Analog/DIgital filter with infinite attenuation
Original Publication Date: 2000-Aug-01
Included in the Prior Art Database: 2003-Jun-19
Disclosed is a method to design an analog filter (any low pass, high pass and bandpass filters) with very high attenuation characteristics , very accurate and without high precision components. As described on figure 1, a low pass filter comprises an oscillator and a frequency to voltage converter (F/V), the output of this converter being the reference voltage of the alternative signal making the cutting frequency of the filter. The other side of the schematic is made up of an Automatic Gain Control (AGC) whose inputs are the Input signal to be filtered and a feed -back (the reference frequency). The comparator is comparing voltage (in this case a voltage is the traduction of a frequency). As the input signal may have a variable amplitude, the AGV is used so that the inputs of the F/V converters presents similar amplitudes. Starting from a low frequency, the FET is ON , thus the input signal is found at the output and there is no attenuation. When the cutting frequency is reached, the comparator toggles and the FET is blocked , so the attenuation is max. It is to be noticed that the only difference between a low pass filter and a high pass filter lies in the position of the comparator inputs. Thus, to design a high pass filter, it is necessary to reverse the and inputs of the comparator. To design a bandpass filter, a low pass filter and a high pass filter have to be connected in serial.