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

Abstract

The invention discloses an improved five-order low-pass filter which comprises two two-order low-pass Butterworth filters, a low-pass RC filter and an operational amplifier unit connected in series, wherein the filters have a cut-off frequency of 8.85 MHz; and the input signal is attenuated to -12dB at 11.5 MHZ, and to -40 dB at 22 MHz. The filter provided by the invention is smooth in passband, larger in stopband attenuation and good in group delay characteristic, and has the advantages of good filtering effect, greatly improved video signal quality, reduced glitches, and stable signal. The improved five-order low-pass filter lays a critical theory and application foundation for improving the quality of the input video signal of a video optical transceiver, and has a wide application prospect.

Description

A kind of follow-on five rank low pass filters

Technical field

The invention belongs to electric and electronic technical field, relate to a kind of follow-on five rank low pass filters.

Background technology

Advantages such as optical-fibre communications is big with its bandwidth, capacity, decay is little have been brought innovation to the communications field, use more and more widely based on the video signal transmission of optical fiber.Traditional video optical multiplexer generally adopts the LC passive filtering to the method for incoming video signal filtering, the very big shortcoming that this filtering method exists, and promptly passband is smooth inadequately, and each frequency can produce bigger group delay difference in the passband.These antijamming capabilities to incoming video signal are less, influence follow-up transmission and processing.

Summary of the invention

The objective of the invention is to overcome the deficiency of prior art, design a kind of follow-on five rank low pass filters, be applicable to Signal Processing at the video inputs of optical transceiver, its characteristic again in:

This filter is made up of two step low-pass Butterworth filters, a low pass RC filter and an amplifier units in series; Described signal is given low pass RC filter through the low-pass signal of two step low-pass Butterworth filter outputs, by the amplifier unit signal is exported at last again.

Described amplifier unit has adopted high-speed video amplifier MAX4450, and bandwidth is 210MHz.

It is 8.85 MHz that described filter has cut-off frequency

; Can realize that input signal decays to-12 dB at 11.5 MHz, be-40 dB during 22 MHz.

Described filter is that its transfer function is behind the amplifier unit through delay stages:

Group delay difference is reduced in 2 ns in the passband of described delay stages adjustment back.

The advantage of the follow-on five rank low pass filters of the present invention:

1. this filter passband is smooth, stopband attenuation big, the group delay characteristic is good;

2. good wave filtering effect, video signal quality obviously changes, and burr reduces, signal stabilization.

Description of drawings

Fig. 1 is the amplitude-frequency characteristic figure that does not add five rank Butterworth filters of delay stages.

Fig. 2 is the group delay performance plot that does not add five rank Butterworth filters of delay stages.

Fig. 3 is five rank Butterworth filter circuit figure behind the adding delay stages.

Fig. 4 is five rank Butterworth filter amplitude-frequency characteristic figure behind the adding delay stages.

Fig. 5 is five rank Butterworth flora of filters time-delay characteristics figure behind the adding delay stages.

Fig. 6 records the vision signal behind the five rank Butterworth filter ripples with oscilloscope.

Embodiment

Embodiment

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

(1) realizes the filter depression of order through improving sample frequency

Stipulated the parameter of video input signals frequency overlapped-resistable filter in the ITU-R.BT601 standard of International Telecommunications Union, promptly filter cutoff frequency is 5.75 MHz, and the Insertion Loss of input signal at 6.75 MHz places is 12 dB.According to this parameter designing Butterworth filter, calculate its exponent number and do

(1)

Simple big with this higher order filter of Realization of Analog Circuit very difficulty and volume, however for frequency overlapped-resistable filter, can lead to

Cross and improve the cut-off frequency that sample frequency improves filter, thereby reach the purpose that reduces exponent number.

Find through OrCAD software observes filter response characteristics; Adopting five rank Sallen-Key structure cut-off frequencies is that the Butterworth low pass filter of 8.85 MHz can realize that input signal decays to-12 dB at 11.5 MHz; Be-40 dB during 22 MHz, satisfy the selectivity requirement of frequency overlapped-resistable filter for frequency.Adopt 4 times of over-samplings sample frequency to be adjusted to

, i.e. 17.7 MHz simultaneously.

Five rank Butterworth filters are to be made up of 2 step low-pass Butterworth filters and 1 low pass RC filter, and the transfer function of second-order active filter device does

(2)

In the formula:

is passband gain;

carries out obtaining the canonical form that the second order active low-pass filter passes letter after normalization is handled to denominator

(3)

In the formula:

is the quality factor of filter;

is the cut-off frequency of filter;

is frequency scaling factor.If, promptly constituted 1 active low pass Butterworth filter of second order with

in the second-order active filter device transfer function and

difference value 1 and 1.414.

Can obtain five rank Butterworth filters

at different levels and

value through searching Butterworth filter parameter table; According to passband gain be 1 and cut-off frequency be the transfer function that 8.85 MHz can calculate anti-normalized filter, as

(4)

Through OrCAD software the filter emulation based on formula (4) is obtained its amplitude-versus-frequency curve and group delay curve, like Fig. 1, shown in 2.

(2) optimization of the group delay difference characteristic of filter

Reached the expection requirement from the frequency selectivity of the visible five rank Butterworth filters of Fig. 1, promptly 11.554 MHz places decay to-12.035dB; But Fig. 2 can find out the group delay difference of this filter and a tangible spike occur; Each frequency delay difference is more than 30 ns in the passband; Can not satisfy the requirement of 3 ns group delay differences in the passband of stipulating in the ITU-R.BT601 design reference, need to add an amplifier unit and change the flora of filters delay difference as the delay balance level.Increased the absolute time-delay of filter after adding the delay balance level, but video signal quality has not been had influence basically.Transfer function behind the adding delay stages, as

(5)

According to the passband gain, cutoff frequency, and levels

and value determines the filter circuit shown in Figure 3.

Amplifier of the present invention has adopted the high-speed video amplifier MAX4450 of Maxim company among Fig. 3, and the bandwidth that it has 210MHz satisfies system requirements.Through its amplitude-frequency characteristic of emulation such as Fig. 4 of OrCAD software, group delay characteristic such as Fig. 5.

Shown in Fig. 3, increased the absolute time-delay of filter after the adding delay balance level, but video signal quality has not been had influence basically.

Shown in Fig. 4; Can know through OrCAD its amplitude-frequency characteristic of software emulation and Fig. 1 contrast; Frequency response is good, passband is smooth, decay 11.823 dB at 11.459 MHz places; Having realized that cut-off frequency of the present invention is that the low pass filter input signal of 8.85 MHz decays to-12 dB at 11.5 MHz, is the designing requirement of-40 dB during 22 MHz.Adopt 4 times of over-samplings sample frequency to be adjusted to simultaneously; I.e. 17.7 MHz; Improve the cut-off frequency of filter through improving sample frequency, thereby reach the purpose that reduces exponent number.

Shown in Fig. 5; Behind the OrCAD software emulation; Its group delay characteristic is compared with Fig. 2, based on the transfer function of formula (5), has realized that each frequency delay difference is in 2ns in the passband after the adding delay balance level; Satisfy the requirement of 3 ns group delay differences in the passband of stipulating in the ITU-R.BT601 design reference, played the low-pass filter effect of expection.

Shown in Figure 6, record with oscilloscope, obviously change through the video signal quality behind the filter filtering of the present invention, burr reduces, signal stabilization.

Claims (4)

1. follow-on five rank low pass filters are applicable to Signal Processing, its characteristic again in:

This filter is made up of two step low-pass Butterworth filters, a low pass RC filter and an amplifier units in series; Described signal is given low pass RC filter through the low-pass signal of step low-pass Butterworth filter output, by the amplifier unit signal is exported at last again.

2. follow-on five rank low pass filters according to claim 1, it is characterized in that: described amplifier unit has adopted high-speed video amplifier MAX4450, and bandwidth is 210MHz.

3. follow-on five rank low pass filters according to claim 1; It is characterized in that: it is 8.85 MHz that described filter has cut-off frequency

; Can realize that input signal decays to-12 dB at 11.5 MHz, be-40 dB during 22 MHz.

4. follow-on five rank low pass filters according to claim 1, it is characterized in that: described filter transfer function is:

Follow-on five rank low pass filters according to claim 1 is characterized in that: group delay difference is reduced in 2 ns in the described filter passband.

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