# Single EX-CCCII-Based First-Order Versatile Active Filter
## 4 Experimental Results  
Because the EX-CCII is not commercially available and the circuit in Figure 2 has not been constructed, an experimental setup was designed using the commercially available AD844 ICs to confirm the theoretical analysis of the proposed filters. The AD844 is a commercially available IC of the CFOA [52], and its symbol is shown in Figure 14a. The CFOA has four terminals and the ideal characteristic can be given by \(V_{x}=V_{y}\), \(V_{w}=V_{z}\), \(I_{y}=0\), and \(I_{x}=I_{z}\)[53]. It should be noted that the ideal CFOA expresses a voltage buffer between the y- and x-terminals, a current follower between the x- and z-terminals, which is the ideal characteristic of a CCII, and a latter voltage buffer (VB) between the z- and w-terminals. Its internal structure is shown in Figure 14b. To realize the EX-CCCII, only the properties of the CCII are used.  
Figure 14: AD844 IC: (**a**) symbol, (**b**) internal structure, and (**c**) realization of the EX-CCCII using the AD844 ICs.  
Figure 14c shows the implementation of the EX-CCCII using AD844 ICs. The resistors in the circuit were set as \(R_{1}=R_{2}=R_{3}=R_{4}=R_{5}\) to obtain \(I_{x1}=I_{z1+}=-I_{z1-}\), and \(R_{1c}=R_{2c}=R_{3c}=R_{4c}\) to obtain \(I_{x2}=I_{z2+}=-I_{z2-}\). The resistors \(R_{1}\), \(R_{2}\), \(R_{3}\), \(R_{4}\), \(R_{5}\), \(R_{1c}\), \(R_{2c}\), \(R_{3c}\), and \(R_{4c}\) were selected as 1 k\(\Omega\). The resistors \(R_{x1}\) and \(R_{x2}\) work as intrinsic resistances of the EX-CCCII. The VM filter in Figure 4c was chosen for measurement. The resistances \(R_{in}\) and \(R_{out}\) were selected as 10 k\(\Omega\), and the circuits were supplied by \(\pm\)5 V. The capacitance \(C_{1}\) of 27 nF was used.  
A photograph of the experimental setup of the first-order VM filter is shown in Figure 15. A Keysight DSOX1204G oscilloscope with a built-in signal generator was used to measure the filter performance. The supply voltage for the AD844 integrated circuits was set to \(\pm\)5 V.  
The measured frequency characteristics of the HPF, LPF, and APF with resistor settings \(R_{x1}=R_{x2}=5\) k\(\Omega\) are shown in Figure 16. The measured natural frequency was 0.87 kHz, while the designed pole frequency of the filter was 1.17 kHz.  
Figure 15: First-order VM filter using the AD844-based EX-CCCII: (**a**) experimental setup and (**b**) close-up image of the circuit.  
Figure 16: _Cont._  
From (7), the pole frequency can be varied by \(R_{x}\) (\(R_{x}=R_{x1}=R_{x2}\)). Figure 17 shows the measured magnitude frequency responses of the HPF, LPF, and APF when the resistances \(R_{x1}\) and \(R_{x2}\) (\(R_{x1}=R_{x2}=R_{x}\)) were set to 1 k\(\Omega\), 3 k\(\Omega\), 5 k\(\Omega\), and 10 k\(\Omega\). Bysetting the resistor's values to 1 k\(\Omega\), 3 k\(\Omega\), 5 k\(\Omega\), and 10 k\(\Omega\), the measured pole frequencies were 4.6 kHz, 1.5 kHz, 0.87 kHz, and 0.39 kHz, respectively, while the filter's designed pole frequencies were, respectively, 5.89 kHz, 1.96 kHz, 1.17 kHz, and 0.58 kHz. This experimental result can be used to confirm the tuning capability of the proposed first-order filter expressed in (7). However, the measured pole frequencies and the designed pole frequencies are slightly different, which is the effect of the IC AD844's gain error.  
Figure 17: Cont.  
Figure 18 shows the measured transient responses of the APF with setting resistances \(R_{x1}=R_{x2}=5\) k\(\Omega\). This shows the phase difference of about 90 degrees at the pole frequency of the input and output signals. The measured pole frequency was about 0.87 kHz. The experimental results in Figures 16-18 show that the proposed VM first-order filter can provide non-inverting and inverting HPFs, LPFs, and APFs. This confirms the functionality of the proposed filter.  
Figure 17: Measured frequency responses with different values of R\({}_{\rm x}\) for the (**a**) HPF, (**b**) LPF, and APF: (**c**) \(V_{o1}+V_{o4}\) (AP+), and (**d**) \(V_{o3}+V_{o2}\) (AP\(-\)).