JPS61108217A - Filter device - Google Patents

Abstract

PURPOSE:To obtain a variable pass band characteristic filter device acting excellently against a broad band input by providing a low pass filter having a cut-off frequency lower than the internal clock frequency of a switched capacitor filter (SCF) to the pre-stage of the SCF having a required characteristic. CONSTITUTION:The cut-off frequency of a switched capacitor low pass filter LPF4 changes depending on the clock frequency fed from a frequency divider 5. The frequency divider 5 frequency-divides the 1st clock frequency fed from a frequency divider 2 to a prescribed ratio to generate the 2nd clock frequency signal. Through the constitution above, the 1st clock fed from a clock oscillator 1 is changed to shift the center frequency of an LPF3, then the cut-off frequency fc of the LPF4 is shifted accordingly. Since a signal of output frequency f3 of the LPF4 produced by an input frequency f4 is apart from the cut-of frequency f1 by 2-3 octaves, the gain is attenuated by -24dB--36dB by the LPF3.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a switched capacitor filter (hereinafter referred to as SCF).
This invention relates to a filter device using a filter.

BACKGROUND ART A switched capacitor filter is characterized in that its passing frequency changes depending on the frequency of a supplied clock signal.

Figure 1 shows a low-pass filter (hereinafter referred to as LP) including an SCF.
(referred to as F) is shown below. In FIG. 1, a clock oscillator 1 supplies a clock signal of a predetermined frequency to a branching unit 2. In FIG. The divider 2 changes the division ratio and generates a first clock signal of a desired frequency from the supplied clock signal. The first clock signal is passed through a low pass filter (hereinafter referred to as LP).
(referred to as F) 3. By changing the frequency division ratio, the required cutoff frequency can be obtained. Generally, in an integrated circuit SCF, the first
The clock signal is further frequency-converted to generate an internal clock signal, which is used as a trigger signal.

FIG. 2 shows the input frequency-output level characteristics when a single sine wave signal whose frequency is swept at a constant level is supplied to the signal input terminal of the SCF and LPF 3. In FIG. 2, frequency f1 is the cutoff frequency of LPF 3, and f2 indicates the internal clock frequency. As is clear from the characteristic curve in the figure, the output has a peak value when the input frequency is around f2. The frequency of this peak output is fl. In other words, when an input signal of frequency f2 is supplied to the SCF, a peak output of frequency f1 is generated. Further, when the cutoff frequency f1 is changed, the peak value generation frequency f2 also changes, so it is inappropriate to use the SCF as a filter for wideband input signals.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a variable pass characteristic filter device that works well for wideband inputs.

In order to achieve the above object, in the filter device according to the present invention, a low-pass filter having a cutoff frequency lower than the internal clock frequency of the SCF is provided upstream of the SCF having the required characteristics.

Example An embodiment of the present invention will be described with reference to FIG.

In FIG. 3, the same parts as those in FIG. 1 are designated by the same reference numerals, and the explanation of these parts will be omitted.

In FIG. 3, the cutoff frequency of the switched capacitor rotor filter LPF4 changes depending on the clock frequency supplied from the branching unit 5. In FIG.

The frequency divider 5 divides the first clock frequency supplied from the frequency divider 2 by a predetermined ratio to generate a second clock frequency signal.

In the above configuration, when the first clock wave number supplied from the clock oscillator 1 is changed and the center frequency of the LPF 3 is moved, the cutoff frequency fc of the LPF 4 is also moved accordingly.

Here, if the cutoff frequency of LPF4 is f3 and the internal clock frequency of LPF4 is f4, for example, f4=n
f3. It is set so that f2=-nf, fl<f3<f2/2. Here, n is usually several tens. FIG. 4 shows the input frequency-output level characteristics of the LPF3 and LP40 using a dotted line α and a solid line O. f 1 , f as above
Since the relationship of 2 + f3 holds, the frequency components of the input signal frequency that are higher than the cutoff frequency 13 of the LPF 4 are suppressed, and therefore the frequency f2 component in the input signal is attenuated and the frequency of the LPF 3 output due to the frequency f2 is suppressed. This suppresses the occurrence of f1 configuration. Also, the frequency f3 generated at the output of LPF 4 due to the frequency f4 configuration of the input signal
Since the configuration is attenuated by the LPF 3 whose upper limit cutoff frequency is fl, the frequency characteristics of the final output level are as shown in FIG.

The cutoff frequency f3 of LPF4 is the above f1<f3<f
Within the range of 2/2, the frequency that has the least influence on the output in relation to its attenuation characteristics is selected. For example, when n-32 and the attenuation characteristic is 12dB10ut, f2=32f1゜f
4=32f3, f, < f3 < 1.6 f1 < 1.6
It becomes f3, but f3=4. If the division ratio of the divider 50 is selected so that f, ~8f, the input signal frequency f2
Since the component is 2 to 3 octaves away from the cutoff frequency f3, the attenuation is about -24 to -36 dB. Also, the output frequency f of LPF4 caused by the input frequency f4
Since the signal of 3 is 2 to 3 octaves away from the cutoff frequency f1, it is -24 to -36 dB by LPF3.
The degree of attenuation is reduced.

FIG. 6 shows another embodiment, in which the third
The low-pass filter 3 in the illustrated embodiment is a band t4 filter.

As explained above, in the filter device of the present invention, a desired pass characteristic is obtained by the SCF, and the SC
Since another filter that cuts off frequencies near the internal clock frequency of F is added before the SCF, variable pass characteristics can be achieved even for wideband input signals without producing unnecessary frequency components. It is.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional switched CANO i9 filter, FIG. 2 is a same wave number-output level characteristic diagram of a switched CANO 4 filter, and FIG. 3 is a block diagram showing an embodiment of the present invention. 4 is a diagram explaining the frequency-output level characteristic of the embodiment in FIG. 3, FIG. 5 is a diagram showing the frequency-final output level characteristic of the embodiment in FIG. 3, and FIG. 6 is a diagram showing another embodiment of the present invention. Block Diagram. Explanation of symbols of main parts

Claims (3)

[Claims] (1) a first switched capacitor filter having a first upper limit cutoff frequency and triggered at a first clock frequency; and a low pass filter having a cutoff frequency smaller than 1/2 of the first clock frequency. (2) The filter device according to claim 1, wherein the low-pass filter is a switched capacitor filter triggered at a second clock frequency. (3) The filter device according to claim 1 or 2, wherein the first switched capacitor filter is a bandpass filter.