frequency. In other words, the noise level increases with frequency. Of course, that might be nothing to do with it, but it could be worth looking at.

spice simulation. R7 and C4,(integrator feedback)simulate integrator gain=1.

[Ferric Toes]

I measured p-p noise level at post amplifier out with the coil input shorted with 1kHz and 5kHz sample rate. p-p noise increased about 2 times at 5kHz. Target, ground and EF signal should increase 5 times giving an increase in S/N of 2.5. Why doesn't the noise signal increase 5 times? My guess in above reply. What is the reason?

Several things going on here so let's go back a bit. You appear to be saying that you are increasing the whole cycle of TX pulses and RX sampling from 1000 operations per sec. to 5000. As you say, this should give 5 times the signal at the integrator output. It appears that you are keeping the integrator TC the same. If you are happy with the response speed at 1kHz (in radar we used kpps which is more accurate for pulses), then you can increase the TC by a factor of 5 and you will see some improvement in S/N.

Eric.

[Qiaozhi]

I haven't studied the circuit closely, but the increased noise might be due to the differentiator, which amplifies the input in direct proportion to the frequency. In other words, the noise level increases with frequency. Of course, that might be nothing to do with it, but it could be worth looking at.

[moodz]

...not sure why it should increase by 5 times ... dont forget your sampling is synchronous .. the noise is not.

[green]

Thanks for the replies.

I have tested the detector in the yard. The ground signal is a lot higher than amplifier noise. The detector isn't usable without GEB. Noise goes up when I add GEB so I'm trying to minimize the noise.

I wonder why some detectors add switches to eliminate the 1k input resistor if coil noise is always higher than 1k resistor noise.

I'm learning so what I'm thinking might not be correct. Target signal is less than 10Hz, ground and EF maybe closer to 1Hz so maybe other than line frequency(50 or 60Hz)most other frequencies are above the sampling rate, if not they would be filtered anyway. The maximum frequency after sampling is 1/2 the sampling frequency. If we filtered all frequencies above 10Hz the only higher frequencies that would matter are 10Hz and below and within 10Hz of multiples of the sampling frequency. Sample rate 1kHz(1010,2010,3010,3990 etc.) 5kHz(4990,10010,19990,30010 etc.) maybe one of the reasons higher sampling frequency might have less noise, fewer frequencies to alias on.

I measured p-p noise level at post amplifier out with the coil input shorted with 1kHz and 5kHz sample rate. p-p noise increased about 2 times at 5kHz. Target, ground and EF signal should increase 5 times giving an increase in S/N of 2.5. Why doesn't the noise signal increase 5 times? My guess in above reply. What is the reason?

[moodz]

The impact of EMI including very low frequencies down to DC should not be underestimated in a PI. The front end of a PI is a natural modulator and signal frequencies in the the frontend are mixed or undergo CONVOLUTION. The pictures below illustrate this ... the first pic is the spectrum of a signal from a PI operating at 1kHz with substantially no noise. The next two pics show the effect of a 60 Hertz signal on the PI signal. You should be able to see that the 60 Hertz is modulated onto the spectral components of the PI signal. The thing about CONVOLUTED signals is that they cannot be removed with a filter. Even though I have used a 60 Hertz signal to illustrate in practice ALL signals ( ie noise / EMI etc etc) from DC to up will be convoluted / modulated onto the PI signal and demodulated with your "wanted" signal.

[Ferric Toes]

Eeek! Maybe I was too economical with words again. Coil noise is EMI. EMI includes power line noise, radio stations, mobile phones, lightning, static generated by hot windblown sand, waves crashing on a seashore, micropulsations in the earth's field; in fact any signal that blows into the detector via the coil when the sampling gates are open. Except of course, the coherent, response from a metal target. You might question the waves thing, but, you have rotating conductive water in the earth's magnetic field. Yes, EMI is generated. A coil of even a few ohms resistance generates little noise of itself compared to the preamp input resistor.

A mono coil picks up a large amount of emi even if well shielded. I use one of my 11in shielded mono coils as an antenna on my workshop radio. Noticeable attenuation only comes in at upward of 7MHz. A figure 8 coil is good for workshop testing but even that becomes unbalanced upwards of 50kHz unless you are very careful about each half being an exact mirror image of the other. This can be overcome with printed circuit coils as I did for an industrial application a while back. This was a four layer board with electrostatic comb shielding as top and bottom layers. Not something for hobby experimentation though.

Another technique is to use a mono coil as TX/RX1 and an identical mono coil mounted rigidly above it as an RX2. TX/RX1 is fed to one preamp input, say the non-inverting, and RX2 fed to the inverting. EMI cancels nicely right at the front end. You have to have sufficient spacing between coil so as not to cancel the target signal and the coil array is a bit unwieldy. The front end can be tweeked to give best common mode noise rejection but to every benefit there is a trade-off. Here, it is two coils and two man operation.


Try it with a couple of small coils, say 8in.

I would say that here in the UK we suffer more from man made EMI than you would in Australia. Out in the bush in Victoria I could dispense with the dual coil in the picture, as a single mono was just as quiet and much easier on the arms.

Eric.

[green]

Thanks for the replies.

I have tested the detector in the yard. The ground signal is a lot higher than amplifier noise. The detector isn't usable without GEB. Noise goes up when I add GEB so I'm trying to minimize the noise.

I wonder why some detectors add switches to eliminate the 1k input resistor if coil noise is always higher than 1k resistor noise.

I'm learning so what I'm thinking might not be correct. Target signal is less than 10Hz, ground and EF maybe closer to 1Hz so maybe other than line frequency(50 or 60Hz)most other frequencies are above the sampling rate, if not they would be filtered anyway. The maximum frequency after sampling is 1/2 the sampling frequency. If we filtered all frequencies above 10Hz the only higher frequencies that would matter are 10Hz and below and within 10Hz of multiples of the sampling frequency. Sample rate 1kHz(1010,2010,3010,3990 etc.) 5kHz(4990,10010,19990,30010 etc.) maybe one of the reasons higher sampling frequency might have less noise, fewer frequencies to alias on.

[moodz]

Not sure what the debate here is ... Coil noise is way bigger than resistor noise in all cases except bad design :-).

EMI is everywhere so the no-EMI measurement is not relevant.

PIX shows the comparison of shielded coil ( yellow ) vs 1K resistor ( blue ).

[Qiaozhi]

Hmmmm, coil noise, do we have a definition for that one? I translated that as the noise that the coil picks up is greater than the amplifier noise.

Cheers mick

I read Eric's statement to mean that any noise generated by the amplifier would be low compared to when the coil is connected (in an EMI-free environment).
Eric will no doubt enlighten us further.

[Mechanic]

What Eric actually stated was: "I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor."
He never mentioned EMI.

Hmmmm, coil noise, do we have a definition for that one? I translated that as the noise that the coil picks up is greater than the amplifier noise.

Cheers mick

[Qiaozhi]

Hi Eric,

I would have to disagree on this. Out in the field, sure some days/times of days the emi received is higher than the amplifier noise floor noise(listening to the output noise, not looking at the front end with a scope). However, on other days or late in the day/night time the external emi seems to quieten down to the point where you really only hear noise floor noise of the detector and front end gain can really be cranked up. If I were to be using a 5534 and 1k input resistors, I could not increase the front end gain and get a usable advantage, no matter what the external emi was doing. And this extra gain does make a big difference in target response for both small bits that would not otherwise be heard even if rubbed on the coil and for larger bits at depth.

Cheers Mick

What Eric actually stated was: "I have always found that coil noise is far in excess of amplifier noise and that is the limiting factor."
He never mentioned EMI.