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05/93/70
A Broadband Random Noise Generator – Design Note 70
Jim Williams
Filter, audio and RF communication testing often requires
a random noise source. The circuit in Figure 1 provides an
RMS amplitude regulated noise source with selectable
bandwidth. The RMS output is 300mV with a 1kHz to 5MHz
bandwidth selected in decade ranges.
The A1 amplifier, biased from the LT1004 reference, pro-
vides optimum drive for D1, the noise source. AC coupled
A2 takes a broadband gain of 100. The A2 output feeds a
gain control stage via a simple selectable lowpass filter.
The filter's output is applied to LT1228 A3, an operational
transconductance amplifier. A3's output feeds LT1228
A4, a current feedback amplifier. A4's output, the circuit's
output, is sampled by the A5 based gain control configu-
ration. This closes a gain control loop back at A3. A3's I
SET
input current controls its gain, allowing overall output
level control.
To adjust this circuit, place the filter in the 1kHz position
and trim the 5k potentiometer for maximum negative
bias
at A3, pin 5.
Figure 1. Random Noise Generator with Selectable Bandwidth and RMS Voltage Regulation
15V
–15V
–
+
A1
1/2 LT1013
15V
–15V
1M
5k
6.2k
+
1µF
50k
D1
1N753
1µF
1k
2
3
1
8
4
3
2
–
+
1k 1k
10Ω
–
+
A2
LT1226
A3
1/2 LT1228
3
2
15V
900Ω
–
+
A4
1/2 LT1228
510Ω
10Ω
1µF
NONPOLAR 10k
+
22µF
+
22µF
0.05µF
–
+
A5
1/2 LT1013
0.1µF10k
1M
1N4148s
COUPLE THERMALLY
–15V
1V
P-P
OUTPUT
LT1004
1.2V
4.7k
7
5
7
6
DN70 • F01
5
1
8
6
1.6k
3k
4
–15V
0.001µF
0.01µF
0.1µF
10pF
100pF
5MHz
1MHz
100kHz
10kHz
1kHz
100k
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
●
FAX
: (408) 434-0507
●
TELEX
: 499-3977
LT/GP 0593 190K
LINEAR TECHNOLOGY CORPORATION 1993
For literature on our Voltage References,
call (800) 637-5545. For applications help,
call (408) 432-1900, Ext. 525
Figure 2 shows noise at a 1MHz bandpass while Figure 3
plots amplitude vs RMS noise in the same bandpass.
Figure 4 plots similar information at full bandwidth. RMS
output is essentially flat to 1.5MHz with about ±2dB control
to 5MHz before sagging badly.
Figure 3. RMS Amplitude vs Frequency for the Random
Noise Generator Is Essentially Flat to 1MHz.
Figure 4. RMS Amplitude Holds Within ±2dB Before
Sagging Beyond 5MHz
FREQUENCY (MHz)
0
AMPLITUDE VARIANCE (dB)
3
9
12
0.8
DN70 • F03
–3
–9
0.2 0.4 0.6 1.0
6
0
–6
–12
Figure 2. Figure 1's Output in the 1MHz Filter Position
FREQUENCY (MHz)
0
AMPLITUDE VARIANCE (dB)
3
9
12
8
DN70 • F04
–3
–9
24610
6
0
–6
–12