JP3070988B2 - Automatic tuning circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic tuning circuit for automatically selecting a broadcasting station in a radio receiver, and more particularly to a signal strength condition after tuning when a tuning frequency is changed within a predetermined range. And a fine adjustment station detecting means for performing station detection.

[0002]

2. Description of the Related Art Conventionally, an automatic tuning circuit for automatically changing a tuning frequency in a car radio or the like and fixing the tuning frequency to the frequency when a radio wave from a broadcasting station is received has been provided. When performing automatic channel selection, first, the tuning frequency is changed in accordance with the inter-station frequency, and a first-stage station detection for detecting whether or not a broadcast station exists based on the radio wave intensity at this time; In the first stage, the tuning frequency is changed in a predetermined minute range before and after the frequency at which the station is detected, and the tuning is performed in two stages of fine adjustment to accurately match the tuning frequency to the radio wave of the broadcasting station. Here, the inter-station frequency refers to a minimum interval between radio waves from each broadcasting station. In the normal case, the frequency of the broadcast wave of each broadcasting station is regulated so as to correspond to the inter-station frequency so that radio waves from each broadcasting station do not interfere with each other. Therefore, in order to search for a broadcasting station, the tuning frequency may be changed for each inter-station frequency.

Further, in a normal radio receiver, the adjustment of the tuning frequency for each inter-station frequency is not always accurately determined at every point, and several tracking points (for example, in an AM radio receiver, 600 kHz
z, 1000 kHz, and 1400 kHz). Therefore, the tuning point set for each inter-station frequency often does not exactly match the frequency of the broadcast wave, so it is necessary to make fine adjustments and finally adjust the tuning frequency of the radio to the frequency of the broadcast wave. Is needed.

[0004]

On the other hand, a conventional radio receiver has an AGC (auto gain control) means therein. Since the intensity of the radio wave from the broadcast station changes according to the distance from the broadcast station, the intensity of the received radio wave obtained by the antenna of the radio receiver is not constant. Further, in signal processing inside the radio receiver, it is required that the signal strength be within a predetermined range. Therefore, A inside the radio receiver
The GC means is provided so that the intensity of the radio wave to be processed becomes a predetermined value. And the usual AGC means
When it is detected that the received radio wave is equal to or more than a predetermined value, a damping circuit that attenuates the received radio wave intensity stepwise is controlled to adjust the intensity of the received radio wave.

[0005] In a car radio, since the distance from a broadcasting station changes in accordance with the traveling of a vehicle, the received radio wave intensity may greatly change even during the channel selection operation. When the intensity of the received radio wave exceeds a predetermined value,
The damping circuit starts to operate due to the C means, and the signal strength inside changes abruptly. For this reason, if the AGC means starts operating while performing the above-described fine adjustment, the signal strength for performing the fine adjustment rapidly changes in response to the start of the operation of the damping circuit, and becomes unnecessary. For example, there is a problem that a proper peak of the broadcast wave cannot be detected in the fine adjustment due to occurrence of a peak, and a tracking error occurs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has been made in consideration of the following problems.
It is an object of the present invention to provide an automatic tuning circuit capable of controlling the operation of the AGC means and reliably tuning to the peak point of the signal strength.

[0007]

An automatic tuning circuit according to the present invention comprises a damping means for attenuating a received radio wave, an AGC means for changing an amount of attenuation of the damping means according to the intensity of the received radio wave, A tuning means for extracting a signal of a tuning frequency from a signal, and a tuning means within a predetermined range.
In the automatic tuning circuit having a fine tuning station detection means for changing the tuning frequency in, and detecting the peak according to the change state of the signal strength after tuning at this time to perform station detection,
At the time of station detection using the fine adjustment station detection means, the damping means is forcibly driven to set a predetermined attenuation , and
Characterized in that a forced driving means for maintaining the amount of attenuation is provided.

Further, the above forced drive means Contact to the damping means by the forced driving in accordance with the signal strength after the tuning
And changing the amount of attenuation to be set and held .

[0009]

As described above, the automatic channel selection circuit according to the present invention has the forced driving means. Therefore, at the time of fine adjustment channel selection detection, a predetermined damping amount is forcibly set in advance. Therefore, it is possible to prevent the reception intensity from greatly changing due to the operation of the AGC means at the time of fine adjustment, and to achieve accurate peak detection and tuning. Further, by changing the forced damping amount according to the signal strength, it is possible to efficiently eliminate the adverse effect of the operation of the AGC circuit at the time of fine adjustment.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing the configuration of the embodiment.
F tuning, mixing, IF amplification and detection are performed, and a predetermined reception signal is output from an output terminal 12.

The antenna 10 has an RF amplification FET 20
Are connected via a capacitor 22, where RF amplification of the received signal is performed. Also, FE for RF amplification
A pin diode 26 connected to the ground via a capacitor 24 is connected to the signal input terminal of T20.
The attenuation of the input signal to the amplification FET 20 can be controlled. For this purpose, a pin diode 28 is connected to a connection point between the pin diode 26 and the capacitor 24.

On the other hand, an input side tuning coil 32 is connected to the upstream side of the RF amplification FET 20 via a current adjustment transistor 30. At a position facing the input-side tuning coil 32, an output-side tuning coil 38 having one end grounded and the other end connected to the ground via a variable capacitance diode 34 and a capacitor 36 is arranged. Then, a tuning signal of a predetermined frequency is output from an intermediate tap of the output side tuning coil 38. That is, by changing the potential of the intermediate point between the variable capacitance diode 34 and the capacitor 36, the capacitance of the variable capacitance diode 34 is changed, and the frequency of the tuning output signal from the output side tuning coil 38 can be controlled.

A coil 40 connected in parallel and a resistor 42 are connected to the upstream side of the input side tuning coil 32, and the other end is connected to a power supply _Vcc . Therefore, a non-tuned signal is output from an intermediate point between the coil 40 and the input side tuning coil 32.

The above-mentioned tuning signal and the non-tuning signal are input to the switch 50, and one of them is selected and input to the mixer 52. A local oscillator 54 is connected to the mixer 52, and the mixer 52 mixes an input signal (tuned signal or non-tuned signal) with a signal from the local oscillator 54 and outputs an intermediate frequency signal to a coil 56. One end of the coil 56 is connected to the ground, and the other end of the coil 58 is connected to the ceramic filter 60. The coil 58 is disposed opposite to the coil 58, and an intermediate frequency signal is supplied from the coil 58 to the ceramic filter 60. This ceramic filter 60
Outputs only a signal of a fixed frequency, and only a signal dropped to a predetermined intermediate frequency is supplied to the IF amplifier 62. In order to keep the frequency of the intermediate frequency signal from the mixer 52 constant, the frequency of the output signal of the local oscillator 54 is controlled according to the frequency of the received signal to be extracted.

The IF signal output from the IF amplifier 62 is detected by a detection circuit 64 and output from an output terminal 12.
And the sound is reproduced.

The IF signal from the IF amplifier 62 is also supplied to an electric field strength detection circuit 70. The electric field strength detection circuit 70 detects the signal level of the signal after IF amplification and supplies the signal level to the AGC control circuit 72. The AGC control circuit 72 controls the base voltage of the current adjustment transistor 30 according to the signal level of the IF signal after the IF amplification detected by the electric field strength detection circuit 70, and reduces the amount of current in the RF amplification FET 20. Control. That is, IF
The current adjusting transistor 30 according to the signal level of the signal
Is controlled so that the signal level after RF amplification always becomes a predetermined value.

Further, when the signal level of the IF signal detected by the electric field strength detection circuit 70 is equal to or higher than a predetermined value, the AGC control circuit 72 supplies a predetermined current to the pin diode 28, To attenuate the signal level at the signal input end of the RF amplification FET 20 stepwise. As a result, it is possible to prevent the input signal of the RF amplifier 20 from exceeding a predetermined value.

The local oscillator 54 is connected to a control circuit 80 for controlling the oscillation frequency.
By supplying a control signal about the oscillation frequency from the LL circuit 82 to the control circuit 80 via the low-pass filter 84, the PLL circuit 82 controls the oscillation frequency of the local oscillator 54. Therefore, during normal broadcast wave reception (at the time of tuning), the PLL circuit 82 controls the local oscillator 54 to output a frequency signal corresponding to the tuning frequency. For example, in the case of an AM receiver, the frequency of the intermediate frequency signal is set to 450 kHz, so that the oscillation frequency of the local oscillator 54 is 450 kHz lower than the frequency of the tuning output. The switch control circuit 86 controls the switch 50 to switch a signal input to the mixer 52 to a non-tuned signal or a tuned signal.

Further, the output of the electric field strength detection circuit 70 is also supplied to the station detection circuit 90. This station detection circuit 90
The presence of a station is detected when the value of the electric field strength is equal to or more than a predetermined value, and is used at the time of station detection for each inter-station frequency using a non-tuned signal (station detection at the first stage).

A fine adjustment circuit 100 is also connected to the electric field strength detection circuit 70. This fine adjustment circuit 100
AD converter 102, detection control circuit 104, DA
It comprises a converter 106. Then, the AD converter 102 supplies a signal about the signal level supplied from the electric field strength detection circuit 70 to a detection control circuit 104 which converts the signal into digital data. The detection control circuit 104 is AD
The digital data from the converter 102 is processed to perform peak detection, and outputs data for frequency control of an output signal from the output-side tuning coil 38. That is, the DA converter 106 converts the data supplied from the detection control circuit 104 into an analog signal, and outputs a signal for changing the capacitance of the variable capacitance diode 34. The signal from the DA converter 106 changes the capacitance of the variable capacitance diode 34 to change the tuning output of the tuning output coil 38 within a predetermined range.

Therefore, when the presence of a station is detected in the station detection in the first stage, the tuning frequency in the tuning circuit 32 is changed within a predetermined range before and after the frequency, and the output of the electric field strength detection circuit 70 at that time is changed. The peak at the time of fine adjustment, that is, the frequency of the broadcast station can be detected according to the value.

Further, in this embodiment, there is provided a forced drive circuit 120 to which a signal about the signal level of the IF signal from the electric field strength detection circuit 70 is supplied. As shown in FIG. 2, the forced drive circuit 120 includes a control unit 122
And two constant current sources 124 and 126,
The constant current sources 124 and 126 are turned on and off in accordance with the signal level of the IF signal to control the amount of damping in the pin diode 26 and to control the level of the output signal in the mixer 52.

Next, the operation at the time of automatic channel selection will be described. First, when the automatic tuning button arranged on the radio receiver is operated, the PLL circuit 82 enters the automatic tuning state. That is, the PLL circuit 82 changes the oscillation frequency of the local oscillator 54 via the control circuit 80 at every inter-station frequency. Further, the switch control circuit 86 controls the switch 50 and supplies an untuned signal to the mixer 52. Therefore, the mixer 5
2 outputs an intermediate frequency signal corresponding to the oscillation frequency of the local oscillator 54, and the filter 60 outputs 450 kHz
Only the signal of is selected. Therefore, among the non-tuned signals, those which are 450 kHz higher than the oscillation frequency of the local oscillator 54 are sequentially selected, input to the IF amplifier 62 as an intermediate frequency signal of 450 kHz, and amplified there. Then, the IF signal level is detected by the electric field strength detection circuit 70, and the station detection circuit 90 determines
It is determined whether a station exists. That is, when the signal level is equal to or higher than a predetermined value, it is determined that a station exists.
If the station does not exist, the frequency is changed to the next frequency of the oscillation signal from the local oscillator 54 (a frequency that differs by the inter-station frequency),
Repeat this.

On the other hand, when the presence of a station is detected, a fine adjustment station is detected. At this time, the forced driving circuit 120 is used to forcibly dump the received signal. Therefore, this will be described with reference to FIG. First, the electric field strength detection circuit 70 detects the level of the IF signal output from the IF amplification circuit 62 obtained from the untuned signal supplied via the switch 50, and detects this level.
(S1). Then, the forced drive circuit 120 outputs the voltage value VS at that time, which is the output from the electric field strength detection circuit 70.
It is determined whether M is greater than a first threshold value V1 (S
2). If YES in S2, the 減 衰 attenuation circuit 112 is operated, and
The signal level value supplied to the D converter 102 is reduced by half.
And Thereby, it is possible to prevent the value output from the AD converter 102 from exceeding the upper limit value and overflowing.

Then, the forced drive circuit 120 turns on the constant current source 124 to supply a current of 4 mA to the pin diode 28 to attenuate 30 dB at the pin diode 26 and to supply a signal to the mixer circuit 52. ,
Here, attenuation of 20 dB is performed (S4). The attenuation in the mixer circuit 52 may be performed by switching a plurality of constant current circuits having different current capacities as a constant current circuit that regulates the amount of current in the mixer circuit 52.

If NO in S2, it is next determined whether the voltage VSM is equal to or higher than the second threshold V2. Here, the threshold value V2 is a value smaller than V1. When the VSM is equal to or higher than V2, the constant current source 126 is turned on and the 200 μA
And the amount of attenuation in the pin diode 26 is set to 10 dB (S6). NO in S5
If so, no forced dumping is performed. Thus, predetermined damping is performed according to the intensity of the received radio wave, and the output of the IF amplifier circuit 62 is assumed to be sufficiently small.

Thereafter, the process is switched from the process using the non-tuned signal (wide band detection) to the process using the tuned signal (narrow band detection) (S7). That is, the station detection circuit 90 outputs a signal for fixing the oscillation frequency from the local oscillator 52 to the PL.
Input to the L circuit 82. Therefore, the PLL circuit 82 fixes the oscillation frequency of the local oscillator 54 via the control circuit 80 to the frequency at that time. Next, the PLL circuit 82 supplies a signal about the current tuning frequency to the DA converter 106, whereby the frequency of the tuning output from the output side tuning coil 38 is increased by 450 kHz from the oscillation frequency of the local oscillator 54. Set things. On the other hand, the switch control circuit 86 switches the switch 50 and supplies a tuning signal to the mixer 52.

Then, in this state, fine adjustment station detection is performed. In this fine adjustment station detection, the capacitance of the variable capacitance diode 34 is changed by sequentially changing data sent from the detection control circuit 104 to the DA converter, and the frequency of the tuning signal from the output side tuning coil 38 is stepwise changed. change. The detection control circuit 104 determines the level of the IF signal at this time, and detects a peak. Then, the detected peak is set to the tuning frequency (S8). As a result, the tuning frequency is set, so that the above-described forced damping process in the pin diode or the like is canceled (S9), and the automatic tuning operation ends.

Here, assuming that the current to the pin diode 28 by the forced drive circuit 120 is IA, the relationship between the amount of damping (IA) and the input intensity to the antenna 10 (received radio wave intensity) is as shown in FIG. become. The broken line in FIG. 4 shows the current supplied to the pin diode 28 by the AGC control circuit 72 in the conventional case. According to the present embodiment, since the forced damping is performed in advance, the AG
The timing of the current supply to the pin diode 28 by the C control circuit 72 is delayed. Therefore, this A
The probability that the current supply to the pin diode 28 by the GC control circuit 72 is started is reduced. Also, when the current supply to the pin diode 28 by the AGC control circuit 72 is started, the damping by the AGC control circuit 72 is added to the forced damping. Therefore, AG
The damping amount by the C control circuit 72 is small as a ratio. The adverse effect due to the addition of the damping amount is exponentially exerted. Therefore, by performing the forced damping in advance, the adverse effect due to the start of the damping can be eliminated. Further, a step for inhibiting the current supply to the pin diode 28 by the AGC control circuit 72, such as the stage before S7 in FIG. 3, is provided to prevent the current supply from being started during the fine adjustment station detection (tuning). You may.

[0030]

As described above, according to the automatic tuning circuit of the present invention, when the peak is detected in the fine adjustment,
Forcibly attenuate the received signal. Therefore, it is possible to prevent the peak detection from being performed incorrectly due to the operation of the AGC circuit during the search in the fine adjustment.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of an automatic tuning circuit according to the present invention.

FIG. 2 is a block diagram illustrating a configuration of a forced drive circuit.

FIG. 3 is a flowchart illustrating the operation of the embodiment.

FIG. 4 is a characteristic diagram showing a relationship between an antenna input and a damping amount.

[Description of Signs] 10 Antenna 20 RF amplification FET 26 Pin diode 30 Current adjustment transistor 38 Output side tuning coil 52 Mixer 70 Electric field strength detection circuit 72 AGC control circuit 82 PLL circuit 100 Fine adjustment circuit 104 Detection control circuit 120 Forced drive circuit

Claims (2)

(57) [Claims] 1. A damping means for attenuating a received radio wave, an AGC means for changing an amount of attenuation of the damping means according to the intensity of the received radio wave, a tuning means for extracting a signal of a tuning frequency from a received signal, change the tuning frequency in the tuning means within a predetermined range, that depending on the state of change of the signal intensity after tuning in this
A fine tuning station detecting means for detecting a peak to detect a station; and in an automatic tuning circuit having: a predetermined attenuation amount by forcibly driving the damping means upon detecting a station using the fine tuning station detecting means. set in, its
An automatic channel selection circuit characterized in that a forced driving means for maintaining the amount of attenuation is provided. 2. A automatic tuning circuit according to claim 1, the above-mentioned forcible drive means reduction of Oite set held in the damping means by the forced driving in accordance with the signal strength after the tuning
It is characterized in that the amount of decay is changed.