Shadow filters for orthogonal modification of characteristic frequency and bandwidth

Abstract

The idea of shadow filters for modifying the characteristic frequency f 0 of second-order filter cells via an external amplifier is generalised to the case of independent control of f 0 and bandwidth. This theory is confirmed experimentally on a current-mode biquad.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. 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.

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].…”

“…Among these blocks current difference transconductance amplifier (CDTA) [11] is most suitable for current mode signal processing owing to its low input and high output impedances, respectively. The VDTA is yet another recently introduced building block which works on a principle similar to that of CDTA except that the input current differencing unit is replaced by the voltage differencing circuit.…”

Tuning approach for first‐order filters and new current‐mode circuit example

“…More recently, the concept of employing an amplifier in the feedback loop of a second-order filter has been introduced, to obtain non-interactive tuning of filter parameters, without actually affecting any of the components comprising the filter core. These have been referred to as shadow filters and successfully used in recent literature with effective results [11][12][13][14][15][16]. The motives of such design have been to vary either the characteristic frequency or the quality factor of the second-order filters, independent of each other, and more importantly without varying the actual elements comprising the filter circuit.…”

New voltage-mode bandpass shadow filter

“…The CCCII was also used in [11] for realizing badpass shadow filters. In [2] the authors extended the idea of [1] for controlling the bandwidth and the quality factor without disturbing the center frequency. The authors of [2] presented a Z-copy current inverter transconductance amplifier (ZC-CITA)-based current-mode bandpass filter.…”