JPS6324661Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a low-pass filter, and particularly to a CR-type low-pass filter whose time constant can be freely controlled.

Low-pass filter (hereinafter abbreviated as LPF)
In this case, it is often necessary to switch and control the time constant to vary the passband characteristics, for example, widely and narrowly.
One example is a loop filter in a PLL (phase locked loop) circuit.
In other words, in a transient state where the PLL circuit follows the input signal, the time constant of the loop filter is reduced to widen the passband width, and when the PLL circuit is locked and in a stable state, the time constant is increased. It is necessary to narrow the passband width. For this purpose, an LPF configuration as shown in FIG. 1 is generally used.

So basically resistor R ₁₁ and capacitor C ₁₁
It is a CR type LPF consisting of
OP ₁₁ is provided. Resistors R ₁₃ and R ₁₄ constitute a negative feedback circuit of operational amplifier OP ₁₁ . and,
A resistor R ₁₂ is provided in parallel with the resistor R ₁₁ , and this parallel resistance
It is configured to perform time constant control by controlling _R12 on and off by switching means _S1 .

The control switching means _S1 generally uses an electronic switch, for example a diode switch circuit as shown in FIG. Two circuits consisting of diodes D ₁ , D ₂ and D ₃ , D ₄ connected in series are connected in parallel between current sources I ₁ , I ₂ , and diodes D ₁ , D ₂ and D ₃ , D ₄ are connected in parallel to each other. The series connection point of the two is used as the input/output terminal. By simultaneously controlling the current sources I ₁ and I ₂ on and off, the impedance between the input and output terminals is set to approximately zero and infinite, thereby performing a switching action.

When this switch circuit is used as the time constant changeover switch _S1 of the LPF shown in Fig. 1, its on/off control causes a large change in the DC offset voltage at the output end of the DC amplifier, which is added to the output DC voltage of the operational amplifier _OP11 . In order to become the output of LPF,
It becomes difficult to obtain a phase lock state of the PLL circuit.

Therefore, the present invention has been made to eliminate the above-mentioned drawbacks, and its purpose is to provide an LPF that can control the frequency characteristics by eliminating the influence of the DC offset voltage caused by the time constant changeover switch.
The purpose is to provide.

The LPF of the present invention consists of a resistor inserted in series in a signal line, a variable gain amplifier that receives a signal applied to the signal line as input, and a first filter provided between the output of the amplifier and the output of the resistor. circuit and a second filter circuit having an RC type LPF configuration, the first and second filter circuits are connected in cascade, and the filter characteristics are controlled by changing the amplifier gain. do.

In particular, it is characterized in that the amplifier gain is varied between 1 and 0 to control the pass characteristics to be wide and narrow, respectively.

The present invention will be explained below using the drawings.

FIG. 3 is a circuit diagram of an embodiment of the present invention, which includes a first LPF circuit 1, a second LPF circuit 2, and a high input impedance buffer circuit 3 interposed between these two LPF circuits. It's summery. first
The LPF circuit 1 includes a resistor R ₁ inserted in series with a signal line, a variable gain amplifier 4, and a capacitor C ₁ provided between the output of the amplifier 4 and the output of the resistor R ₁ . ing. This amplifier 4 is
As shown in the figure, it consists of a voltage dividing circuit made up of resistors R ₅ and R ₆ that divides the voltage of a signal input, and an operational amplifier OP ₁ that receives the divided voltage output as an input and has feedback resistors R ₃ and R ₄ .

The high input impedance circuit 3 has a feedback resistor R ₇ ,
It consists of an operational amplifier OP ₂ having R ₈ , and its output is applied to a CR type second LPF circuit 2 consisting only of a resistor R ₂ and a capacitor C ₂ .

Here, for simplicity, resistor R ₁ and capacitor
Assuming that the impedances of C ₁ are Z ₁ and Z ₂ and the gain of the amplifier 4 is K, the transfer function G ₁ S of the first filter circuit 1 is determined (S indicates jω). Now, let the input and output of the first filter circuit 1 be v _i and v _p , and let the current flowing to the ground line through the negative feedback network of the signal line series-inserted resistive element Z ₁ , capacitive element Z ₂ and operational amplifier OP ₁ be expressed as follows: When i is set, the following formula holds true.

v _i =(Z ₁ +Z ₂ )·i+K·v _i ...(1) v _p =Z ₂ ·i+K·v _i ...(2) The following equation is obtained from equation (1).

i=(1-K)·v _i /(Z ₁ +Z ₂ ) (3) Substituting this equation (3) into equation (2) yields the following equation.

G ₁ (S) = v _p / v _i = (Z ₁ · K + Z ₂ ) / (Z ₁ + Z ₂ ) ... (4) In the above equation (4), Z ₁ = R, Z ₂ = 1/SC ₁ By substituting and rearranging, the transfer function G ₁ S of the first filter circuit 1 is expressed by the following equation.

G ₁ (S) = (1+S・K ₁・C ₁・R ₁ ) / (1+S・C ₁・R ₁ ) ……(5) In the above equation (5), when |K|≦1
The amplitude characteristic of G ₁ S, that is, the frequency vs. gain characteristic is |
G ₁ S | ≦ 1, which is negative in logarithmic dB display.
Then, if K changes under the condition |K|≦1,
The roll-off frequency is constant at 1/C ₁ R ₁ , but the turnover frequency is 1/KC ₁ R ₁ , which increases as the high-frequency gain decreases, until K=0, that is, the gain of the variable gain amplifier 4 becomes zero. For example, |G ₁ S|=1/√1+( _{1 1} ) ² ...(6), and it acts as an LPF that attenuates at -6 dB/OCT in a frequency range greater than 1/C ₁ R ₁ .
Further, in the case of K=1, |G ₁ S|=1 (7), and the all-pass characteristic does not have a filter effect.

FIG. 4 illustrates the above characteristics. Therefore, in order to control the gain K of the amplifier 4, the characteristics shown in FIG. 4 can be obtained by making the resistor R ₆ of the voltage dividing circuit variable as shown in FIG. 3.

FIG. 5 shows a specific circuit diagram of the circuit in FIG. 3,
Identical parts are designated by the same reference numerals in both figures. That is, instead of using a variable resistor as the resistor _R6 in Figure 3, a switch _S2 is provided in parallel with the resistor _R6 to control the gain of the amplifier 4 to either K=1 or zero. be. If switch _S2 is closed, the input to operational amplifier _OP1 becomes zero, so K
= 0, and therefore the characteristics of the first filter circuit 1 are the turnover frequency 1/ in FIG.
This is a characteristic when KC ₁ R ₁ becomes infinite, and shows an LPF characteristic that slopes at -6 dB/OCT at a roll-off frequency of 1/C ₁ R ₁ .

Therefore, if the roll-off frequency 1/C ₂ R ₂ of the second LPF 2 is selected to be greater than 1/C ₁ R ₁ ,
The characteristic GS of the entire circuit becomes a curve shown at 51 in FIG. 6, and has a narrow band.

Furthermore, if the switch S ₂ is opened and the constants R ₃ to _{R 6} of the amplifier 4 are set so that K=1, the characteristic G ₁ S of the first filter circuit 1 becomes “1” independent of the frequency. ”. Therefore, the characteristics of the entire circuit
GS becomes a curve shown at 52 in FIG. 6, and has a wide band.

By making the resistor _R6 variable as shown in FIG. 3, characteristics as shown at 53 in FIG. 6 can also be obtained. Further, the same effect can be obtained even if the order of the first and second filter circuits 1 and 2 is replaced.

As described above, the present invention has the advantage that it is possible to control the band of the LPF with an extremely simple configuration without any adverse effect on the output DC offset voltage. Therefore, when used as a loop filter in a PLL circuit, the filter output can be changed by switching the band.
Since the DC component does not change, a phase lock state can be obtained accurately, and it can be applied to PLL circuits with good characteristics. Therefore, if used in the PLL circuit of the synchronous detection circuit in AM and FM receivers, etc.,
It becomes an extremely high-performance detection circuit, and the S/N of the receiver is
can be improved.

[Brief explanation of the drawing]

Figure 1 shows an example of a conventional LPF circuit, Figure 2 shows an example of a conventional LPF circuit.
The figure shows a time constant changeover switch circuit of the circuit in Figure 1, Figure 3 is a circuit diagram of an embodiment of the present invention, and Figure 4 shows a circuit diagram of an embodiment of the present invention.
5 is a diagram showing the characteristics of a part of the circuit shown in FIG. 3, FIG. 5 is a circuit diagram of an embodiment of the present invention, and FIG. 6 is a diagram showing the characteristics of the circuit shown in FIG. Explanation of symbols of main parts: 1...first filter circuit, 2...second filter circuit, 3...high input impedance circuit, 4...variable gain amplifier. _R1 , _R2 ...Resistor, _C1 , _C2 ...Capacitor.

Claims (1)

[Claims for Utility Model Registration] (1) A resistive element inserted in series in a signal line, a variable gain amplifier whose input is the signal applied to this signal line, and between the output of this amplifier and the output of the resistive element. a first filter circuit consisting of a capacitive element provided in the first filter circuit, and a second low-pass filter circuit consisting of a resistive element and a capacitive element, and the first and second filter circuits are connected in cascade. A low-pass filter characterized in that the passband characteristics of the filter are controlled by changing the gain of the amplifier. (2) The filter according to claim 1, characterized in that a high input impedance circuit is interposed between the first and second filter circuits. (3) Claims 1 or 2 of the utility model registration claim characterized in that the gain of the amplifier is changed between 1 and 0 to control the pass band characteristics to be wide and narrow, respectively. filter.