Random noise source is often needed in testing many type of electronic systems such as filter, audio, or RF communication. The circuit presented here generates an RMS amplitude regulated noise source with selectable bandwidth. With 1 KHz to 5 MHz decade ranges selectable bandwidth and 300mV RMS output, this noise generator is suitable for wide range of application.
Noise is generated by D1 that is AC coupled to A2, an amplifier with broadband gain 100. The output of A2 is fed to a simple selectable low-pass filter. The filter’s output is applied to LT1228 operational transconductance amplifier A3. A3’s output feeds current feedback amplifier LT1228 A4. A4’s output, which is also the circuit’s output, is sampled by the A5-based gain control configuration. 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]