JPH0637584A - Filter circuit - Google Patents

Abstract

PURPOSE:To provide a simple filter circuit having constitution capable of preventing a circuit in the rear stage from being saturated even when the level of an input signal is high and also, providing it at a low cost. CONSTITUTION:A pair of resonator type filters 1a, 1b and a resonance circuit 2 inserted between both resonator type filters 1a, 1b and whose resonance frequency can be varied are inserted to a signal transmission line. A level detection circuit 5 detects the level of an output signal outputted through the resonator type filter 1b. A comparator 3 compares a signal level detected by the level detection circuit 5 with a reference level set at a reference voltage generation circuit 4. Also, the comparator 3 increases damping quantity by widening difference between the resonance frequency of the resonance circuit 2 and that of the resonator type filters 1a, 1b when the level of the output signal of the resonator type filter 1b exceeds the reference level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter circuit which passes only signals in a predetermined frequency band.

[0002]

2. Description of the Related Art Generally, in a receiver, if the dynamic range of all the constituent circuits is increased, the cost increases. Therefore, it is desired to reduce the size of the dynamic lens in the constituent circuits except the input portion. In the case of such a configuration, when the electric field strength of the received signal is large, the input level becomes excessive and the output is saturated, which causes a problem that distortion increases and demodulation cannot be performed. In addition, when the reception level is displayed, the problem that the correct level is not displayed occurs.

Considering, for example, a mixer circuit for further converting the intermediate frequency, it can be seen that the output level gradually becomes saturated as the input level (horizontal axis) increases, as shown in FIG. Here, for the mixer circuit, grades are set in order from the one that completely satisfies the specifications to the first grade product and the second grade product, and the linearity of the first grade product (upper line) and the third grade product (lower line) The intersection point of the extension lines of the region will be referred to as an intercept point P, and the input level at the intercept point P will be referred to as an intercept level. Generally, when the input level to the mixer circuit exceeds the intercept level, the S / N ratio deteriorates and it becomes impossible to maintain a desired reception state.

In order to solve the above problems,
Attenuator 1 with variable attenuation as shown in 8
Before the mixer circuit (after the high frequency circuit or at the intermediate frequency
It is considered to be inserted before the circuit). This
Attenuator 1 consists of two capacitors inserted in the transmission path.
SA C₁, C₂And diode D₁And capacitor C ₂
And diode D₁One end is connected to the connection point with the anode of
Choked coil CH₁And a series circuit of power source DC
Equipped with a capacitor C₁And diode D ₁The Kaso
Choke coil CH at the connection point with the cord₂And transistor
Q₁One end of the series circuit of the emitter-collector of
Capacitor C₂And diode D₁Contact with the anode of
Diode D at the continuation point₂And transistor Q₂Emitter of
One end of the series circuit with the collector is connected, and both transistors
Q₁, Q₂Constant current source IS
It is configured to be connected to. Transistor Q₂D
In the series circuit of the mitter collector and the constant current source IS,
Densa C₃Are connected in parallel. Therefore, both transitions
Star Q₁, Q₂The potential difference of the voltage applied to the base of
Accordingly, the transistor Q₁, Q₂The amount of current passing through
As a result, the amount of attenuation between the input and output of the attenuator 1 is adjusted.
It can be saved.

[0005]

The attenuator 1 described above uses a T-type filter, but a configuration using a π-type filter is also conceivable. However, in order to change only the amount of attenuation without changing the input impedance or the output impedance using this type of attenuator 1, it is necessary to change the constants of at least two elements of the components that make up the attenuator 1. There
There is a problem that the circuit scale for control becomes large.

SUMMARY OF THE INVENTION The present invention is intended to solve the above problems, and enables demodulation without saturating the circuit on the subsequent stage even when the input signal level is large, and at low cost. It is intended to provide a simple filter circuit having a configuration that can be realized.

[0007]

In order to achieve the above object, the present invention provides a pair of resonator filters, a resonance circuit inserted between the resonator filters and having a variable resonance frequency, and And a comparison circuit for comparing the level of the output signal output through the resonator type filter with a predetermined reference level, and resonating in the direction of decreasing the maximum value of the level of the output signal based on the comparison result of the comparison circuit. A frequency variable element for adjusting the resonance frequency of the circuit is provided in the resonance circuit.

[0008]

According to the above structure, the resonance frequency of the resonance circuit inserted between the pair of resonator filters is changed in accordance with the level of the output signal, so that the resonance between the resonance filter and the resonance circuit occurs. The amount of attenuation can be adjusted according to the difference in frequency. That is, since the attenuation amount of the pass signal is adjusted in the band pass filter at the same time, it is possible to realize a simple structure at low cost. Moreover, since the attenuation amount is adjusted by adjusting the resonance frequency of the resonance circuit, it is easy to adjust the attenuation amount, and a comparison circuit for comparing the output signal level with the reference level is provided, and the resonance circuit resonance It becomes possible to control with a small-scale circuit configuration such as providing a frequency variable element for adjusting the frequency.

[0009]

(Embodiment 1) FIG. 1 shows a basic configuration showing the concept of the present invention, and FIG. 2 shows a specific circuit. Basically, a resonator type filter 1a, 1 such as a crystal resonator or a ceramic resonator is used.
b is provided as a pair, and L is provided between both resonator filters 1a and 1b.
The resonance circuit 2 which is a C parallel resonance circuit is inserted. Both resonator filters 1a and 1b having the same characteristics are used. As shown in FIG. 2, the resonance circuit 2 is a parallel circuit of an inductor L and a variable capacitance diode VC, and the resonance frequency can be changed by the voltage applied to the variable capacitance diode VC. That is, the variable capacitance diode VC functions as a frequency variable element. Therefore, if the resonance frequency of the resonance circuit 2 is adjusted by controlling the voltage applied to the variable capacitance diode VC,
The amount of attenuation of the output signal level with respect to the input signal level can be adjusted according to the frequency difference between the resonant frequencies of the resonator filters 1a and 1b. That is, as the resonance circuit 2 is detuned, the passing attenuation amount increases.

The output signal from the resonator type filter 1b is
The frequency is converted by the mixer circuit 6, amplified by the intermediate frequency amplifier 7, and then input to a demodulation circuit (not shown). On the other hand, the output signal of the intermediate frequency amplifier 7 is the level detection circuit 5
Is detected and integrated, and the average level of the output signal of the intermediate frequency amplifier 7 is detected. Level detection circuit 5
Is composed of, for example, a diode that rectifies the input signal, a capacitor that smoothes the rectified signal, a resistor that determines the charging / discharging time of the capacitor, and the like. The output of the level detection circuit 5 is input to the comparison circuit 3 and compared with the reference level generated by the reference voltage generation circuit 4. As shown in FIG. 2, the reference voltage generating circuit 4 includes two resistors R ₁ and R ₂ that divide the power supply voltage Vcc. The comparison circuit 3 is
A comparator with hysteresis provided by connecting a resistor R ₃ between the output terminal and the inverting input terminal, which is connected so that the output becomes H level when the output level of the level detection circuit 5 is below the reference level. Has been done.

The output terminal of the comparison circuit 3 is connected to the control terminal of the variable capacitance diode VC via the resistor R ₄ and the choke coil CH. Also, resistor R ₄ and choke coil C
The connection point with H is connected with the connection point between the variable resistor VR and the resistor R ₅ connected between both ends of the power supply. When the output of the comparison circuit 3 becomes H level, one end of the resistor R ₄ is connected to the positive electrode of the power source, and the resistor R ₄ and the resistor R ₅ are connected in parallel and applied to the variable capacitance diode VC. Voltage V
_H becomes V _H = Vcc · VR / {(R ₄ #R ₅ ) + VR} when the power supply voltage is Vcc. Here, R ₄ #R ₅ is R ₄ · R ₅ / (R ₄ +
R ₅ ) is shown. On the other hand, when the output of the comparison circuit 3 is at the L level, one end of the resistor R ₄ is connected to the negative electrode of the power supply, and the resistor R ₄ and the variable resistor VR are connected in parallel to form a variable capacitance diode. The applied voltage V _L to VC is V _L = Vcc · (R ₄ #VR) / {R ₅ + (R ₄ #VR)}. However, R ₄ #VR is R ₄ VR / (R ₄ +
VR) is shown. Here, if R ₄ <R ₅ and R ₄ <VR, then V _H > V _L , and if the output level of the level detection circuit 5 rises, the control voltage of the variable capacitance diode VC drops. .

That is, when the level of the received signal is low and the output level of the level detection circuit 5 is below the reference level, the control voltage of the variable capacitance diode VC becomes V _L as described above. At this time, the resonance frequency of the resonance circuit 2 is set to be equal to the resonance frequencies of the resonator filters 1a and 1b. That is, as shown by the solid line in FIG. 3, there is no attenuation near the center frequency. On the other hand, when a received signal having a large electric field strength is input and the output level of the level detection circuit 5 exceeds the reference level, the control voltage of the variable capacitance diode VC becomes V _H. At this time, the resonance frequency of the resonance circuit 2 deviates from the resonance frequencies of the resonator filters 1a and 1b, and the amount of attenuation increases as shown by the broken line in FIG.

By properly setting the reference level, as shown in FIG. 4, when the level of the input signal to the resonator type filter 1a reaches a predetermined level, the resonator type filter 1b is provided.
The output signal level of (1) (the output signal level of the intermediate frequency amplifier 7) is reduced, and the saturation of the constituent circuit in the subsequent stage of the resonator filter 1b is prevented. That is, unless the attenuation amount is changed, the input signal level is saturated at about -20 dB as shown by the broken line in FIG. 4, but the saturation of the input signal level is adjusted to, for example, 10 d by adjusting the attenuation amount.
Can be raised to B. As a result, the dynamic range viewed from the input side is increased even though a circuit having a small dynamic range is used as a constituent circuit of the subsequent stage of the resonator filter 1b. Since the comparison circuit 3 is provided with hysteresis, even if the output level of the level detection circuit 5 becomes smaller than the reference level when the resonance circuit 2 is detuned, the attenuation amount does not immediately return to the original state, After the received signal level has dropped to a predetermined level, the original state is restored. Therefore, even if the output of the level detection circuit 5 fluctuates many times in a short time near the reference level, the amount of attenuation does not fluctuate and the reception state can be stabilized.

(Embodiment 2) In this embodiment, as shown in FIG. 5, in order to display the reception intensity of the circuit shown in FIG. 2, a display signal generation circuit for extracting a signal of a level corresponding to the reception signal level. 8 is added. That is,
In the configuration of the first embodiment, since the output level of the level detection circuit 5 is discontinuous with respect to the input signal as shown in FIG. 4, when the reception intensity is displayed using the output of the level detection circuit 5, the reception It does not accurately reflect the strength.
Therefore, in the present embodiment, in the display signal generation circuit 8,
The output level of the level detection circuit 5 and the reference level are added, and the added value is displayed on a reception intensity display such as an S meter.

That is, in the display signal generating circuit 8, the output of the level detecting circuit 5 and the output of the reference voltage generating circuit 4 are connected to the resistors R ₆ , respectively at the non-inverting input terminal of the operational amplifier OP by using the operational amplifier OP. Since it is input via R ₇ , the output of the operational amplifier OP is output to the reception intensity indicator. Here, the output voltage of the reference voltage generation circuit 4 is input to the comparison circuit 3 and the display signal generation circuit 8 through the reverse current blocking diodes D ₃ and D ₄ , respectively.

According to the above configuration, the resonator type filter 1
Even if the resonance frequencies of a and 1b and the resonance circuit 2 are changed to adjust the attenuation amount, the output of the display signal generating circuit 8 continuously changes in accordance with the reception intensity, so that the attenuation amount can be adjusted. Despite this, the reception strength can be displayed accurately. If the attenuation amount is not adjusted using the comparison circuit 3, the circuit at the subsequent stage of the resonator filter 1b is saturated as described in the first embodiment, and as shown by the broken line in FIG. When the signal level is relatively low (for example, -20 dB), the display level of the reception strength indicator is saturated, but the attenuation amount is adjusted using the comparison circuit 3 so as to correspond to the input level. 6, the input signal level is relatively high (for example, 0
It is possible to continuously display the data until it reaches dB). Other configurations and operations are similar to those of the first embodiment.

[0017]

As described above, the present invention outputs through a pair of resonator type filters, a resonance circuit inserted between both resonator type filters and having a variable resonance frequency, and one resonator type filter. A frequency for adjusting the resonance frequency of the resonance circuit in the direction of decreasing the maximum value of the level of the output signal based on the comparison result in the comparison circuit. The variable element is provided in the resonance circuit, and the resonance frequency of the resonance circuit inserted between the pair of resonator filters is changed according to the level of the output signal. This has the advantage that the amount of attenuation can be adjusted according to the difference in the resonance frequency of the. That is, since the attenuation amount of the pass signal is adjusted in the band pass filter at the same time, it is possible to realize a simple structure at low cost. Moreover, since the attenuation amount is adjusted by adjusting the resonance frequency of the resonance circuit, it is easy to adjust the attenuation amount, and a comparison circuit for comparing the output signal level with the reference level is provided, and the resonance circuit resonance There is an advantage that control can be performed with a small-scale circuit configuration such that a frequency variable element for adjusting the frequency is provided.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a first embodiment.

FIG. 2 is a specific circuit diagram showing the first embodiment.

FIG. 3 is an operation explanatory diagram of the first embodiment.

FIG. 4 is an operation explanatory diagram of the first embodiment.

FIG. 5 is a circuit diagram showing a second embodiment.

FIG. 6 is an operation explanatory diagram of the second embodiment.

FIG. 7 is an explanatory diagram showing a problem of a mixer circuit.

FIG. 8 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 1a Resonator type filter 1b Resonator type filter 2 Resonance circuit 3 Comparison circuit 4 Reference voltage generation circuit 5 Level detection circuit

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[Procedure amendment]

[Submission date] November 9, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0002

[Correction method] Change

[Correction content]

[0002]

2. Description of the Related Art Generally, the dynamic range of a receiver is
Constrained by the configured amplifier to a constant value,
When configured for sensitivity, the output may saturate even with low input signals.
There is. When the output is saturated, problems occur that distortion increases and demodulation cannot be performed. In addition, when the reception level is displayed, the problem that the correct level is not displayed occurs.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

For example, considering the intermodulation distortion characteristic of a mixer circuit that further converts the intermediate frequency,
As shown in FIG. 7, it can be seen that the converted output level of the desired wave gradually becomes saturated as the input level (horizontal axis) increases . Furthermore, the third-order distortion component also increases. here,
The intersection of the extension line of the linear region between the converted output level of the desired wave and the third-order distortion component will be referred to as an intercept point P, and the input level at the intercept point P will be referred to as an intercept level. Generally, when the input level to the mixer circuit exceeds the intercept level, the S / N ratio deteriorates and it becomes impossible to maintain a desired reception state.

[Procedure 3]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 7

[Correction method] Change

[Correction content]

[Figure 7]

Claims (1)

[Claims] 1. A pair of resonator filters, a resonance circuit inserted between both resonator filters and having a variable resonance frequency, and a level of an output signal output through one of the resonator filters to a predetermined level. A resonance circuit is provided with a comparison circuit for comparing with a reference level, and the resonance circuit is provided with a frequency variable element for adjusting the resonance frequency of the resonance circuit in the direction of decreasing the maximum value of the level of the output signal based on the comparison result in the comparison circuit. A filter circuit characterized by being formed.