Hum Bug Reference Manual Page 9
Quest Scientific
SOURCES OF COMPLEX NOISE
Fluorescent lamps, dimmers and related noise sources can induce very narrow spikes of
interference which repeat at some multiple of 60 Hz (or 50 Hz). Under worse case conditions the
frequency components within these spikes can approach the upper limit of your signal
conditioning filters.
The Hum Bug will eliminate spikes with frequency components up to 4 kHz. If you
routinely set the upper limit of your signal conditioning filters to a value greater than this limit,
you may occasionally find that high frequency noise components are not fully eliminated. If so,
try adjusting your signal conditioning filter to a lower setting. This will remove some of the
highest frequency harmonics so that the remaining noise can be eliminated by the Hum Bug.
Whether this trade off between noise elimination and frequency response is acceptable will
depend on your application. Feel free to consult Quest Scientific for more information.
ADAPTATION RATE
The Hum Bug adapts to changing noise characteristics in the input signal. Under normal
conditions these characteristics evolve slowly and the Hum Bug is able to maintain effective
noise elimination. However, noise will appear in the output signal following a sudden change in
noise characteristics. This output noise will then decay with a time constant related to the
adaptation rate of the Hum Bug. Under ideal conditions this time constant is in the order of 5 to
10 seconds. In practice, the exact time constant is difficult to predict because it depends on the
change in noise amplitude, the harmonic content of the noise, and the magnitude of the noise
relative to ongoing physiological activity. The Hum Bug adapts rapidly when the input signal is
dominated by noise and proceeds more slowly when low amplitude noise is imbedded within
continuous physiological activity. Adaptation is also slower for harmonics with frequencies
greater than 1 kHz.
