JPH01202914A - Anti-aliasing filter - Google Patents

Abstract

PURPOSE:To form an inexpensive anti-aliasing filter with small size tacking to a sampling frequency by using a switched capacitor filter and a programmable analog filter. CONSTITUTION:A low pass filter section 10 consists of a low pass filter LPF1 (11) of an anti-aliasing filter of the sampling system of a switched capacitor filter itself, a 2nd low pass filter LPF 2(12) comprising the switched capacitor filter being a core of the low pass filter section and a 3rd low pass filter LPF 3(13) being a low pass filter for smoothing. A control section 20 gives a control signal and a control data required for each filter or the low pass filter section 10. Thus, the anti-aliasing filter for a switched capacitor filter and the smoothing filter whose impedance is varied with a sampling frequency FS, are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an antialiasing filter, and particularly to a cutoff frequency control method.

[Prior Art] Conventionally, there is an anti-aliasing filter whose cutoff frequency can be controlled by an external signal.

[Problems to be Solved by the Invention] However, in the conventional anti-aliasing filter, (1) a relatively wide frequency range (for example, 10 to 20 KH);
(2) The force/ant-off frequency changes in accordance with the change in the sampling frequency, (3) The attenuation slope is thick (more than 48 dB10 ct), (
4) It was impossible to meet the requirements of small scale and low cost at the same time, which was a major obstacle, especially when used in a multi-point tracking analysis device.

The object of the present invention has been made in view of these points, and is to provide a small-scale and inexpensive anti-aliasing filter that follows the sampling frequency using a switched capacitor filter and a programmable analog filter. It is something to do.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides a second low-pass filter formed by a switched capacitor filter and serving as the core of a low-pass filter system, and an anti-aliasing filter. 2, a first low-pass filter connected before the second low-pass filter and having a cutoff frequency characteristic that can be controlled by an external signal; and a first low-pass filter connected after the second low-pass filter for smoothing. a third low-pass filter that is connected to the third low-pass filter and whose cutoff frequency characteristic is controllable by an external signal; The present invention is characterized in that it includes a control section that generates control signals and data to be applied.

[Function] In the present invention, the first
The cutoff frequencies of the low-pass filter, the second low-pass filter, and the third low-pass filter can be changed according to the sampling frequency, thereby realizing an anti-aliasing filter that follows the sampling frequency.

[Example code] Embodiments of the present invention will be described in detail below with reference to the drawings.

When sampling the target signal at frequency Fs, an anti-aliasing filter with a cutoff frequency characteristic of k<1/2> is required. When performing tracking analysis on a source or a rotating body, it is required that Fc be controllable.

Switched filters are suitable for easily controlling Fc.
Although there is a capacitor filter, since the switched capacitor filter itself is a sampling system, an anti-aliasing filter and a smoothing filter for the switched capacitor filter are also required.

There is a relationship between the external clock frequency Fe and the cutoff frequency Fc of the switched capacitor filter: Fe=:m-FC (e.g., m=100), and the internal sampling frequency of the switched capacitor filter Between Fs' and FO, Fs'=n-Fe (e.g., n=L/2)
There is a relationship between

Now, Fc (max) is 20 as shown in Figure 7 (a).
Assuming KH2, Fe(rnax)=m −Fc(lna×)=2M (
Hz)Fs'(max) = n-Fe(
max)=I M (Hz)FcMm
ax) = to'-Fs' (may, ) < 500K (
07), and as shown in Figure 7 (b), FC (Pin)
If it is 10Hz, then Fafmin) = I K (Hz)F
s'(IIlin) = 500 (H2)Fc'[m
1n) =k'・Fs'(min)<250 (
Hz), and the allowable range (shaded area) overlaps the frequency range of the main filter.For this reason, the frequency of the anti-aliasing filter of the switched capacitor filter must also be variable.

Furthermore, since a low-order filter is desirable in terms of implementation, it is necessary to change the Fc of the main filter. In other words, the anti-aliasing filter of the switched capacitor filter is sufficiently higher than FC so as not to affect the passband characteristics of the main filter, and Fs
', a cutoff frequency characteristic exhibiting sufficient attenuation is required.

The anti-aliasing filter of the present invention shown in FIG. 1 is a principle configuration diagram of a sampling frequency tracking type anti-aliasing filter that satisfies the above conditions. In the figure, 10 is a low-pass filter section, and 20 is a control section.

The low-pass filter section 10 is composed of filters connected in cascade in three stages, and 11 is a switched capacitor.
A first low-pass filter L, P F 1 is an anti-aliasing filter based on the sampling system of the fill itself, and is composed of a programmable analog filter.
．． 12 is a second low-pass filter LP consisting of a switched capacitor filter, which is the core of the low-pass filter section.
F2.13 is a third low-pass filter LPF3 that is a low-pass filter for smoothing. This third
The low-pass filter LPF3 is a programmable analog filter like the first low-pass filter LPF1. The programmable analog filter in this case is an analog filter that uses a multiplication type digital-to-analog converter (hereinafter referred to as a multiplication type D/A converter) as a variable resistance element, and whose frequency characteristics can be controlled by a digital signal.

An example of an analog filter that uses such a multiplication type D/A converter and whose frequency characteristics can be controlled is as shown in FIG. 2. Here, a configuration of an 8th-order simultaneous Chievishi flow-bass filter is adopted, with la, lb, and IC.

1d is a second-order active filter with a transmission zero point, which has the same configuration although the constants of each part are different. Each active filter is described in Section 1. Section 2. Section 3. This is called the active filter in Section 4.

The Chiebishief flow-bass filter is a low-pass filter configured according to a synthesis rule called the Chiebishiev characteristic. The synthesis rules are described, for example, in the January 1986 issue of the magazine "Interface", pages 276-280, but in short, multiple sets of low-pass filters with different cutoff characteristics are combined, and the overall amplitude in the passband is Although there is some ripple in the characteristics, this is a coupling method that makes the cutoff characteristics extremely steep. Although the same magazine shows an example of synthesis of a 6th-order Tiewissiev filter, the present invention uses an 8th-order Tiewissief flow filter.

Also, a second-order active filter with a transmission zero point is
means an active filter with transmission zero characteristics,
The characteristics of the transmission zero point and its circuit are explained in the January 1986 issue of the magazine "Interface", page 279. ), and exhibits a cut-off characteristic that is even steeper than the Tievisiev characteristic.

FIG. 3 is a basic configuration diagram of a second-order active filter circuit with a transmission zero point, which is a basic component of an eight-order simultaneous Thievesier flow-bass filter. The second-order active filter circuit having each transmission zero point also has the same configuration, and in the present invention, the configuration is as shown in FIG. 4 in order to make the crotch resistance RF variable.

Returning to FIG. 3 again, its characteristics will be explained.

In the circuit with the basic configuration shown in Figure 3, the input/output transfer function H
(s) is, where the amplitude characteristic of this circuit becomes zero at a certain frequency,
Attenuation 1 becomes infinite. If the angular frequency is 0g, then ωM=kω0.

This is an 8-channel system in which 2nd-order active filter circuits with transmission zero points, each with different characteristics, are combined into a 4-stage surface array.
The simultaneous Tievisiev filter has ripples in the amplitude characteristics in the passband and attenuation band. Instead, the cutoff characteristic is steeper than that of other filters of the same order.

In such a circuit, ω. is determined only by the value of RF-C, so if RF is made variable, ω. can be changed.

The circuit of the present invention shown in FIG. 4 is such that ω2 is changed by making the RF variable.

In FIG. 11, the difference from the conventional 2nd-order active filter with a transmission zero point is that the operational amplifiers AI and A2
．． The point is that a circuit composed of a multiplication type D/A converter or the like is used as a variable resistance element that determines the cutoff frequency between the stages of A3.

In Fig. 4, A and B are multiplying type D/A converters using an R-2R ladder, which are controlled based on an externally applied control signal, and which are controlled based on externally applied digital data and operational amplifiers A1 or A2. It outputs the multiplied value of the analog signal (voltage) by the analog signal (voltage) as an analog current.

BA4 is an amplifier for converting the current output signal of the multiplication type D/A converter A into a voltage output, and is composed of an operational amplifier 0PA4 and a feedback resistor R11. Note that the resistance value of this feedback resistor R11 is set to the same value as the value of R of the R-2R ladder.

Manari A5 is also an amplifier for converting the current output signal of the multiplication type D/A converter B into a voltage output, and is composed of an operational amplifier 0PA5 and a feedback resistor R21, and the value of the feedback resistor R21 is R- It is the same value as the R value of the 2R ladder.

In short, each amplifier BA4. BA5 is an amplifier for converting the current output value of the multiplication type D/A converter into a voltage output value, and each output is inputted to the next stage operational amplifier via a resistor R, respectively.

In this case, the low resistance value corresponding to RF in the circuit of FIG. is the number of hits of digital input.

Digital setting values for each section are controlled by the control section 20.
Each is given separately.

In such a configuration, the cutoff frequency control of the 8th order simultaneous Chievishi flow bus (2) obtained by appropriately setting the constants of the active filter elements in each section is TTI, (transistor-transistor-logic) 256-step control is possible using the level signal (8 bits). (3) Attenuation characteristics are 90 dB/
(4) The passband ripple is maximum 0.0
It is 1 dB.

The control section 2o in FIG.
It provides necessary control signals and control data to each filter of 0, and its configuration is shown in FIG. In the figure, 5
Reference numeral 1 denotes a clock generator that generates a reference clock, and 52 a control data generation circuit that generates control data n, which is constituted by a frequency dividing circuit that divides the frequency of the reference clock and an up counter that counts the frequency-divided clock.

A down counter 53 is a town counter that is preset by the output P of the control data generation circuit 52 and counts down based on the reference clock.

Its output is learned by the second low-pass filter L P F 2 . 54 are first and third low-pass filters LPFI from the control data generation circuit 52;

This is an arithmetic circuit that obtains control data for the LPF3.

The operation in such a configuration will be explained next.

Second low-pass filter LPF2 (main filter section)
If the cutoff frequency of is Pc, then Fc = k −F
s = Fe /m That is, a frequency of -m-FS must be supplied to the switched capacitor filter at Ff3=.

The frequency of the reference clock is f, and the control data generation circuit 5
The output P (ratio between the reference clock f and the frequency Fs of the sampling pulse) obtained by 2 is as follows: P=f/Fs =k −m −f/Fe (where k,
m is a constant). When the above formula is modified, it becomes Fe=lc-m-f/P. By supplying this to the second low-pass filter LPF2, a main filter section in which Fc changes according to Fs is realized.

On the other hand, in the arithmetic circuit 54, from Fc<Fc'<Fs'/2, k-f/P<Fc'<Fe/2-q=-m
-f/CP-q) Find Fc'.

Here, Fc'=r-f/P (r is a constant), and the first and third low-pass filters LPFI, L
Since PF3 is a program analog filter, F
It can be set as c'=s/N (where S is a constant and N is a setting value for the multiplication type D/A converter), so in the arithmetic circuit 54, N=s-P/<r-f ) = -P (where K = s/(r-f)
Give F 1 and L P F 3 as set values.

Thereby, in the low-pass filter section 1o, as shown in FIG. 6, an antialiasing filter and a smoothing filter for a switched capacitor filter that change according to the sampling frequency FS can be realized.

[Effects of the Invention] As described above in detail, the present invention has the following effects.

(1) By using a multiplication type D/A converter as a variable resistance element that determines the cutoff frequency, TTL
The cutoff frequency can be adjusted over a wide range (10) [
Z~20KH7), (2) changes following the change in sampling frequency, and (3) has a large degree of attenuation (
(4) It can be constructed on a small scale and at low cost.

Further, according to the present invention, by configuring the low-pass filter section and the control section separately, when configuring a multi-point anti-aliasing filter, the control section can be shared and only the low-pass filter section can be used in multiple pieces. It has the advantage that it can be configured on a small scale.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the principle of the anti-aliasing filter that follows the sampling frequency according to the present invention, Figure 2 is a diagram showing the configuration of an analog filter whose frequency characteristics can be controlled, and Figure 3 is a diagram showing the configuration of an analog filter that can control frequency characteristics.
A basic configuration diagram of a second-order active filter circuit with a transmission zero point, which is the basic component of the following simultaneous Chievishi flow-pass filter. FIG. 4 is an example configuration diagram of a second-order active filter circuit with a transmission zero point. FIG. 5 6 is a block diagram showing the details of the control section, FIG. 6 is a frequency characteristic diagram for explaining the present invention in detail, and FIG. 7 is a frequency characteristic diagram for explaining the frequency of the anti-aliasing filter. 10...Low pass filter section, 11...First low pass filter, 12...Second low pass filter, 1
3... Third low-pass filter, 20... Control unit, 51... Clock generator, 52... Control data generation circuit, 53... Down counter, 54...
Arithmetic circuit, la, lb, lc, ld... 2nd order active filter circuit with transmission zero point, Al, A2゜A3・
...Operation amplifier, A, B...Multiplication type D/A converter, BA4. BA5...buffer amplifier. Fig. 1 2n Fig. 2 Control bow PJ 1 (7, ~ ld, 1) LFFI to No. 5 Lf'FZ. To LPF3

Claims (1)

[Claims] An antialiasing filter configured to allow cutoff frequency control by an external signal, the antialiasing filter being a switched filter.
a second low-pass filter formed by a capacitor filter and serving as the core of the low-pass filter system; and an anti-aliasing filter connected before the second low-pass filter, the cut-off frequency characteristic of which is configured to be controllable by an external signal. a third low-pass filter that is connected after the second low-pass filter for smoothing and whose cutoff frequency characteristics can be controlled by an external signal; and a sampling pulse that is applied. a control section that generates control signals and data to be applied to the first low-pass filter, second low-pass filter, and third low-pass filter based on the first low-pass filter, the second low-pass filter, and the third low-pass filter; An anti-aliasing filter characterized in that the cutoff frequency of the low-pass filter of No. 3 can be changed according to the sampling frequency.