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Noise generator

I want to build a noise generator from a Zener diode. I designed the above circuit. The noisy voltage from the diode is filtered via a high-pass filter and input into a 92x gain non-inverting amplifier. (I replaced the 10k resistor by a 100k to increase the gain). The power sources are from 2 AC-DC adaptors at 11.6V. The op-amp is an LM318P and the diode is a 1N755A

Although I am using a diode with Vz=7.5V, the voltage actually drops by 9V. The current through the 100 ohm resistor is 16mA, well below the max current of the diode. At last I filter the output to limit the band to below the Nyquist frequency of a cheap PC-based oscilloscope and limit its max voltage to protect it. Results are shown below.

My problem is that the output signal exhibits a periodic signal of about 3kHz:

Noise output with periodic component

What is wrong? Hints would be very appreciated.

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  • \$\begingroup\$ Have you tried another zener? This is kind of an off-label application. If you replace the 100pF with 10nF do you still see the 3kHz? \$\endgroup\$ – Spehro Pefhany Dec 21 '18 at 4:55
  • \$\begingroup\$ Thx. Will try your suggestions tmr. I will let you know. \$\endgroup\$ – Antoine Dec 21 '18 at 5:38
  • \$\begingroup\$ I would try other diodes of the same voltage, but also 5 volt zener diodes. There is a possibility your noise source may have a resonate frequency based on temperature and your load capacitance. \$\endgroup\$ – Sparky256 Dec 21 '18 at 5:39
  • \$\begingroup\$ Your gain is only 9X. Nine. \$\endgroup\$ – analogsystemsrf Dec 21 '18 at 8:45
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Consider this circuit

schematic

simulate this circuit – Schematic created using CircuitLab

Your oscillation could be

1) thermal oscillation inside the Opamp, as the output current causes changes in the silicon heating, and the input bipolar diffpair becomes imbalanced; however the delta_V_out is 1 volt, so I doubt thermal feedback is the cause

2) how about 0.1 volts of I*R drop inside the battery, as the 10 or 20mA thru the Zener starts up, and the opamp amplifies that 0.1 volts by 10X to produce the 1 volt output? Yes. An internal resistance of 10 ohms or 5 ohms will cause this.

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