Abstract
To eliminate a low frequency noise contained in a step DC signal, usually a lowpass filter is used, but when the frequency of the noise is very low, a great deal of time is required to get the final value of the step DC signal. The author has tried turning his attention in these circumstances to the rise-time interval of the DC signal. That is, the author has replaced the resistor ofthe ordinary RC onestage filter with a photoconductor-lamp composite element whose resistance can be controlled by a lamp current. The time-constant of this filter is made very small at the beginning of the DC step rise, and is increased proportionally to the time elapsed from the beginning. After a short time, the time-cnstant is made very large and is fixed. This procedure is triggered by the rise of the DC signal so the author calls this filter a “trigger filter”.
As a result of this, it has been found, theoretically and experimentally, that the output of this filter rises to within a given allowable range faster than the output of an ordinary RC one-stage low-pass filter. For example, when the noise is sine wave, the time required for the trigger filter to reach its response within the range of ±36% of the final value of the step response is less than that required for the ordinary RC filter.
