JP2008236419A - Front tuner circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a tuning frequency from being deviated when AGC control is applied. <P>SOLUTION: When a reception signal level becomes high and an output signal level of a differential amplifier 24 exceeds a fixed value, AGC control current output from an AGC circuit 26 increases and impedance of diodes D2, D1 becomes small. At such a time, since capacitance of a capacitor C12 has been reduced, impedance separation between an attenuator circuit 22 and a tuner circuit 23 can be actualized and a tuning frequency can be prevented from being deviated. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a front tuning circuit of a radio receiver.

  FIG. 4 is a circuit diagram of a front tuning circuit 10 of a conventional radio receiver. In the front tuning circuit 10, a capacitor C1, an inductor L1, and a capacitor C2 are connected in series with an antenna 11, and a capacitor C3 is connected to the other end of the capacitor C2. The tuning coil T1 is connected to the other end of the capacitor C3.

  Variable capacitance diodes D3 and D4 are connected in series to both ends on the secondary side of the tuning coil T1, and a control voltage VT for selecting a receiving station is applied to the cathode side of the variable capacitance diodes D3 and D4.

  An attenuating circuit including PIN diodes D1 and D2, a resistor R1, and a capacitor C4 is connected to a connection point a between the capacitors C2 and C3. The anode of the PIN diode D1 is connected to the connection point a, and the cathode of the diode D1 is grounded. The cathode of the PIN diode D2 is connected to the connection point a, and the anode is connected to the resistor R1 and one end of the capacitor C4. The other end of the capacitor C4 is grounded. An AGC control current FMAGC is supplied to the other end of the resistor R1.

  In the above circuit, the AGC control current is almost zero when it is not necessary to suppress the received signal level. When it is necessary to suppress the reception signal level, a positive AGC control current is supplied to the anode of the diode D2, and the impedances of the diodes D2 and D1 are reduced. When the impedances of the diodes D2 and D1 viewed from the connection point a are reduced, the reception signal level at the connection point a is suppressed.

When the L and C series resonance type front tuning circuit 10 is used as an FM signal tuning circuit, the capacitance of the capacitors C2 and C3 is about 0.1 μF.
However, in the conventional front tuning circuit 10 described above, since the capacitance of the capacitor C3 is as large as about 0.1 μF and the impedance in the frequency band of the received signal is small, a change in the impedance of the attenuation circuit affects the tuning circuit. End up. That is, when the AGC control current is supplied to the attenuation circuit in order to suppress the received signal level in a state in which the tuning circuit is tuned, the impedances of the diodes D2 and D1 become small, thereby tuning the tuning circuit. There was a problem that the frequency shifted.

Patent Document 1 discloses an input tuning circuit in which one end of two tuning coils connected in series is connected to a filter, the other end is grounded, and a connection point of the tuning coil is connected to a high-frequency amplifier circuit.
Patent Document 2 includes a first inductor connected between the input of the capacitor connected to the antenna and the ground, and a second inductor connected between the output of the capacitor and the ground, and VHF. A matching device is disclosed that can switch the values of the first and second inductors when receiving signals in the band and the UHF band.

  Patent Document 3 discloses a low-pass filter circuit including two inductors connected in series, a capacitor connected between a connection point of the inductor and the ground, two capacitors connected in series, and a capacitor. There is disclosed a television tuner that takes out a television signal in the VHF band and the UHF band by switching a high-pass filter circuit composed of an inductor connected between the connection point of the circuit and the ground.

  Patent Document 4 discloses that in a radio receiver, a signal obtained by adding a narrowband AGC signal and a wideband AGC signal is compared with a reference signal, and the attenuation amount of the antenna attenuation circuit is controlled based on the comparison result. .

Patent Document 5 discloses that two attenuation circuits are connected in series to an antenna, and the attenuation amount of each attenuation circuit is controlled based on the output of a high-frequency amplifier.
In the above Patent Documents 1 to 5, there is no description that suggests a problem of reducing the deviation of the tuning frequency when the impedance of the attenuation circuit is changed by applying AGC control and means for solving the problem.
Japanese Patent Laid-Open No. 4-196925 JP 2004-172958 A JP-A-61-2433 JP-A-5-206885 Japanese Patent Laid-Open No. 9-46153

  An object of the present invention is to provide a front tuning circuit that is not affected by a shift in tuning frequency even during AGC operation in a configuration in which a received signal is attenuated by AGC operation.

  The front tuning circuit of the present invention includes an antenna, an inductor connected between the antenna and the ground, a first capacitor connected to the antenna and one end, and an anode on the other end of the first capacitor. A first diode having a cathode connected to the ground and a second diode having a cathode connected to an anode of the first diode, the first and second AGC circuits built in a semiconductor integrated circuit An attenuation circuit to which a current for changing the impedance of the second diode is supplied, a tuning circuit for tuning a received signal, the other end and one end of the first capacitor are connected, and the other end of the tuning circuit A second capacitor connected to the input side and set to a value capable of separating the impedance of the attenuation circuit and the tuning circuit in a frequency band of a received signal; That.

According to the present invention, it is possible to prevent the tuning frequency from shifting when the impedance of the attenuation circuit is changed by applying AGC control.
A front tuning circuit according to another aspect of the present invention includes an antenna, an inductor connected between the antenna and the ground, a first capacitor connected to the antenna at one end, and the first capacitor. An attenuation circuit having a first diode having an anode connected to the end and a cathode grounded, and a second diode having a cathode connected to the anode of the first diode, and a tuning circuit for tuning a received signal And the other end and one end of the first capacitor are connected to each other, the other end is connected to the input side of the tuning circuit, and the attenuation circuit and the tuning circuit can be separated from each other in the frequency band of the received signal. The set second capacitor and a built-in semiconductor integrated circuit that controls the current supplied to the anode of the second diode of the attenuation circuit to control the attenuation circuit. And a AGC circuit for changing the impedance.

According to the present invention, it is possible to prevent the tuning frequency from shifting when the impedance of the attenuation circuit is changed by applying AGC control.
In the front tuning circuit of the above invention, the semiconductor integrated circuit is composed of a MOS transistor, a differential amplifier for amplifying a tuning signal of the tuning circuit is built in the semiconductor integrated circuit, and the AGC circuit includes the differential amplifier. The impedance of the attenuating circuit is controlled so that the output signal level becomes constant.

  With this configuration, it is possible to prevent the tuning frequency from shifting when the impedance of the attenuation circuit is changed so that the output signal level of the differential amplifier becomes constant.

  According to the present invention, it is possible to prevent an influence such as a shift in tuning frequency when the AGC operates and the impedance of the attenuation circuit is changed.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a front tuning circuit 21 of the FM radio receiver according to the embodiment. FIG. 1 also shows a front tuning circuit 21 provided outside the radio tuner IC, and a differential amplifier 24, a mixer circuit 25, and an AGC circuit 26 built in the radio tuner IC. In the following description, the same components as those of the conventional front tuning circuit 10 of FIG.

  In FIG. 1, an inductor L11 and a capacitor C11 are connected to an antenna 11 in parallel. The other end of the inductor L11 is grounded, the other end of the capacitor C11 is connected to one end of the capacitor C12, and the other end of the capacitor C12 is connected to the primary side of the tuning coil T1. In this embodiment, for example, the capacitance of the capacitor C11 is designed to be 0.1 μF, and the capacitance of the capacitor C12 is designed to be about several tens of pF.

  An attenuation circuit 22 is connected to a connection point a between the capacitors C11 and C12. The attenuation circuit 22 includes two diodes D1 and D2, a resistor R1, and a capacitor C4. The anode of the diode D1 of the attenuation circuit 22 is connected to the connection point a between the capacitors C11 and C12, and the cathode of the diode D1 is grounded. The cathode of the diode D2 is connected to the connection point a, and the output current (AGC control current) of the AGC circuit 26 is supplied to the anode via the resistor R1. A capacitor C4 is connected to the anode of the diode D2, and the other end of the capacitor C4 is grounded. The capacitor C4 is a noise removing capacitor.

  Between the secondary terminals of the tuning coil T1, variable capacitance diodes D3 and D4 connected in series are connected in parallel. A control voltage determined by the frequency of the receiving station is applied from the IC to the connection point of the variable capacitance diodes D3 and D4.

  Capacitors C5 and C6 are connected to the secondary side terminal of the tuning coil T1, and the other ends of the capacitors C5 and C6 are connected to a differential amplifier 24 built in the IC. The output of the differential amplifier 24 is output to the mixer circuit 25 and the AGC circuit 26.

  The AGC circuit 26 supplies an AGC control current that makes the output signal level of the differential amplifier 24 constant to the attenuation circuit 22 to change the impedance of the attenuation circuit 22. The received signal level is suppressed by changing the impedance of the attenuation circuit 22.

  Next, the operation of the front tuning circuit 21 configured as described above will be described. When the output signal level of the differential amplifier 24 is below a certain value, the AGC control current output from the AGC circuit 26 is almost zero. At this time, since the diode D1 and the diode D2 of the attenuation circuit 22 are turned off, the impedances of the diodes D1 and D2 viewed from the connection point a have a large value, and the received signal is output to the tuning circuit 23 without being attenuated. .

  When the received signal level increases and the output signal level of the differential amplifier 24 exceeds a certain value, the AGC control current output from the AGC circuit 26 increases in accordance with the output signal level. When the AGC control current flowing through the diodes D2 and D1 of the attenuation circuit 22 increases, the impedance (resistance value) of the diodes D2 and D1 decreases. At this time, the resistance values of the diodes D2 and D1 are very small values (for example, several Ω) compared to the resistance values when the diodes D2 and D1 are in the off state.

  Accordingly, the resistance value of the attenuation circuit 22 viewed from the point a in FIG. 1 is about several Ω, and the signal level at the point a decreases as the resistance value decreases. Thereby, the input signal level of the tuning circuit 23 is suppressed, and the output signal level of the differential amplifier 24 is kept constant.

  Since the front tuning circuit 21 of the embodiment has a small capacitance (several tens of pF) of the capacitor C12, the impedance of the capacitor C12 becomes very large in the frequency band of the received signal. Therefore, the impedance of the attenuation circuit 22 and the tuning circuit 23 can be separated by the capacitor C12. Thus, even when the AGC control is performed to reduce the impedance of the attenuation circuit 22, the impedance of the attenuation circuit 22 viewed from the tuning circuit 23 hardly changes.

  Thus, even if the impedance of the attenuation circuit 22 is changed by performing AGC control to suppress the input signal level by separating the impedance of the attenuation circuit 22 and the tuning circuit 23 by the capacitor C12 having a small capacity, the attenuation is suppressed. The change in the impedance of the circuit 22 does not affect the tuning circuit 23. Therefore, the tuning frequency of the tuning circuit 23 is not shifted. Further, when comparing the conventional front tuning circuit 10 of FIG. 4 with the front tuning circuit 21 of the embodiment of FIG. 1, the resonance circuit of the conventional front tuning circuit 10 uses three capacitors. Since the front tuning circuit 21 of the embodiment requires only two capacitors, the number of capacitors having a large capacity can be reduced.

  2A and 2B are graphs comparing the voltage gains of the conventional front tuning circuit 10 and the front tuning circuit 21 of the embodiment. 2A and 2B, the vertical axis represents voltage gain [dB], and the horizontal axis represents frequency displacement Δf [kHz] with respect to the tuning frequency.

  The gain characteristics shown on the upper side of FIGS. 2A and 2B (characteristics indicated by the diamond points in FIG. 2) show changes in voltage gain when AGC control is off, and the gain shown on the lower side. The characteristics (characteristics indicated by square points in FIG. 2) indicate changes in voltage gain when AGC control is on.

  As shown in FIG. 2A, the conventional front tuning circuit 10 has different frequency characteristics of the voltage gain when the AGC control is on and the voltage gain when it is off. This indicates that the difference between the voltage gain when the AGC control is off and the voltage gain when the AGC control is on, that is, the attenuation of the front tuning circuit 10 depends on the frequency. That is, in the conventional front tuning circuit 10, since the attenuation changes when the AGC control is turned on, the tuning frequency of the tuning circuit is shifted.

  On the other hand, in the front tuning circuit 21 of the embodiment, as shown in FIG. 2B, the frequency characteristics of the voltage gain when the AGC control is off and when the AGC control is on are substantially the same. Therefore, the difference between the voltage gain when the AGC control is off and the voltage gain when the AGC is on, that is, the attenuation amount is almost constant without depending on the frequency.

Therefore, since the attenuation amount of the front tuning circuit 21 does not change depending on the frequency, the tuning frequency of the tuning circuit 23 does not shift when the AGC control is turned on.
FIG. 3A is a diagram showing the voltage gain for each receiving station of the conventional front tuning circuit 10, and FIG. 3B shows the voltage gain for each receiving station of the front tuning circuit 21 of the embodiment. FIG.

  As shown in FIG. 3A, the conventional front tuning circuit 10 includes a voltage gain of the front tuning circuit 10 when a signal of the receiving station 1 is received, and a voltage gain when a signal of the receiving station 2 is received. The voltage gains when receiving signals from the receiving station 3 are different. That is, the receiving sensitivity differs for each receiving station.

  On the other hand, as shown in FIG. 3B, the front tuning circuit 21 of the embodiment uses the voltage gain of the front tuning circuit 21 and the signal of the receiving station 2 when the signal of the receiving station 1 is received. The voltage gain when receiving the signal and the voltage gain when receiving the signal of the receiving station 3 are substantially equal. That is, the reception sensitivity for a plurality of receiving stations is substantially constant.

  According to the above-described embodiment, when the received signal level is controlled by changing the impedance of the attenuation circuit by performing AGC control, it is possible to prevent the tuning frequency from shifting. Further, the number of capacitors can be reduced by one as compared with the conventional series resonance type front tuning circuit 10.

It is a circuit diagram of the front tuning circuit of an embodiment. 2A and 2B are diagrams showing voltage gains of the conventional front tuning circuit and the front tuning circuit of the embodiment. FIGS. 3A and 3B are diagrams showing voltage gains for each receiving station of the conventional front tuning circuit and the front tuning circuit of the embodiment. It is a circuit diagram of the conventional front tuning circuit.

Explanation of symbols

L1, L11 Inductors C1-C6, C11, C12 Capacitors D1, D2 Diodes D3, D4 Variable capacitance diode R1 Resistance T1 Tuning coil 11 Antenna 10, 21 Front tuning circuit 22 Attenuation circuit 23 Tuning circuit 24 Differential amplifier 25 Mixer circuit 26 AGC circuit

Claims (3)

An antenna,
An inductor connected between the antenna and ground;
A first capacitor having one end connected to the antenna;
A first diode having an anode connected to the other end of the first capacitor and a cathode grounded; and a second diode having a cathode connected to the anode of the first diode, and a semiconductor integrated circuit An attenuation circuit to which a current for changing impedances of the first and second diodes is supplied from an AGC circuit built in
A tuning circuit that tunes the received signal;
The other end and one end of the first capacitor are connected, the other end is connected to the input side of the tuning circuit, and the attenuation circuit and the tuning circuit are set to a value capable of impedance separation in the frequency band of the received signal. And a second tuning circuit. An antenna,
An inductor connected between the antenna and ground;
A first capacitor having one end connected to the antenna;
An attenuation circuit comprising: a first diode having an anode connected to the other end of the first capacitor and a cathode grounded; and a second diode having a cathode connected to the anode of the first diode;
A tuning circuit that tunes the received signal;
The other end and one end of the first capacitor are connected, the other end is connected to the input side of the tuning circuit, and the attenuation circuit and the tuning circuit are set to a value capable of impedance separation in the frequency band of the received signal. A second capacitor;
A front tuning circuit comprising: an AGC circuit that is built in a semiconductor integrated circuit and controls an electric current supplied to an anode of the second diode of the attenuation circuit to change an impedance of the attenuation circuit. The semiconductor integrated circuit is composed of MOS transistors, and a differential amplifier for amplifying the tuning signal of the tuning circuit is built in the semiconductor integrated circuit, and the output signal level of the differential amplifier is constant in the AGC circuit. The front tuning circuit according to claim 2, wherein an impedance of the attenuation circuit is controlled.