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:
What is wrong? Hints would be very appreciated.