JPH03292011A - Automatic gain control circuit for automatic channel selection am tuner - Google Patents

Abstract

PURPOSE:To avoid an event of a distorted demodulation signal due to an AGC voltage mixed with a modulation signal component by controlling the time constant of a low pass filter so as to be increased more than a time constant at an automatic channel selection when the automatic channel selection is stopped and the reception state is arrived. CONSTITUTION:When automatic sweeping is implemented, a PLL control microcomputer 12 turns off a transistor(TR) Q1 to decrease the time constant TA of an LPF 15 at the time of automatic sweeping as TA=R1.C1. Thus, when the reception signal level reaches a proper level, the channel selection is stopped. When the level of the reception signal inputted to a carrier level detection circuit 8 reaches a setting level or above, the TR Q1 is turned on, a capacitor C2 is connected in parallel with the circuit C1 and the time constant is larger than the time constant TA at automatic sweeping. Thus, the event of a distorted demodulation signal outputted from an AM detection circuit 7 due to the mixture of a modulation signal component into the AGC voltage is avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is applicable to, for example, PLL (phase 1 locked
1oop) This invention relates to an automatic gain control circuit for an automatic channel selection AM tuner that performs channel selection using a frequency synthesizer method.

[Conventional technology]

For example, an automatic channel selection AM tuner that performs channel selection using a PLL frequency synthesizer method has a configuration shown in FIG. The AM tuner uses an antenna (hereinafter referred to as ANT).
) 44 is transmitted to the ANT tuning circuit 31.
The signals extracted by this are
It is amplified by a high frequency amplification circuit (hereinafter referred to as an RF amplification circuit) 32, mixed with the output signal of a voltage controlled local oscillation circuit 34 by a frequency mixing circuit 33, and converted to an intermediate frequency.

The tuning frequency of the ANT tuning circuit 31 and the oscillation frequency of the frequency mixing circuit 33 are controlled by a PLL circuit 41 controlled by a PLL control microcomputer (hereinafter referred to as PLL control microcomputer) 42, that is, via a PLL circuit LPF 40. PL supplied
The output signal of the L circuit 41 is controlled to correspond to the frequency of the selected broadcast signal.

That is, by inputting a tuning signal specifying a broadcasting station to be received using a tuning key or the like to the PLL control microcomputer 42, an automatic control system that controls the tuning frequency of the ANT tuning circuit 31 and the oscillation frequency of the frequency mixing circuit 33 is input. Automatic channel selection is performed by a sweep operation.

Further, the output signal of the frequency mixing circuit 33 is amplified by an IF amplifier circuit 36 via an intermediate frequency (hereinafter referred to as IF) filter 35, detected by an AM detection circuit 37, and demodulated into an audio signal.

AM tuners generally use automatic gain control (AGC) to prevent audio output from fluctuating due to changes in input signal strength due to fluctuations in electric field strength due to fading, etc.
It is equipped with a circuit called This AGC circuit includes a detection rectifier circuit provided in the above AM detection circuit 37,
AGC amplifier circuit 39, resistor R1 and capacitor Cs
+ low-pass filter (hereinafter referred to as LPF)
45, the signal rectified by the AM detection circuit 37 is amplified by the AGC amplification circuit 39, and the LPF 4
5 and becomes a DC AGC voltage, which is fed back to the RF amplification circuit 32 and the IF amplification circuit 36, thereby controlling the amplification degrees of both the amplification circuits 32 and 36. That is, when the received signal level of the AM tuner is high, the AGC voltage becomes high and the degree of amplification becomes small, and when the received signal level is small, the AGC voltage becomes high and the amplification degree becomes low.
By lowering the Ct pressure and increasing the degree of amplification, a demodulated signal of a constant level can be obtained.

(Problem to be Solved by the Invention) However, in the above conventional configuration, the resistor R3 of the LPF 45
The following problems arise depending on the magnitude of the time constant determined by + and capacitor C1.

That is, during automatic tuning, while the frequency of the ANT tuning circuit 31 and the oscillation frequency of the voltage-controlled local oscillation circuit 34 are different from the frequency specified by the tuning signal, the IF signal output level of the frequency mixing circuit 33 is Therefore, the AGC voltage is low, and the amplification degree of the RF amplifier circuit 32 and the IF amplifier circuit 36 is high. on the other hand,
When the frequency of the ANT tuning circuit 31 and the oscillation frequency of the voltage controlled local oscillation circuit 34 match the frequency specified by the tuning signal, the IF signal output level of the frequency mixing circuit 33 becomes high, so the AGC voltage tends to rise. However, if the time constant of the LPF 45 is large, the AGC voltage will be delayed in following changes in the IF signal level, and the amplification degrees of both the amplifier circuits 32 and 36 will be greater than in the steady reception state. Therefore, the carrier level detection circuit 38 outputs a sweep stop signal instructing to stop the automatic sweep operation when the signal level is considerably lower than the minimum lighting level of the tuning indicator 43 in the steady reception state. The automatic sweep operation is stopped by the PLL control microcomputer 42 that receives the input, and a steady reception state is entered. That is, the minimum signal level at which the automatic sweep operation can be performed is lower than the minimum level at which the tuning indicator 43 lights up, and even if the automatic sweeping operation is stopped, the tuning indicator 43 will not light up.

On the other hand, when the time constant of the LPF 45 is small, the cutoff frequency of the LPF 45 becomes high. In this case, the ACC voltage is A
Since it is obtained by filtering the M-modulated IF signal, if the frequency of the modulation signal is low, the modulation signal component will be mixed into the AGC voltage, and when such an AGC voltage is applied to the IF amplifier circuit 36, The degree of amplification changes with the modulation signal frequency, and the IF
The demodulated signal obtained from the AM detection circuit 37 at the next stage of the amplifier circuit 36 will be distorted.

In this way, the above-mentioned problems arise depending on the size of the time constant of the LPF 45, and in conventional AGC circuits, priority is given to preventing automatic stop malfunction during automatic channel selection, or Currently, the time constant is set with priority given to reducing the distortion rate in the low frequency range, or as a compromise between the two. However, with such a configuration, it is not possible to improve the overall quality of the tuner, and there is a need to develop a configuration that solves the above problems.

[Means to solve the problem]

The automatic gain control circuit of the automatic channel selection AM tuner of the present invention includes:
In order to solve the above problem, automatic tuning operation is performed when a tuning command is input, and when the level of the received signal becomes higher than the set level due to this automatic tuning operation, automatic tuning operation is performed. The following measures are taken in the automatic gain control circuit of the automatic tuning AM tuner which is provided in the automatic tuning AM tuner which enters the reception state when the AGC voltage is stopped and supplies the AGC voltage to the amplifier circuit through the low-pass filter.

That is, the above-mentioned low-pass filter has an element that determines a time constant, such as a switching means that connects or disconnects a capacitor, such as a transistor, and is configured such that the time constant changes according to the ON/OFF operation of this switching means. , when the automatic tuning operation is started, the above switching means is controlled so that the time constant of the low-pass filter becomes small, and when the automatic tuning operation is stopped and the receiving state is entered, the low-pass filter is A time constant control means is provided for controlling the switching means so that the time constant is larger than the time constant during the above-mentioned automatic channel selection operation.

[For production]

According to the above configuration, the automatic channel selection AM tuner performs an automatic channel selection operation when a channel selection command is input. At this time, the time constant control means controls the switching means so that the time constant of the low-pass filter becomes small. Therefore, the AGC voltage can rise immediately following the rise in the received signal level when a channel is selected. Thereby, the channel selection operation can be stopped when the received signal level is at an appropriate level. That is, reception can be performed with appropriate channel selection.

Thereafter, when the received signal level becomes equal to or higher than the set level, the automatic channel selection AM tuner stops the automatic channel selection operation and enters the receiving state. At this time, the time constant control means controls the switching means so that the time constant of the low-pass filter becomes larger than the time constant during the above automatic channel selection operation. Therefore, in normal reception conditions, the cutoff frequency of the low-pass filter is set low, so that the situation where the demodulated signal is distorted due to the modulation signal component being mixed into the AGC voltage and the amplification degree changing with the modulation signal frequency is avoided. Avoided.

[Example] An example of the present invention will be described below based on FIGS. 1 and 2.

A synthesizer AM tuner that performs automatic tuning using the PLL frequency synthesizer method is constructed as shown in FIG. That is, this tuner includes an ANT tuning circuit 1 that extracts a signal of a selected frequency from among the signals received by the ANT 14, an RF amplification circuit 2 that amplifies the signal selected by the ANT tuning circuit 1, P mentioned later
Controlled by the LL circuit 11, LPF for PLL circuit
A voltage controlled local oscillation circuit 4 that oscillates a signal of a frequency corresponding to the output voltage of the PLL circuit 11 obtained via the RF amplifier circuit 10 and applies it to the RF amplifier circuit 2 is provided. Similar to the voltage controlled local oscillation circuit 4, the ANT tuning circuit 1 is a PLL circuit L
The tuning frequency is changed based on the output voltage of the PLL circuit 11 obtained via the PF 10.

Moreover, this tuner outputs from the RF amplifier circuit 2,
The frequency mixing circuit 3 generates a signal with a frequency that is the difference between the signal selected by the ANT tuning circuit 1 and the output signal of the voltage controlled local oscillation circuit 4, the F filter 5, and the IF filter 5. It includes an IF amplifier circuit 6 that amplifies the IF signal, and an AM detection circuit 7 that extracts a demodulated signal, which is an audio signal, from the output signal of the 1F amplifier circuit 6.

In addition, this tuner lights up the tuning indicator 13 when the received signal level obtained from the AM detection circuit 7 exceeds the set level, and sets the received signal level to LOW when the received signal level is lower than the set level. The carrier level detecting circuit 8 supplies a high level output to the PLL control microcomputer 12 via the tuning indicator 13 when the level is higher than the level, and the tuning indicator 13 displays the tuning state. The above set level in the carrier level detection circuit 8 is usually set near practical sensitivity. The above-mentioned high level signal output from the carrier level detection circuit 8 is sent to the PLL control microcomputer 12 as a sweep stop signal.
supplied to

This tuner also includes an AGC amplifier circuit 9 that amplifies the rectified output of the AM detection circuit 7, a resistor R1, a capacitor C1,
C2 and a transistor Q as a switching means,
It includes an LPF 15 that supplies the AGC voltage to the RF amplifier circuit 2 and the IF amplifier circuit 6. The capacitor C2 is grounded via a transistor Q, and a time constant control signal output from the PLL control microcomputer 12 is input to the base of the transistor Q. Then, the A G
A C circuit is configured.

In addition, this tuner has a PLL control microcomputer 12.
P that controls the oscillation frequency of the voltage controlled local oscillation circuit 4 and the tuning frequency of the ANT tuning circuit 1 based on the control signal of
LL circuit 11, PLL control microcomputer 12 as time constant control means for controlling 0N10FF of PLL circuit 11 and transistor Q of LPF 15;
The PLL circuit LPF 10 supplies only the low frequency component of the output voltage of the LL circuit 11 to the ANT tuning circuit 1 and the voltage controlled local oscillation circuit 4. The PLL control microcomputer 12, for example, by operating a channel selection switch,
When a channel selection signal as a channel selection command indicating which broadcast signal to receive is input, the PLL circuit 1 is controlled in accordance with this channel selection signal, and a low-level time constant control signal is output to transistor Q1. On the other hand, when a sweep stop signal is manually applied via the tuning indicator 13, the sweep operation is stopped and the reception state is turned off, and a high level time constant control is applied to the base of the transistor Q. A signal is supplied to turn on the transistor Q1.

In the above configuration, the signal received by ANT14 is A
The NT tuning circuit 1 selects and extracts only signals with preselected frequencies. This signal is amplified by the RF amplifier circuit 2, mixed with the output signal of the voltage controlled local oscillation circuit 4 by the frequency mixing circuit 3, and converted to an intermediate frequency. The signal is amplified, detected by the AM detection circuit 7, and demodulated into an audio signal. The intertuning frequency of the above-mentioned ANT tuning circuit and the oscillation frequency of the frequency mixing circuit 3 are made to correspond to the frequency of the selected broadcast signal by the PLL circuit 11° controlled by the PL, L control microcomputer 12. controlled by.

Next, the channel selection control operation in the PLL control microcomputer 12 and the operation as the time constant control means shown in FIG. 2 will be explained.

When a channel selection signal is input to the PLL control microcomputer 12 by operating a channel selection switch, for example, in a steady reception state in which a certain broadcast signal is being received (St
), the PLL control microcomputer 12 sets the PLL control so that the tuning frequency of the ANT tuning circuit 1 and the oscillation frequency of the frequency mixing circuit 3 correspond to the input tuning signal.
Controls the LL circuit 11. This allows automatic tuning operation,
That is, an automatic sweep operation is performed. At the same time, the PLL controller 12 supplies a low-level time constant control signal to the base of the transistor Q of the LPF 15 (S2). As a result, transistor Q is turned off.
F, and the time constant TA of the LPF 15 during automatic sweeping becomes a small value, such as time constant TA=R1·C. In this embodiment, this time constant T. -R1·C2 is set so that the signal level at which the automatic sweep operation is stopped matches the minimum signal level at which the tuning indicator 13 lights up in a steady state.

As described above, when the time constant TA of the LPF 15 during automatic sweep becomes a small value, the AGC voltage can rise immediately following the rise in the received signal level when a channel is selected. Thereby, the channel selection operation can be stopped when the received signal level reaches an appropriate level.

Thereafter, when the level of the received signal input to the carrier level detection circuit 8 becomes equal to or higher than the set level, the carrier level detection circuit 8 lights up the tuning indicator 13 and sends a sweep stop signal to the PLL control microcomputer 12 via the tuning indicator 13. (Am) When the PLL control microcomputer 12 receives the sweep stop signal (S3)
, stops the automatic sweep operation and enters the receiving state, and supplies a high-level time constant control signal to the base of transistor Q (S4). As a result, transistor Q1
is turned ON, and the capacitor C2 is connected in parallel to the capacitor CI. Therefore, L during steady reception state
The time constant T of PF15 is the time constant Ts = Rt (C
t+Cz), which is a value larger than the time constant TA during automatic sweep. In this example, time constant Ts =
R1 (C+ +Ct) is set so that distortion in the low frequency range of the demodulated signal in a steady reception state is minimized.

As mentioned above, when the time constant of the LPF 15 becomes large in the steady reception state, the cutoff frequency of the LPF 15 is set low, so that modulation signal components are mixed into the AGC voltage, causing the RF amplifier circuit 2 and the IF amplifier circuit 6 A situation in which the demodulated signal output from the AM detection circuit 7 is distorted due to the amplification degree changing with the modulation signal frequency can be avoided.

〔Effect of the invention〕

The automatic gain control circuit of the automatic channel selection AM tuner of the present invention includes:
As described above, the low-pass filter has a switching means for connecting/disconnecting the element that determines the time constant, and is configured such that the time constant changes by the ON10FF operation of this switching means, and the automatic channel selection operation is started. The switching means is controlled so that the time constant of the low-pass filter becomes small when the automatic tuning operation is stopped and the time constant of the low-pass filter becomes small when the automatic tuning operation is stopped and the reception state is entered. This configuration includes a time constant control means for controlling the switching means so that the time constant becomes larger than the time constant of the switching means.

Thereby, in the channel selection operation, the channel selection operation can be stopped when the received signal level reaches an appropriate level, and reception can be performed in a state where the channel is appropriately selected. Further, in a normal reception state, a situation in which a modulated signal component is mixed into the AGC voltage and the demodulated signal is distorted is avoided, and a good demodulated signal can be obtained.

This is a flowchart.

FIG. 3 shows a conventional example, and is a block diagram showing a PLL frequency synthesizer type automatic channel selection AM tuner.

7 is an AM detection circuit, 8 is a carrier level detection circuit, 12
is a PLL control microcomputer (time constant control means), 15 is a low-pass filter, R is a resistor, and C3
is a capacitor, C2 is a capacitor, and Ql is a transistor (switching means).

Claims (1)

[Claims] 1. Automatic tuning operation is performed when a tuning command is input, and when the level of the received signal becomes equal to or higher than a set level due to this automatic tuning operation, automatic tuning operation is performed. In the automatic gain control circuit of the automatic tuning AM tuner that is installed in the automatic tuning AM tuner that is stopped and enters the receiving state, and supplies the AGC voltage to the amplifier circuit through the low-pass filter, the above-mentioned low-pass filter has a time constant of It has a switching means that connects and disconnects the connection of the element to be determined, and is configured such that the time constant changes depending on the ON/OFF operation of this switching means, and when the automatic channel selection operation is started, the time constant of the low-pass filter changes. The switching means is controlled so that the time constant of the low-pass filter becomes larger than the time constant during the automatic tuning operation when the automatic tuning operation is stopped and the reception state is entered. An automatic gain control circuit for an automatic channel selection AM tuner, characterized in that the automatic gain control circuit includes a time constant control means for controlling a switching means.