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| LM324 Noise Source |
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| Kleinstein:
A sampling rate of 1 Ms/s suggests an upper limit of 500 kHz. This would mean 50 kHz per div. One should be able to more the maker away from the left edge to somewhere in the center to get a more meaningful number to estimate the scale. Otherwise one could check the scale with a known signal (e.g. 1 kHz cal signal for the probes). Even if colored noise, it could still be useful to create something like true random number, e.g. with a µC reading the noise with an ADC. |
| Zero999:
--- Quote from: ogden on September 17, 2019, 01:42:48 pm --- --- Quote from: Zero999 on September 17, 2019, 01:03:48 pm ---Increase the resistor values for more noise. --- End quote --- Don't bother. Pk-pk amplitude of existing noise is 1.4V already, no need to increase it further. As noise is "colored" towards low frequencies, this "noise generator" does not have any meaningful value other than learning tool - opamp noise demonstrator. If you are looking for noise source, you better build it out of zener and broadband amplifier (not for gods sake LM324). --- End quote --- I'm talking about simplifying things by using the LM358, rather than the LM324, so more gain would be required, with only two stages, rather than three. Something like this will do. It has a similar lower cut-off and noise gain to the original poster's circuit, but it's crammed into two stages, each with a noise gain of 1+820/3.3, rather than three stages with a noise gain of 40, so the upper cut-off frequency will be lower. R7 & R8 are there to match the bias currents at the inputs. They can be omitted but the DC output voltage will increase by about 75mV and will be less stable, over the temperature range. C3 helps to prevent noise from the supply being coupled to the circuit's output. Is the spectrum really that important if all the original poster needs is to seed a random number generator? |
| ogden:
--- Quote from: Kleinstein on September 17, 2019, 02:03:17 pm ---A sampling rate of 1 Ms/s suggests an upper limit of 500 kHz. This would mean 50 kHz per div. One should be able to more the maker away from the left edge to somewhere in the center --- End quote --- Thing is that marker already is in the center, showing 5.9 KHz frequency. [edit] Yes, I agree that this kind of noise source potentially can be used for RNG application. |
| Dabbot:
--- Quote from: ogden on September 17, 2019, 03:22:56 pm --- --- Quote from: Kleinstein on September 17, 2019, 02:03:17 pm ---A sampling rate of 1 Ms/s suggests an upper limit of 500 kHz. This would mean 50 kHz per div. One should be able to more the maker away from the left edge to somewhere in the center --- End quote --- Thing is that marker already is in the center, showing 5.9 KHz frequency (attach). [edit] Yes, I agree that this kind of noise source potentially can be used for RNG application. --- End quote --- That's just the scope's inbuilt frequency measurement being very confused. It's all over the place while running. Sorry, I should have turned that measurement off. The FFT is 0 to 100KHz, with 50KHz center. |
| Dabbot:
--- Quote from: Zero999 on September 17, 2019, 03:13:18 pm --- --- Quote from: ogden on September 17, 2019, 01:42:48 pm --- --- Quote from: Zero999 on September 17, 2019, 01:03:48 pm ---Increase the resistor values for more noise. --- End quote --- Don't bother. Pk-pk amplitude of existing noise is 1.4V already, no need to increase it further. As noise is "colored" towards low frequencies, this "noise generator" does not have any meaningful value other than learning tool - opamp noise demonstrator. If you are looking for noise source, you better build it out of zener and broadband amplifier (not for gods sake LM324). --- End quote --- I'm talking about simplifying things by using the LM358, rather than the LM324, so more gain would be required, with only two stages, rather than three. Something like this will do. It has a similar lower cut-off and noise gain to the original poster's circuit, but it's crammed into two stages, each with a noise gain of 1+820/3.3, rather than three stages with a noise gain of 40, so the upper cut-off frequency will be lower. R7 & R8 are there to match the bias currents at the inputs. They can be omitted but the DC output voltage will increase by about 75mV and will be less stable, over the temperature range. C3 helps to prevent noise from the supply being coupled to the circuit's output. (Attachment Link) Is the spectrum really that important if all the original poster needs is to seed a random number generator? --- End quote --- The discrete component count is actually the same as my original circuit. None the less, I'll build this and give it a spin this afternoon. :) |
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