CN102790845A - Improved five-order low-pass filter - Google Patents

Abstract

The invention discloses an improved fifth-order low-pass filter, which is composed of two second-order low-pass Butterworth filters, a low-pass RC filter and an op-amp unit in series, and the filter has a cut-off frequency

It is 8.85MHz, and the input signal attenuation is -12dB at 11.5MHz, and -40dB at 22MHz. The passband of the filter is flat, the attenuation of the stopband is large, and the group delay characteristic is good; the filtering effect is good, the quality of the video signal is obviously changed, the burr is reduced, and the signal is stable. The invention will lay a key theoretical and application foundation for improving the quality of the input video signal of the video optical transceiver, and has very broad application prospects.

Description

An Improved Fifth-Order Low-Pass Filter

technical field

The invention belongs to the technical field of power electronics and relates to an improved fifth-order low-pass filter.

Background technique

Optical fiber communication has brought innovations to the field of communication due to its advantages of frequency bandwidth, large capacity, and low attenuation. Optical fiber-based video signal transmission has become more and more widely used. Traditional video optical transceivers generally use LC passive filtering to filter the input video signal. This filtering method has a big disadvantage, that is, the passband is not flat enough, and each frequency in the passband will produce a large group delay difference. . These have little anti-interference ability to the input video signal, which affects subsequent transmission and processing.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies in the prior art, and design a kind of improved five-order low-pass filter at the video input end of optical transceiver, be applicable to the processing of signal, its feature is again in:

The filter is composed of two second-order low-pass Butterworth filters, a low-pass RC filter and an op-amp unit connected in series; Pass the RC filter, and finally output the signal by the op amp unit.

The operational amplifier unit is a high-speed video operational amplifier MAX4450 with a bandwidth of 210MHz.

为8.85 MHz，可以实现输入信号在11.5 MHz衰减为-12 dB，22 MHz时为-40 dB。 The filter has a cutoff frequency

It is 8.85 MHz, and the attenuation of the input signal is -12 dB at 11.5 MHz, and -40 dB at 22 MHz.

After the filter passes through the delay stage, that is, the operational amplifier unit, its transfer function is:

After the delay stage is adjusted, the group delay difference in the passband is reduced to within 2 ns.

The advantage of the improved fifth-order low-pass filter of the present invention:

1. The filter has flat passband, large stopband attenuation, and good group delay characteristics;

2. The filtering effect is good, the video signal quality is significantly changed, the glitch is reduced, and the signal is stable.

Description of drawings

Fig. 1 is the amplitude-frequency characteristic diagram of the fifth-order Butterworth filter without delay stage.

Fig. 2 is a group delay characteristic diagram of a fifth-order Butterworth filter without a delay stage.

Fig. 3 is the circuit diagram of the fifth-order Butterworth filter after adding the delay stage.

Figure 4 is a diagram of the magnitude-frequency characteristics of the fifth-order Butterworth filter after adding the delay stage.

Fig. 5 is a characteristic diagram of the group delay of the fifth-order Butterworth filter after adding the delay stage.

Fig. 6 is the video signal after the fifth-order Butterworth filter wave measured with an oscilloscope.

Detailed ways

Example

Below in conjunction with accompanying drawing and embodiment the present invention is further described.

(1) Realize filter order reduction by increasing the sampling frequency

The ITU-R.BT601 standard of the International Telecommunication Union stipulates the parameters of the video input signal anti-aliasing filter, that is, the filter cut-off frequency is 5.75 MHz, and the insertion loss of the input signal at 6.75 MHz is 12 dB. Design the Butterworth filter according to this parameter, and calculate its order as

                       (1)

(1)

It is difficult and bulky to implement such a high-order filter purely with analog circuits, but for anti-aliasing filters, it is possible to

By increasing the sampling frequency to increase the cut-off frequency of the filter, so as to achieve the purpose of reducing the number of orders.

，即17.7 MHz。 Observing the filter response characteristics through the OrCAD software, it is found that the Butterworth low-pass filter with the fifth-order Sallen-Key structure cut-off frequency of 8.85 MHz can realize the attenuation of the input signal to -12 dB at 11.5 MHz and -40 dB at 22 MHz, satisfying Anti-aliasing filter for frequency selectivity requirements. At the same time, 4 times oversampling is used to adjust the sampling frequency to

, which is 17.7 MHz.

The fifth-order Butterworth filter is composed of two second-order low-pass Butterworth filters and one low-pass RC filter. The transfer function of the second-order active filter is

                            (2)

(2)

为通带增益；

，对分母进行归一化处理后得到二阶有源低通滤波器传函的标准形式 In the formula:

is the passband gain;

, after normalizing the denominator, the standard form of the transfer function of the second-order active low-pass filter is obtained

                                   (3)

(3)

为滤波器的品质因数；

为滤波器的截止频率；

为频率比例因子。如果将二阶有源滤波器传递函数中的

和

分别取值1和1.414，即构成了１个二阶有源低通Butterworth滤波器。      In the formula:

is the quality factor of the filter;

is the cut-off frequency of the filter;

is the frequency scaling factor. If the transfer function of the second-order active filter is

and

Values of 1 and 1.414 respectively constitute a second-order active low-pass Butterworth filter.

与

值，根据通带增益为1以及截止频率为8.85 MHz可计算出反归一化的滤波器的传递函数，如 By looking up the Butterworth filter parameter table, the fifth-order Butterworth filter stages can be obtained

and

value, the transfer function of the denormalized filter can be calculated according to the passband gain of 1 and the cutoff frequency of 8.85 MHz, such as

             (4)

(4)

The amplitude-frequency characteristic curve and group delay curve of the filter based on formula (4) are simulated by OrCAD software, as shown in Figures 1 and 2.

(2) Optimization of the group delay difference characteristics of the filter

It can be seen from Figure 1 that the frequency selectivity of the fifth-order Butterworth filter meets the expected requirements, that is, the attenuation at 11.554 MHz is -12.035dB; but Figure 2 shows that there is an obvious peak in the group delay difference of the filter. The delay difference of each frequency in the band is more than 30 ns, which cannot meet the requirement of 3 ns group delay difference in the passband specified in the ITU-R.BT601 design reference. It is necessary to add an operational amplifier unit as a delay equalization stage to change the filter group delay difference. After adding the delay equalization stage, the absolute delay of the filter is increased, but it basically has no effect on the quality of the video signal. The transfer function after adding the delay stage, such as

     (5)

(5)

与值确定滤波器电路如图3。 According to the passband gain, cutoff frequency and

and The value determination filter circuit is shown in Figure 3.

The operational amplifier of the present invention in Fig. 3 has adopted the high-speed video operational amplifier MAX4450 of Maxim Company, and it has the bandwidth of 210MHz, satisfies the system requirement. Through the simulation of OrCAD software, its amplitude-frequency characteristics are shown in Figure 4, and the group delay characteristics are shown in Figure 5.

As shown in Figure 3, adding the delay equalization stage increases the absolute delay of the filter, but basically has no effect on the video signal quality.

As shown in Fig. 4, it can be seen that its amplitude-frequency characteristic is compared with Fig. 1 through OrCAD software simulation, and frequency response is good, passband is flat, attenuates 11.823 dB at 11.459 MHz place, has realized that the cut-off frequency of the present invention is the low-pass of 8.85 MHz The attenuation of the filter input signal is -12 dB at 11.5 MHz and the design requirement of -40 dB at 22 MHz. At the same time, 4 times oversampling is used to adjust the sampling frequency to , namely 17.7 MHz, by increasing the sampling frequency to increase the cut-off frequency of the filter, so as to achieve the purpose of reducing the order.

As shown in Figure 5, after OrCAD software simulation, its group delay characteristics are compared with those in Figure 2. After adding the delay equalization stage, based on the transfer function of formula (5), the delay difference of each frequency in the passband is realized. Within 2ns, it meets the requirement of 3 ns group delay difference in the passband specified in ITU-R.BT601 design reference, and achieves the expected low-pass filtering effect.

As shown in Figure 6, it is measured with an oscilloscope that the quality of the video signal after filtering by the filter of the present invention is significantly changed, the burrs are reduced, and the signal is stable.

Claims (4)

1. A fifth-order low-pass filter of an improved type, suitable for signal processing, characterized in: The filter is composed of two second-order low-pass Butterworth filters, a low-pass RC filter and an op-amp unit connected in series; the signal is sent to the low-pass RC through the low-pass signal output by the second-order low-pass Butterworth filter filter, and finally the signal is output by the operational amplifier unit. 2. The improved fifth-order low-pass filter according to claim 1, characterized in that: the operational amplifier unit has adopted a high-speed video operational amplifier MAX4450, and the bandwidth is 210MHz. 3. The improved fifth-order low-pass filter according to claim 1, characterized in that: said filter has a cut-off frequency

It is 8.85 MHz, and the attenuation of the input signal is -12 dB at 11.5 MHz, and -40 dB at 22 MHz. 4. The fifth-order low-pass filter of improved type according to claim 1, is characterized in that: described filter transfer function is:

The improved fifth-order low-pass filter according to claim 1, characterized in that: the group delay difference in the passband of the filter is reduced to within 2 ns.

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