Versatile noise generator tests signal recovery gear

-May 19, 2014

I sometimes need a noise generator for testing equipment, and useful signals must often be mixed with the noise, such as when testing equipment designed to extract a clean signal from a contaminated one. This Design Idea implements such a piece of test equipment.




Figure 1
  Noise generator schematic

The source of white noise is avalanche diode D2 (Russian 2?401?): the spectral density of the noise is not less than 30µV/vHz@(25±10)°C, and irregularity of spectral density less than 4dB from 20Hz to 1MHz.



Figure 2  The spectral density of the 2G401V vs. operating current
 

The SSM2166S microphone preamp and compressor (IC2) (with “A” mark on package; without "A" needs another adjustment – see the datasheet) provides normalization of RMS noise level. The compressor stabilizes the output noise level at 775mV RMS (0dB), even as the input of IC2 varies from 3mV-30mV – much more variation than that expected from D2.

A filter can be placed between D2 & C2 if desired. In my generator, a 4th-order Butterworth LPF at 5kHz is used.

 

Figure 3 The optional 4th-order 5kHz Butterworth LPF

 

Without the filter, the frequency range of the noise will be approximately 20Hz-20kHz as set by IC2 & IC3.

Output level can be adjusted by R22 & R7. Values of the other parts around IC2 were established experimentally, by criteria of stability of characteristics of the generator (additional gain in its buffer amplifier: 20dB, compression ratio: 15:1, total gain: 34dB, rotation point: 150mV, noise gate: 10mV). The standard output resistance of 600O is established by R12 and the output stage resistance of the generator.

The test generator allows mixing of noise signals with another signals, for example, a low-frequency sine. The external signal is connected to the X1 “INPUT LF” socket. The switch BH1 turns off the noise signal, letting only the clean input through.

The signal level can be reduced in decade steps using an additional scaling amplifier. Switches S2-S5 select the gain.


Figure 4  The scaling amplifier

 

The generator uses power from an external PSU of 12VDC@100mA.

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