| Electronics > Beginners |
| LM324 Noise Source |
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| Zero999:
It has the same part count, if you include R7 & R8, which probably aren't needed. Try omitting them and see what happens. |
| ogden:
--- Quote from: Dabbot on September 17, 2019, 08:55:54 pm ---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. --- End quote --- Frequency measurement shall be confused while measuring *noise*. That's whole point of noise :D It's my fault, I missed 50KHz display for "FFT channel" measurement @cursor because of unusual placement. Anyway - if frequency span displayed is 0 - 100KHz, then drop of noise amplitude we see is not due to BW limit, i'ts corner/knee between 1/f and broadband noise. |
| 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 --- ...and the results are in! I built the circuit and checked it out on the scope. Lots of oscillations and even some clipping (not shown), but this is not due to the circuit itself, rather the circuit's sensitivity to the environment. It's on a breadboard and the metal base was not grounded. I took another shot of the scope with said base grounded and you can see it all but got rid of the oscillations. As you can see, the amplitude of the noise is lower, but still very usable for my intents and purposes. I performed the same tests with the breadboard's base on my original circuit and it did not show the same susceptibility. Edit: Fix typos in attachments. |
| Zero999:
I'm surprised the output is that much lower. Did you test it both with R7 & R8 and without? The circuit I posted will be more susceptible to EMI, simply because it uses higher value resistors, than your circuit. |
| Dabbot:
Taking out R7 and R8 causes the output to diminish substantially. Don't these chips share biasing circuitry? It might explain the smaller amount of noise, given there's less circuitry to generate it. |
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