JPS6237567B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a CR active type notch filter that can adjust the Q of a notch characteristic that removes a specific frequency component in an input signal based on a fixed transmission ratio.

Fig. 1 shows a notch filter using a well-known parallel T-type circuit of this type of CR. The characteristic Q can be set, and by adjusting the partial pressure ratio, it is possible to adjust the Q while keeping the transmission ratio constant. However, in this notch filter, the total capacity of the parallel T-type circuit 1 is large, which is inconvenient when miniaturizing the design by making it into a hybrid IC, and also poses a problem in terms of cost.
Furthermore, when switching the resistance value R of the parallel T-type circuit 1 in order to switch the notch frequency into two stages, it is not only necessary to add series resistors at three locations and analog switches parallel to these, but also to Since the input impedance must be sufficiently large compared to the output impedance of the parallel T-type circuit 1, the analog switch added to the input side of the buffer amplifier A1 must be one that can be driven with a correspondingly high impedance. I had to. Furthermore, a notch filter is often used in combination with a bandpass filter. For example, when a notch filter is used in a biological signal amplification circuit to remove hum, the step response becomes oscillatory as shown in Fig. It was necessary to add a low-pass filter as shown by the dotted line, making it increasingly difficult to miniaturize the filter.

Therefore, the present invention can be made more compact,
It is an object of the present invention to provide a notch filter of the kind mentioned at the outset, which is also inexpensive. And this purpose is
According to the present invention, as mentioned above, in view of the usefulness of using it in combination with a bandpass filter, the Vienna bridge is used for downsizing and cost reduction, and the bandpass filter is used to avoid changes in the transmission ratio due to Q adjustment. This problem can be solved by performing Q adjustment within the range of the attenuation amount of the filter.

Next, the present invention will be explained based on the illustrated embodiments.

In FIG. 3, a first circuit consisting of a series circuit of resistor R11 and capacitor 11, a parallel circuit of capacitor C12 and resistor R12, and resistor R13 is shown.
and a second resistance circuit consisting of series-parallel resistors R14 to R16 constitute a Wien bridge with each as a side, and the connection point of the side of resistor R13 and series circuits R11 and C11 is a signal As input terminal, series circuit R11, C11 and parallel circuit C1
2, the side connection point of R12 is connected to the - input terminal of the differential amplifier A11, and the side connection point of the free end of the resistor R13 and the series-parallel resistor R4 is connected to its + input terminal, and the parallel circuit C12, R12 and a differential amplifier A at the connection point of the free end of the series-parallel resistor R15.
11 output terminals are connected. Also, among the series and parallel resistors R14 to R16, both parallel resistors R1
A low-pass filter as a bandpass filter consisting of a resistor R17 and a capacitor C13 and a buffer amplifier A12 are connected between the free ends of the resistors R17 and R16. and these resistors R15 and R16
The free ends of are made low impedance at the output ends of amplifiers A11 and A12, respectively, so that the combined resistance value of the series-parallel resistors R14 to R16 is equal to the side resistance of the aforementioned second resistor circuit of the Vienna Bridge. value.

If the resistance value of resistors R11 and R12 is R, and the capacitance of capacitors C11 and C12 is C, then the equilibrium frequency of the Wien bridge, that is, the notch frequency f ₀ =
1/2πCR, and in order to satisfy the equilibrium condition, the ratio of the resistance values of the resistor R13 and the series-parallel resistors R14 to R16 is the same as that of the series circuit R11, C11 and the parallel circuit R12 for the notch frequency _f0 . , 2:1 as well known, corresponding to the impedance exhibited by C12.
is set to . Also, low pass filter R1
7. The cutting frequency f ₁ of C13 is the notch frequency f ₀
For example, f ₁ =
Set to f ₀ /2.

The operation will be explained below with reference to FIG.

When the input signal is DC, the output voltage of the differential amplifier A11 is applied to the resistor R16 without being attenuated by the low-pass filters R17 and C13. Therefore, the output voltage is directly fed back positively without being divided, and then fed back negatively. Since the ratio is also 1, the transmission ratio is 1. When the frequency of the input signal gradually increases and attenuation begins to occur in the low-pass filters R17 and C13, a voltage drop occurs between the resistors R15 and R16, and the positive feedback voltage decreases in response to the voltage division ratio, so the differential The output of amplifier A11 begins to gradually decrease. In other words, when the voltage division ratio of the voltage dividers R15 and R16 is changed (the combined resistance value of the series-parallel circuits R14 to R16 is always kept constant), the voltage division ratio changes from small to large, that is, when Q changes from small to large. Figure 4, a
When the voltage division ratio is zero and the frequency is sufficiently high for the frequency _f1 , the combined resistance of resistor R13 and series and parallel resistors R14 to R16 (in this case R
14 is grounded), that is, 2:1.
Correspondingly, the transmission ratio becomes 1/3. Buffer amplifier A1 via low pass filter R17, C13
The frequency response of the output voltage of No. 2 is attenuated in the low range as shown in FIG. 4, b. During this period, when the frequency of the input signal approaches the notch frequency f ₀ , the differential input approaches zero, and at the notch frequency f ₀ the input signal becomes in-phase and zero input, and the output voltage of the differential amplifier A11 changes in a dip shape. Since attenuation is provided by the low-pass filters R17 and C13 in this frequency f ₀ region, Q can be adjusted by adjusting the voltage division ratio, that is, adjusting the positive feedback rate, by the resistors R15 and R16. In other words, as the voltage division ratio becomes larger, the levels on both sides of the notch frequency f ₀ rise, so that a higher Q can be obtained. Thus, while the transmission ratio of the pass band remains at 1, it is possible to adjust the Q of the notch characteristic by setting the resistors R14 to R16.

For example, when using the notch filter shown in Figure 3 in an electrocardiograph, the transmittance ratio is kept constant at 50 and 60
It is now possible to set the Q to remove hum at both frequencies of Hz at the desired level, and at this time, the high frequency is attenuated, eliminating the oscillation of the step response as shown in Figure 2, and reducing the distortion of the ST wave. Misdiagnosis caused by this will also be eliminated. Similar characteristics can be obtained by adding a low-pass filter indicated by a dotted line to the notch filter in FIG. If the values are made the same, the total capacitance can be reduced to 1/4, and the buffer amplifier A2 becomes unnecessary.

In addition, depending on the case, series/parallel circuits R14 to R16
It is also possible to eliminate the series resistor R14 and form the side resistance only with a voltage divider formed by the parallel resistors R15 and R16. If the notch frequency is to be switched between, for example, 50 and 60 Hz, two analog switches S11 and S12 in parallel with additional resistors R11' and R12' can be connected to low impedance points, respectively, as shown in FIG. In order to make Q arbitrarily adjustable, as shown in FIG.
Consisting of a buffer amplifier A21 and a variable resistor RV21 functioning as a parallel resistance, the variable resistor RV
The divided voltage of 21 is connected to resistor R2 with low impedance.
Positive feedback is given through 1.

Figure 7 shows a low pass filter R1 in Figure 3 in order to make the pass band of the band filter high.
7, C13 is replaced with a high pass filter R31, C31. In this case, in the low range, the output voltage of the buffer amplifier A12 is attenuated, so the cutoff frequency _f1 is higher than the notch frequency _f0 , for example, 2
If it is set to double, as shown in FIG. 8, at the output end of the buffer amplifier A12, the transmission ratio in the high pass band is constant, and a response characteristic whose Q can be adjusted is obtained.

Note that the Vienna bridge circuit according to the present invention can also be configured by replacing the series circuit and parallel circuit of the capacitor and resistor with each other.
In this case, referring to FIG. 3, these side connection points are connected to the + input terminal of the differential amplifier A11, and the resistor R13 and series-parallel resistors R14 to R
The ratio of the resistance values of 16 is also reversed, and these side connection points are connected to the - input terminal. The resistors R11, R12 and the capacitors C11, 12 may have different values, respectively, and the resistance ratios on the other two sides may be set to the impedance ratio in the balanced state determined by these values.

As is clear from the above explanation, a differential amplifier is connected to the Vienna Bridge in a balanced state so that the differential output is zero, and a bandpass filter is connected to this, and the difference between the differential output and the bandpass filter output is By feeding back the divided voltage, it is possible to adjust the Q according to the voltage division ratio, and at this time, the transmission ratio for the signal in the pass band can be kept constant. At the output end of the bandpass filter, a transfer characteristic with bandpass and notch characteristics is obtained, and at the output end of the differential amplifier, a transfer characteristic close to the notch characteristic is obtained. Moreover, the number of required parts of the CR element of the notch filter is small, and the total capacity is also small, making it possible to create a hybrid IC at low cost and small size despite the combination with bandpass characteristics. Switching of the notch frequency can also be performed using a common inexpensive analog switch.

[Brief explanation of the drawing]

FIG. 1 shows a well-known parallel T-type notch filter, FIG. 2 shows its step response waveform, FIG. 3 shows an example of the Wien bridge-type notch filter of the present invention, and FIG. 4 shows the frequency response characteristics of the notch filter according to FIG. 3. 5, 6 and 7 show a modification of the notch filter according to FIG. 3, and FIG. 8 shows a frequency response characteristic of the notch filter according to FIG.

Claims (1)

[Claims] 1. A Vienna Bridge is constructed with four sides including a series circuit of a capacitor and a resistor, a parallel circuit of a capacitor and a resistor, a first resistance circuit, and a second resistance circuit, and the series circuit and the parallel A first side connection point of the circuit and a second side connection point of the first and second resistance circuits are differentially connected to an input terminal of a differential amplifier, and the series circuit or the parallel circuit A third side connection point with the first resistance circuit is an input terminal, and an output terminal of the differential amplifier is connected to a fourth side connection point between the parallel circuit or the series circuit and the second resistance circuit. The fourth side connection point is followed by a bandpass filter that also attenuates the signal at the balanced frequency of the Wien Bridge, and the second resistor circuit converts the output voltage of the bandpass filter into the fourth side. 1. A notch filter comprising a resistor that provides positive feedback to a connection point on two sides.