A Broadband Random Noise Generator – Design Note 70 Jim Williams

Filter, audio and RF communication testing often re- output feeds LT1228 A4, a current feedback amplifi er. quires a random noise source. The circuit in Figure 1 A4’s output, the circuit’s output, is sampled by the A5 provides an RMS amplitude regulated noise source with based gain control confi guration. This closes a gain selectable bandwidth. The RMS output is 300mV with control loop back at A3. A3’s ISET input current controls a 1kHz to 5MHz bandwidth selected in decade ranges. its gain, allowing overall output level control. The A1 amplifi er, biased from the LT®1004 reference, To adjust this circuit, place the fi lter in the 1kHz position provides optimum drive for D1, the noise source. AC and trim the 5k potentiometer for maximum negative coupled A2 takes a broadband gain of 100. The A2 bias at A3, Pin 5. output feeds a gain control stage via a simple selectable L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks lowpass fi lter. The fi lter’s output is applied to LT1228 of Linear Technology Corporation. All other trademarks are the property of their respective owners. A3, an operational transconductance amplifi er. A3’s

1M 0.1μF 15V

1kHz 100k 2 – 8 0.01μF A1 1

1μF 10kHz

1/2 LT1013 3 + 50k 3 5k + 0.001μF 4 + A2 1.6k 1μF D1 100kHz –15V 1k LT1226 6.2k 1N753 2 – 100pF 1k 1k 1MHz 10pF 10Ω 5MHz

15V 3 + 7 A3 1 1/2 LT1228 + 2 A4 6 1V – 900Ω 1/2 LT1228 P-P 5 8 OUTPUT – 4 510Ω 1μF NONPOLAR –15V 10k 3k

10Ω 22μF 22μF + +

0.1μF 0.05μF 10k

15V – 6 1M 4.7k –15V 7 A5 %/
t
'

1/2 LT1013 1N4148s + 5 LT1004 COUPLE THERMALLY 1.2V

–15V

Figure 1. Random Noise Generator with Selectable Bandwidth and RMS Voltage Regulation

05/93/70_conv Figure 2 shows noise at a 1MHz bandpass while Figure 3 plots amplitude vs RMS noise in the same bandpass. 12 Figure 4 plots similar information at full bandwidth. 9 RMS output is essentially fl at to 1.5MHz with about 6 ±2dB control to 5MHz before sagging badly. 3 0 –3 –6

AMPLITUDE VARIANCE (dB) –9 –12

0 0.2 0.4 0.6 0.8 1.0 FREQUENCY (MHz)

%/
t
' Figure 3. RMS Amplitude vs Frequency for the Random Noise Generator Is Essentially Flat to 1MHz.

12 9 6 Figure 2. Figure 1's Output in the 1MHz Filter Position 3 0 –3 –6

AMPLITUDE VARIANCE (dB) –9 –12

0 2  6 8 10 FREQUENCY (MHz)

%/
t
' Figure 4. RMS Amplitude Holds Within ±2dB Before Sagging Beyond 5MHz

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