Shadow filters – new family of second-order filters

Design of CDTA and VDTA Based Frequency Agile Filters

“…The CDTA [11][12][13][14][15][16][17][18] consists of a unity-gain current source controlled by the difference of the input currents and a transconductance amplifier providing electronic tunability through its transconductance gain. The CDTA symbol is shown in Figure 4 and its terminal characteristics in matrix form are given by…”

“…The frequency agile filter (FAF) [1][2][3][4][5][6][7][8][9][10] characterized by adjustment range, reconfigurability, and agility may be used in transceivers. The term shadow filters is sometimes used in literature to refer to FAF [11,12]. The literature survey shows that a limited number of topologies of active FAF are available and are based on op-amp [1] and current mode active block [2,3] and CMOS [4].…”

Shadow filters for orthogonal modification of characteristic frequency and bandwidth

“…Introduction: The idea of the so-called shadow filter published in [1] enables an additional modification of the characteristic frequency ( f 0 ) of existing second-order filters via simply modifying the gain of an external amplifier, connected in a feedback path from the lowpass output to the input of the filter. This method can be useful in many applications, such as tunable filters for telecommunications or for additionally trimming the f 0 after its prospective modification due to non-ideal effects.…”

“…This method can be useful in many applications, such as tunable filters for telecommunications or for additionally trimming the f 0 after its prospective modification due to non-ideal effects. It is shown in [1] that f 0 can be tuned over a relatively large range with a simultaneous modification of the quality factor (Q) such that the 23 dB bandwidth (B ¼ f 0 /Q) remains constant. Also, the bandpass gain remains unchanged but the lowpass gain is decreased with increasing f 0 .…”

“…This Letter extends the above idea to two methods for a simple control of B and Q via an external amplifier without disturbing f 0 . Combining the first method with the f 0 control from [1], one can control f 0 and B orthogonally.…”