Simple Magnetic Delay unit with High Accuracy and Reliability For Digital Filters

The delay unit for digital filters can be realized with hardware in either of two ways, i.e., the analog and digital types. If accuracies between 0.1 and 1 percent are required, then it is known that the analog methods simplify the construction of the digital filters. This paper presents the delay unit for the digital filters using a single core which circumvents the problem of core matching. The magnetic delay unit has essentially no practical limitation on the long delay time. Therefore, the digital filters for very low frequencies can be easily realized. The characteristics of the core material to be required for such a unit are discussed and a method of practical realization is shown. In the practical unit, the delay time of 1 hour is realized without trouble. As to the relation between the input and output signals, an accuracy of within approximately 0.05 percent can be maintained when the whole supply voltage is changed by ±4 percent. Under the conditions of the temperature change from 20 to 50°C, there appears no drift caused by the change of temperature. For large variations of time during 24 hours, the drift is not observed experimentally, so that the above mentioned accuracy can be maintained. These facts show that the delay unit is fairly stable and reliable. As an example of a digital filter to be realized for the noise rejection, the simple first-order low-pass filter has been constructed by the method of the bilinear transformation.