Realization of Programmable Filter Based on MAX267

Filtering is an indispensable and even crucial link in the fields of signal processing, data transmission and interference suppression. The most commonly used filter is the RC active filter, which is composed of resistors, capacitors and operational amplifiers, and the frequency characteristics of the filter are changed by selecting different R and C values ​​through analog switches. For high-order active filters, due to the large number of analog switches required, the circuit is complex, the distributed parameters are large, and the cut-off frequency accuracy is not high. The use of programmable switched capacitor filters with variable frequency characteristics and the method of filtering signals with large bandwidth by programmable methods can overcome the above shortcomings. Since the switched capacitor programmable filters produced by MAXIM have a high cost performance and are easy to program, this article uses the MAX267 bandpass active filter in the linear radar level meter test system.

1 MAX267 Overview

MAX267 contains 2 independent second-order switched capacitor bandpass filters. It has 12 programmable input terminals, 5 of which are used to set the filter center frequency, and the other 7 are used to set the filter quality factor Q. Therefore, no external components are required, and only an external clock is required to achieve the bandpass filtering function, which is extremely convenient to use. MAX267 uses a 24-pin narrow DIP package, and the functions of the main pins are shown in Figure 1.


INa, INb (5, 1) are the signal inputs of two bandpass filters respectively;
BPa, BPh (2, 24) are the signal outputs of two bandpass filters respectively;
Q0~Q6 (13, 14, 19, 20, 23, 6, 7) are the quality factor setting inputs, connected to low level or high level respectively, the quality factor of the filter can be set between 0.5 and 64;
F0~F4 (22, 15, 21, 10, 9) are the center frequency setting inputs of the filter, connected to low level or high level respectively, the center frequency can be set to 1/197.92~1/100.53 of the clock frequency;
CLKa, CLKb (11, 12) are the clock inputs of two bandpass filters respectively.
 
2 Design of Adaptive Bandpass Filter

The microwave unit reflection signal and the transmission wave signal of the radar level meter are mixed in the mixer detection circuit to generate a certain difference frequency signal, which contains the distance information of the target, but also contains many interference signals. In order to filter out clutter, the programmable filter circuit is realized by a programmable switched capacitor bandpass filter chip, a true RMS conversion chip, an A/D converter and a single-chip microcomputer. Its principle block diagram is shown in Figure 2.


The program-controlled filter circuit is realized by the dual second-order universal switched capacitor active bandpass filter MAX267, the true RMS conversion chip AD637, the A/D conversion chip ADC0809 and the single-chip microcomputer AT89C51. The circuit schematic is shown in Figure 3.


2.1 Filter circuit

The single-chip microcomputer AT89C51 sends data F0~F5 to the filter MAX267 through the P1 port (P1.0~P1.4). The 32 center frequencies of F0~F5 are set in sequence through a 32-bit one-dimensional array. The signal output by the filter is converted into a DC voltage signal through the true effective value of AD637, and then converted into a digital signal readable by the single-chip microcomputer through ADC0809, and read by the P0 port (P0.0~P0.7). The single-chip microcomputer uses the bubble sorting method to store the maximum value of the data. The center frequency of the maximum value of the output waveform of the bandpass filter is the optimal cutoff frequency of the programmable filter. This is a cyclic determination process. And connect INb and BPa of MAX267, input from INa, output from BPb, cLKa and cLKb are connected, and the external clock signal is input by the active 1 MHz crystal oscillator. In this way, a fourth-order bandpass filter is formed. By looking up the table, we know that the low level of -5 V is set for Q0, Q1, and Q2, and the high level of +5 V is set for Q3, Q4, Q5, and Q6. The quality factor Q=8, and the bandwidth and gain at this time meet the design requirements.

2.2 Comparator circuit

Because MAX267 uses higher than +0.5 V and lower than -0.5 V as high and low levels, a voltage of about 1.25 V is input to the 2nd pin of UA741 through a series connection of 3 kΩ and 1 kΩ resistors, and the 3rd pin is connected to the P1 port (P1.4~P1.0) of the microcontroller to form a comparator to control the filter. When the microcontroller outputs a high level (+5 V), the comparator outputs a +5 V voltage, and when the microcontroller outputs a low level (0 V), the comparator outputs a -5 V voltage. 2.3 True RMS conversion circuit

The integrated chip AD637 adopts the standard true RMS voltage conversion connection method. Pin 3 and 4 are grounded, pin 6 and 9 are connected, pin 10 is connected to the negative power supply, and pin 11 is connected to the positive power supply. There are only two external components, one is the DC blocking capacitor C5 connected to the input pin, which is 103, and the other is the filter capacitor C6 connected between pin 8 and pin 9, which is 10μF. Connecting the original output pin 9 to the input pin 1 of the chip's built-in buffer amplifier and outputting from the buffer output pin 14 can improve the output load capacity.

2.4 A/D conversion circuit

ADC0809 is a typical 8-bit 8-channel successive approximation A/D converter, CMOS process. Since there is only one analog input, the channel selection signals A, B, and C are all grounded to select channel IN0. The acquisition frequency is 6 MHz, divided by 6 inside the microcontroller, and then divided by two by the chip 14024 to obtain 500 kHz. The start signal START and ALE signals are connected to P2.2, so that the channel address is written (latched) at the leading edge of the signal, and the conversion is started immediately at its trailing edge. The conversion end signal EOC is connected to P2.1, and the query method is used to wait until it is high before continuing to execute the program. The output enable signal OE controls the three-state output latch to output the converted data to the single-chip microcomputer. When OE=1, the converted data is output. The 8 data output ports are connected to the 8 ports of the single-chip microcomputer P0 port respectively, and the obtained data is stored in the intermediate variable vol for comparison.
　
3 Experimental results

After debugging, the system program is actually tested in the laboratory. Taking the input frequency of 6.2 kHz and 0.1 V as an example, the result of running the program automatically searching is: The output waveform of MAX267 observed by the oscilloscope is shown in Figure 4. Waveform 2 is the input waveform, and the amplitude of each grid is 50 mV. Waveform 1 is the output waveform, and the amplitude of each grid is 2 V. The horizontal axis is time, and each grid is 50μs. The multimeter measures the output voltage of AD637 to be 3.56 V.


Imax is the input of the microcontroller to the final control word of the filter, and is also the control word of the filter with the largest output voltage value. Imax = 0x13. The table shows that the N value of 0x13 is 19, fclk/f0 = 160.22, and the external clock uses a 1 MHz crystal oscillator, so through calculation, it is known that its center frequency f0 should be 6.241 kHz, which is consistent with expectations.

The frequency of the input signal is changed many times, and the results are shown in Table 1. Comparing the input signal with the optimal filter center frequency searched by the program automatically, the results are similar. Therefore, it can be determined that the bandpass filter has a good filtering effect and meets the design requirements.


4 Computer flow chart

Keil C is used for programming. The software flow chart is shown in Figure 5.

5 Conclusion This paper

introduces the program-controlled filter based on MAX267 switched capacitor filter chip, AD637 true RMS conversion chip, and 89C51 single-chip microcomputer. The test shows that the filtering effect of the system is good and meets the design requirements.