Below is the schematic of a simple, audio frequency white noise generator. This simple piece of test
equipment is very useful for measuring the frequency responses of audio systems.
It uses a Zener diode as a white noise source. The Zener diode is biased into breakdown to produce
approximately 0.1 ma of average current through the 47kohm resistor. More current is ok... if you wish to use a higher voltage
to bias the Zener diode. In this case, two 9-volt batteries are used as a bias voltage source for a 12 volt Zener diode.
The current flowing through the 47kohm resistor is (as desired) very noisy, because the
Zener diode's avalanche breakdown process greatly increases the shot noise associated with the average current.
The op-amp acts as an impedance converter (~ 50kohm input impedance, and ~ 10 ohm output impedance)
and also provides a voltage gain of 23 dB.
The 100pF capacitor limits the bandwidth of the white noise to an upper frequency of around 50
kHz. (taking into account the input inpedance of the op-amp). The 0.22 uF coupling capacitor (for DC blocking) limits
the lower frequency of the white noise to around 15 Hz. The 16 uF output coupling capacitor limits the lower frequency
of the white noise to around 100Hz (with the 100 ohm output load shown in the schematic). A larger coupling capacitor
(~80 uF) would be useful for producing a lower frequency (~20Hz) cutoff of the white noise. To drive an 8 ohm load,
an even larger output coupling capacitor (~1000 uF) would be needed to produce a low frequency cutoff of the white noise
of 20 Hz. .
The output of the op-amp can drive the line level input of a mixer.
