All-Pass RC-Filters Architecture with Independent Adjustment of the Main Parameters Based on Differential Difference Amplifiers

A R T I C L E I N F O A B S T R A C T Article history: Received:28 May, 2019 Accepted:04 July, 2019 Online: 11 July, 2019 We have suggested an architecture of the second-order all-pass active RC-filter (ARCF), based on three differential difference amplifiers (DDA’s), which provides full set of amplitude-frequency responses (AFR’s) (low pass filter (LPF), high pass filter (HPF), band pass filter (BPF), rejection filter (RF)). We have given the basic equations, which allow ARCF’s. The results of computer simulation of the LPF, HPF, BPF and RF’s are discussed. In case of small volume production it is possible to produce the given ARCF in a form of specialized structural arrays, in which LPF, HPF, BPF and RF characteristics are realized by using one of the seven inputs, to which the processed signal is supplied, and four outputs, from which the said signal is outputted. The ARCF is noted, because its transfer ratio and pole frequency are not changed, when the pole Q-factor is adjusted.

The present article's purpose and novelty are analysis of allpass ARCF's new structure's properties [34]. The filter is provided with independent adjustment of base parameters (for example, with digital potentiometers or passive elements' digital switching. A full set of LPF, HPF, BPF, RF's characteristics are implemented by switching seven inputs, on which the signal is supplied, and four outputs, form which the said signal is outputted.

ARC-Filter Base Architecture's Properties
There is a circuit of the suggested all-pass ARC-filter (LPF, HPF, BPF, RF) on Fig. 1 [34]. It provides an independent adjustment of pole Q-factor, when pole frequency and transfer ratio, which depend on other elements' parameters, are constant. The independent adjustment is achieved by controlling resistors' resistance. It significantly simplifies a process of frequency selection devices' trimming and adjustment, based on the suggested ARCF circuit. The buffer amplifier (BuffAmp) in scheme Fig. 1 is implemented on a base of traditional operational amplifiers (OpAmps) or DDA, it provides a wide range of R3, R4 resistors' resistance change.
All types active RC-filters' (LPF, HPF, BPF, RF) generalized transfer function is written as (the filters are implemented on the circuit, given on where ai, bj are equation (1) numerator's and denominator's coefficients, which depend on components' parameters and inputs and outputs in circuit on Fig. 2.
A set of transfer function's numerator coefficients ai defines a type of ARC-filter (LPF, HPF, BPF, RF).
There are transfer functions' (1) numerator coefficients ai, which are realized in particular ARCF of the Fig. 1 connection circuits, given in Table 1.
The sensitivity functions of the basic parameters of ARCF ( Fig.  1) easily correspond to formulas (1) and Table 1.
In the proposed ARCF schemes, typical schemes are applicable, including digital potentiometers or digital switching passive elements, which are described in detail in [35,36].
The implemented filters' (LPF, HPF, BPF, RF) transfer functions' (1) denominator coefficients bj relate to elements of the circuit on Fig. 1 by the following equations: Here the following notation are used in formulas (1), (2): where Rij are resistance of ij-resistor, C1, C2 are capacitance of capacitors С1 and С2.
The active RC-filters, shown in Table 1 and defined as LPF (+) , HPF (+) , BPF (+) , RF (+) , have the pole Q-factor, transfer ratio's and pole frequency's independent adjustment properties. Here the pole Q-factor adjustment does not change the filter's transfer ratio and its pole frequency. These filters have the greatest practical interest.
The active RC-filters, defined in Table 1 as LPF, HPF, BPF, RF, do not have the Q-factor, transfer ratio's and pole frequency's independent adjustment properties. Here the transfer ratio and pole frequency coefficients may change, when the pole Q-factor is changed.
The active RC-filters, defined in Table 1 as LPF (-) , HPF (-) , BPF (-) , RF (-) , have an amplitude-frequency response slope, which corresponds the first-order transfer function. It limits application scope for these circuit solutions.
There are variants of ARCF Fig. 1 inputs and outputs application on Fig. 2-Fig. 6, they provide AFR different modifications' implementation. In this case, connecting a signal source, for example, to the first input (In.1) assumes that unused inputs are connected to a common bus.       The computer simulation of the ARCF Fig. 1 different modifications, which correspond Fig. 2-Fig. 6, was made in MicroCap on AD830 DDA. It has confirmed the above properties of the suggested ARCF's circuit solution.

Low-Pass Filter LPF (+)
ARCF's inputs (Fig. 1) are connected according to the circuit, given on Fig. 7a, it provides amplitude-frequency responses of HPF (-) &BPF (-) , LPF (+) , LPF on outputs Out.1, Out.2(4), Out.3. Analysis of the AFR's curves (Fig. 7b) has shown, that transfer ratio and pole frequency are not changed in LPF (+) , implemented for output Out.2(4), when the pole Q-factor is adjusted (by changing R3 and R4 resistors resistance). At the same time, the pole frequency in this circuit can be tuned using resistors R5 and R6.

Band-Pass Filter BPF (+)
ARCF's inputs (Fig. 1)  Based on the BPF (+) 's AFRs curves (Fig. 8b) for output Out.3, can be concluded, that transfer ratio and pole frequency are not changed, when the pole Q-factor is adjusted (by changing R3 and R4 resistors' resistance). In this case, the pole frequency in this circuit can be tuned with resistors R5 and R6, and the transfer ratio change with resistors R1 and R2.

Rejection Filter RF (+) and Low-Pass Filter LPF (+)
ARCF's inputs (Fig. 1) are connected according to the circuit, given on Fig. 9a, it provides amplitude-frequency responses of LPF (+) , BPF, RF (+) on outputs Out.1, Out.2(4) and Out.3. So the ARC's curves (Fig. 9b) show, that transfer ratio and pole frequency are not changed in the implemented LPF (+) and RF (+) for outputs Out.1 and Out.3, when the pole Q-factor is adjusted by changing R3 and R4 resistors' resistance. The pole frequency in this circuit can be tuned with resistors R5 and R6, and the transfer ratio with resistors R1 and R2.
It should be noted that when the pole Q-factor is adjusted, the transfer ratio and pole frequency of HPF (+) for output Out.2(4) are not changed, which follows from AFR on Fig. 10b. At the same time, the pole frequency in this circuit can be tuned using resistors R5 and R6.
The given AFRs' curves' (Fig. 11b) analysis has shown, that the transfer ratio and pole frequency in LBF (+) and RF (+) for outputs Out.1 and Out.3 are not changed, when the pole Q-factor is adjusted by changing R3 and R4 resistors' resistance. At the same time, the pole frequency in this circuit can be tuned with resistors R5 and R6, and the transfer ratio with resistors R1 and R2.
When the pole Q-factor is adjusted (by changing R3 and R4 resistors resistance), the transfer ratio and pole frequency of LPF (+) and RF (+) (for outputs Out.1 and Out.3 correspondently) are not changed. At the same time, the pole frequency in this circuit can be adjusted using resistors R5 and R6, and the transfer ratio using resistors R1 and R2. From the analysis of the AFRs' curves (Fig. 13b), it may be concluded, that the transfer ratio and pole frequency are not changed in the RF (+) , implemented for output Out.2(4), when the pole Q-factor is adjusted (by changing R3 and R4 resistors resistance). At the same time, the pole frequency in this circuit can be tuned using resistors R5 and R6.

Conclusion
We have developed a structure of all-pass ARCF with pole Qfactor, transfer ratio and pole frequency independent adjustment for the whole spectrum of amplitude-frequency responses of the second-order filter (LPF, HPF, BPF, RF) implemented. This is a significant advantage of the considered circuit design solutions in comparison with the known ARCF of this class. We recommend using the suggested ARCF as a specialized chip of a structural array because of its universality. It is possible to implement a wide range of frequency selection devices in the said ARCF, because the signal source is connected to one of seven inputs and four outputs.

Conflict of Interest
The authors declare that there is no conflict of interests regarding publication of this paper.