答案请以分号分隔 (1)What is a filter?A filter is a device that passes electric signals________certain frequencies or frequency ranges-preventing the passage of others. Filter circuits are used________a wide variety of applications.________(在电信领域),band________pass filters are used in the audio frequency range(0 kHz to 20 kHz)for________(调制 解调器和语音处理).High________frequency band________pass filters(several hundred MHz)are used for________(信道选择)in telephone central offices.Data acquisition systems usually require________(抗混叠低通滤波器)as well as low________pass noise filters in their preceding________(信 号调理)stages.System power supplies often use band________rejection filters to suppress the 60________Hz line frequency and high frequency transients.In addition,there are filters that do not________(滤除)any frequencies of a complex input signal,but just add________(线性相移)a linear phase shift to each frequency component,thus contributing________a constant________(时延).These are called________(滤波器).t high frequencies(>1 MHz),all of these filters usually consist________passive components________inductors(L),resistors(R),and capacitors(C).They are then________LRC filters.In the lower frequency range(1 Hz to 1 MHz),however,the inductor value becomes very large and the inductor itself gets quite bulky,________economical production difficult.In these________,________(有源滤波器)become important.Active filters are circuits that use________(运算放大器)as the active device________combination with some________(电阻和电容)to provide an LRC________like filter performance at low frequencies.