Programmable Active Filter Circuit Based on Single Chip Microcomputer Control

Programmable Active Filter Circuit Based on Single Chip Microcomputer Control

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Source: foreign electronic components Author: Zhao shiqiang, Zhou yijian, Liu Xia

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1 Introduction
Active Filter is widely used in digital signal processing, communication, and automatic control, but it is difficult to design a variable wide band active filter. The dual-Second-Order General Switched-Capacitor Active Filter with Programmable Single-Chip Microcomputer Control parameters is used to precisely set the central frequency fO of the active filter, the quality factor Q, and the working mode of the active filter.

2 working principle of active capacitor Filter
The basic principle of the Active Filter of the switching capacitor is that a capacitor with a high-speed switch is indirectly connected between two nodes of the circuit, which is equivalent to a resistor connected between two nodes. Figure 1 (a) is an active RC Integrator. Figure 1 (B) uses a ground capacitor C1 and a CMOS Switch T1 and T2 to replace the input resistor R1. Figure 1 (c) is a non-overlapping two-phase clock pulse used to drive T1 and T2. The clock frequency fCLK is higher than the signal frequency. When Phi 1 is high, T1 conduction (depending on Short Circuit) and T2 cutoff (depending on open circuit), C1 is connected to the input signal VI and charged (1 (d ), there is a charge qcl = C1V1. When is high, T1 ends and T2 is turned on, C1 is disconnected from the input signal VI and connected to the input end of the Operational Amplifier. C1 is discharged, and the charging charge qc1 is transmitted to C2. In each cycle Tc, the charge qcl = C1VI extracted from the signal source is provided to the integrated capacitor C2. Therefore, the average current flowing between nodes 1 and 2 is iav = C1VI/TC. If TC is short enough, this process can be considered as continuous. Therefore, an equivalent resistance Req can be defined between two nodes, that is, Req = VI/iav and Req = Tc/C1. In this way, the time constant of the equivalent integrator is obtained, that is, Tau = C2Req = TCC2/C1. Obviously, the time constant of the filter's frequency response is dependent on the TC and the capacitance ratio C2/C1 in the clock period, but it is independent of the absolute value of the capacitor. It is easy to implement the small capacity Capacitor by using the integrated circuit process. When the capacitance ratio is constant and the clock frequency is changed, the cut-off frequency of the filter can be changed, that is, the basic working principle of the Switched Capacitor filter.

3 MAX262 Introduction
MAX262 is a dual-Second-Order General Switched Capacitor Active Filter manufactured by Maxim. Its pin function and internal Diagram 2 are shown in. MAX262 is integrated with two second-order General Switched Capacitor Active Filters with the same structure and one independent OP. The two source filters can be used independently or in series. The working method of the filter, center frequency fO, quality can be through D0D1, A0 ~ The A3 port Line writes the selected internal unit in the rising edge of WR to set parameters such as the filter operation mode, center frequency fO, and quality factor Q. For more information, see table 1. The central frequency fO ranges from 1Hz ~ 140 kHz. FCLKA and FCLKB are the external clock required by the Internal Switch Capacitor Network, usually dozens to hundreds of times of the center frequency fO.

In table 1, MOM1 is the working mode and can be written only when the address is A3A2A1A0 = 0000 (or 1000. F0 ~ F5 is the fO control word, Q0 ~ Q6 is the control word of quality factor Q. Working Mode 1 when D0D1 = 00, LP (low-pass), BP (band-pass), and N (ripple) functions: Working Mode 2 when DOD1 = 01, implement LP (low-pass), BP (band-pass), and N (Band-trap) functions different from working method 1; Work Mode 3 when D0D1 = 10, implement LP, BP, HP (Qualcomm) function; When D0D1 is set to 10, if the independent OP amp OP is set to 3A, the LP, BP, and HP functions are implemented different from 3. Working Mode 4 when D0D1 = 11, implements LP, BP, and AP (all-pass) functions. MAX262 has two signal inputs: INA and INB. The maximum input voltage amplitude is ± 4. 7 V.

4 program-controlled active filter hardware circuit design
Figure 3 shows the hardware circuit diagram of the programmable active filter. Control the keyboard display 8279, programmable counter 8254, and Programmable Active Filter max262. Among them, 8254 is a 3-Channel 16-bit programmable counter with a counter clock frequency of up to 10 MHz. It can provide MAX262 with a precise clock frequency fCLKA and fCLKB.

The system design requires that the filter can be set to low-pass. The-3 dB cutoff frequency of FC is between 1 kHz and ~ When the step value of the 20 kHz adjustment frequency is 1 kHz, the amplitude-frequency characteristics of the filter at 2fc must be reduced by no less than 10 dB. At the same time, the filter can be set as a high-pass filter, the FC range and step values are the same as those in the low-pass mode. The amplitude-frequency characteristics of the FC filter must be reduced by no less than 10 dB. Set Filter A to work mode 1 and filter B to work mode 3. The transfer function of the second-order active low-pass filter is: Gl (S) = holp [(ω 0/(s2 + 2 ω 0/q + ω 0)], (holp is the DC gain ). Order | GL (s) | if the frequency at 0.707holp is FC

After calculation, when Q = 0.7, the amplitude-frequency gain at fc = f0 and 2fc is-12 dB (measured below-12 dB), which meets the requirements. When q = 0.7, a high-pass filter also requires that the gain be reduced by 3 dB at fc = f0, while the gain at 0.5fc is-12 dB. According to the reference data, the ratio of fclka/f0 (or fclkb/f0) must be higher than 40.84. When the filter operates in method 1 or method 3, fclk/f0 = [π (26 + N1)]/2, N1 is F0 ~ Fclk/fo = 56.55 if N1 is set to 10. Q = 64/(128-n2), because Q = 0.7, so n2 = q0 ~ Q5 = 37. When fo = 1 kHz, fclkb = 56.55 kHz.

5 program-controlled implementation
According to the working principle of max262 ~ F5, q0 ~ After the q5 value is written to max262, if you want to implement a step-by-step control for the filter's operating frequency fo, you only need to set the ratio of fclka/fo (or fclkb/FO) to 56.55. The circuit design uses a 8254 programmable counter to generate fclka and fclkb. 8254 is 3 channels
A 16-bit programmable counter with a maximum counter pulse frequency of 10 MHz. When it is set to work mode 3, it is the output of the square wave generator, the output of channel 0 is fCLKA, and the output of Channel 1 is fCLKB. Because 8254 is a 16-bit programmable counter, the ratio of fCLKA to fCLKB to fO is close to 56.55 after the split, if fCLKA and fCLKB are fixed values, F0 ~ FCLKA, fO, or fCLKB/fO have only 64 options, obviously with low accuracy. After the fO value is input on the keyboard, the MCU calculates the corresponding frequency division factor MA and writes the MB value to 8254. If the step value is a fixed value, store the MA and MB of the corresponding fO in the form of a table and directly look up the table.

6 conclusion
The Programmable Active Filter Based on Single Chip Microcomputer can realize low-pass, high-pass, band-pass, ripple, and full-pass filters. Under the control of the program, the bandwidth, center frequency and quality factor Q of the filter can be precisely changed. The actual test results are good. When MAX262 is used as a low-pass filter, in order to suppress the noise produced by the fCLKA switching frequency, a low-pass filter with a cutoff frequency less than the minimum fCLKA can be connected at the output end of the filter, which has a better effect.