Broadband Noise Generator

To test some electronic systems such as RF communication, filter, or audio we often need random noise source. This article shows the circuit that generates an RMS amplitude regulated noise source with selectable bandwidth. This circuit is suitable for wide range of application since it has 1 KHz to 5 MHz decade ranges selectable   bandwidth and 300mV RMS output. Here is the circuit:

D1 generates noise, that is AC coupled to A2. A2 is an amplifier with broadband gain 100. It yields to a simple selectable low-pass filter output. The output of this filter is applied to A3,which is LT1228 operational transconductance amplifier. The output from A3 feeds A4 which is current feedback amplifier(LT1228). The A5-based gain control configuration samples the output of this circuit which also A4’output. This closes a gain control loop to A3. A3’s ISET current controls gain, allowing overall output level control. [Schematic diagram source: Linear Technology Application Notes]