Pocket-Sized White-Noise Generator Quickly Tests Circuit’s Response (.PDF Download)
The behavior of analog circuits can often be characterized by sweeping their inputs with a signal going across a range of frequencies and then observing the output. These sweeps can be composed of discrete input frequencies or a swept sine.
For the first option, extremely low-frequency sine waves (below 10 Hz) are difficult to produce cleanly, and usually requires a processor, digital-to-analog converter (DAC), and some complex, precise filtering to produce relatively clean sine waves. Further, for each frequency step, the system must settle, making the sequential full sweep with many frequencies into a slow process. The faster option is to test at fewer discrete frequencies, but increases the risk of skipping over critical frequencies where high-Q phenomena reside.
In contrast, a white-noise generator is simpler and faster than a swept sine wave because it effectively produces all frequencies at the same time with the same amplitude. Imposing white noise at the input of a device under test (DUT) can quickly produce an overview of the frequency response over an entire frequency range. In this case, there’s no need for expensive or complex swept sine-wave generator—simply connect the DUT output to a spectrum analyzer and watch. Using more averaging and longer acquisition times produces a more accurate output response across the frequency range of interest.
The expected response of the DUT to white noise is frequency-shaped noise. Using white noise in this fashion can quickly expose unexpected behavior such as weird frequency spurs, strange harmonics, and undesirable frequency-response artifacts. Furthermore, a white-noise generator allows a careful engineer to test a tester. Lab equipment that measures frequency response should produce a flat noise profile when measuring a known, flat white-noise generator.
On the practical side, a white-noise generator is easy to use, small enough for compact lab setups, portable for field measurements, and inexpensive. Quality signal generators with myriad settings are attractively versatile. However, versatility can hamper quick frequency-response measurements. A well-designed white-noise generator requires no controls, yet produces a fully predictable output.