Preamp

[green]

See if the comments above are adequate. If not just ask...

Thanks for reply #81. Shows me how to get a good FFT. What I'm missing in the PI example is how to use the FFT plots. It still looks easier to see if the peak to peak signal is grater than the peak to peak noise. The FFT could show high 60 Hz noise when it could have a low 60 Hz peak to peak because it's continuous.

[Old cart]

Always wanting to learn something. We used FFT where I worked, sometimes it made sense to use FFT and sometimes not. Could you give an example using FFT to determine signal to noise for a PI detector?

See if the comments above are adequate. If not just ask...

[Old cart]

Thanks, I got it and it seems to work fine on Windows 10

that is nice software. It will also let you try out various filters and see the effect on the noise.

[Old cart]

FFT usage and hints

FFT's are a mathematical way to convert from time domain (amplitude vs time) to frequency domain (amplitude vs frequency).While these are just different views of the same signal (object) each domain has advantages when it comes to looking for different things. If the signal is complex waveform it is difficult to decompose it into the various frequency components. In the case of our PI's the signal is ideally a rectangular pulse of width t. The Spectrum of this ideal pulse would be a Flat line starting at DC with the first zero in the response at 1/t. There would be successively smaller cosine shaped humps after that and successive zeros in the response at 2,3,4,5...times 1/t.

There used to be some great online calculators to help you visualize what any time domain signal looks like in the frequency domain. I suggest you do a google search for fourier component calculator to find one that works it's your computer. This wil help give Aquino understanding about how FFT's work. A general search for FFT's yields lots of information but much of it is very math heavy.

However there are few basic rules for using the FFT's in any device, like a scope that has this function.

1. Time and frequency are inverse domains so good time resolution wil give poor frequency resolution.
2. Set the scope record length to around 10000 points.
3. Set the sample rate so that it is around 4 times the highest frequency component you expect to observe. For example if you want to look at all the noise coming out of the first stage of the preamp and want to see all the signal to say 250Khz set the sample rate to 1 Ms/S.
4. Make the signal fill the screen vertically to the extent this is possible.
5. Turn on FFT. You should see a signal in the frequency domain. Many scopes will allow you to place cursors on the spectrum and measure amplitudes at various frequencies. Use haning or hamming windowing if this is adjustable on your scope and if the signal contains periodic components. No windowing is necessary if the signal contains broadband noise.
6. You may have to adjust the sweep speed slightly to show the best frequency domain detail.
7. Since FFT's are just mathematical transforms of the time series of the data they have limitations.
a. You wil not be able to se signal frequencies higher than 1 / sample rate. ( 1 MS shows signals up to 500KHz BUT it is best to ignore anything great than 250 KHz in this example)
b. The best resolution you can get is the sample rate /2 / .5 times the record length so with the setting above this becomes
1MS/s / 2= 500KHz/ 5000 points = 100 Hz. So with thes setting it would not be possible to differentiate the difference between 50 Hz and 100Hz, they would just blob together on the display.
c. The best vertical dynamic range you can get is approximately 6 x the number of bits your a to d converter has ( 8 or 48 dab) in a general purpose scope.

Using a sound card with a PC can give much broader range in amplitude but not in the frequency range since most sound cards are limited to 96KHz sample rate and give a frequency range of 48KHz. This is usually adequate to look at the output after the integrator since it severely band limits the signal. I use a pice of great software from Croatia cal ARTA. Even the free version if fully functional except you can not save waveforms or spectrum displays.

[green]

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...

Always wanting to learn something. We used FFT where I worked, sometimes it made sense to use FFT and sometimes not. Could you give an example using FFT to determine signal to noise for a PI detector?

[Monolith]

http://www.qsl.net/dl4yhf/spectra1.html

try here

Thanks, I got it and it seems to work fine on Windows 10

[6666]

http://www.qsl.net/dl4yhf/spectra1.html

try here

[Monolith]

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.

I admire your systematic, methodical approach. To be able to detect a 1uVolt target response would be excellent.

The 3mV noise that Eric was talking about was taken at the end of signal processing, before the signal voltage is converted to the audio output. The same applies to the screen shots I posted. So this is the accumulation of all noise. The input and pre-amp noise can then only be inferred, by changing the front-end, coil, shielding, op-amp etc.

[Monolith]

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...

I would very much like to get your tips on FFT. My pre-amp signal input must have a minimum bandwidth of 1MHz.
Some years ago, I used SPECTRUMLAB, to look at my signal. Attached is a screen shot. You can see the FFT. It shows distinct noise at the 60Hz mains and 120Hz harmonic.
Unfortunately, as it uses the sound card of the computer, it's frequency range is limited.
I am going to search for this software again, hope it works with Windows 10.

[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...

[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.

[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.

[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.

[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.

[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.