JP3172377B2 - AM radio receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A circuit having an AGC circuit.
The present invention relates to an improvement in an M radio receiver, and more particularly to an AM radio receiver suitable for use in an integrated circuit (IC).

[0002]

2. Description of the Related Art In general, there is known an AM radio receiver having an AGC circuit for detecting a level of a detection output signal and controlling a gain of an RF amplifier circuit and an IF amplifier circuit in accordance with the detected level. Such a radio receiver is configured as shown in FIG. In FIG. 2, a received RF signal is amplified by an RF amplifier circuit (1), mixed with a local oscillation signal generated from a local oscillation circuit (3) in a mixing circuit (2), and converted into an IF signal. The IF signal is I
The signal is amplified by the F amplifier circuit (4) and then detected by the AM detection circuit (5). The detection output signal of the AM detection circuit (5) is amplified by a low-frequency amplifier circuit (not shown) at the subsequent stage. The output signal of the AM detection circuit (5) is applied to an AGC circuit (7) after being smoothed by a low-pass filter (6) including a resistor and a capacitor. Then, according to the output signal of the low-pass filter (6), the AGC signal
The RF signal generated from the GC circuit (7) is generated by the AGC signal.
The gains of the amplifier circuit (1) and the IF amplifier circuit (4) are controlled. Therefore, the gains of the IF amplifier circuit (1) and the RF amplifier circuit (4) are controlled by the detection output signal level, and the IF
The average level of the signal is kept substantially constant.

[0003]

By the way, if the output signal of the low-pass filter (6) includes a ripple component, R
The gains of the F amplifier circuit (1) and the IF amplifier circuit (4) change due to the ripple. As a result, since the IF signal level changes, there is a problem that the distortion rate of the detection output signal subjected to AM detection deteriorates. Therefore, in order to sufficiently smooth the output signal of the AM detection circuit (5) and suppress the ripple component, the cut-off frequency of the low-pass filter (6) is set to a value lower than the audio signal band, that is, several Hz. I had to. In order to set the cutoff frequency low, the capacitance of the low-pass filter (6) must be increased, so that the circuit of FIG.
There is a problem that even if it is C, the size of the set cannot be reduced.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has an AM detection circuit for detecting an IF signal, a low-pass filter, and an AGC circuit to which an output signal of the low-pass filter is applied. In an AM radio receiver comprising: a detection circuit for detecting an audio signal component from the detection output of the detection circuit; and mixing the output signal of the detection circuit with the detection output of the detection circuit, Means for removing an audio signal component, wherein an output signal of the removing means is supplied to the low-pass filter.

Further, the removing means comprises an inverting circuit for inverting the detection output of the detection circuit, and an adding circuit for adding the detection output of the detection circuit and the output signal of the inversion circuit. Further, the removal means includes an inversion circuit for inverting a detection output of the detection circuit, and an addition circuit for adding output signals of the detection circuit and the detection circuit.

Further, the removing means comprises a subtraction circuit for subtracting a detection output of the detection circuit and an output signal of the detection circuit.

[0007]

According to the present invention, the detection circuit detects the audio signal component from the detection output generated from the detection circuit,
In the removing means, the audio signal component in the detection output of the detection circuit can be removed by mixing the audio signal component and the detection output of the detection circuit.
Thereafter, the output signal of the removing means is applied to the low-pass filter, and the AGC circuit generates an AGC signal. Therefore,
The cutoff frequency of the low-pass filter can be set in the audio signal band, and the capacitance of the capacitor forming the low-pass filter can be reduced.

[0008]

FIG. 1 shows an embodiment of the present invention.
(8) is a low-frequency amplifier circuit for amplifying the detection output signal,
(9) is a detection circuit for detecting an audio signal component from the detection output signal, (10) is an inversion circuit for inverting the output signal of the detection circuit (9), and (11) is a detection output signal and the output of the inversion circuit (10). It is an addition circuit for adding the signal. still,
Circuits that are the same as those in the related art are given the same reference numerals as those in the related art, and descriptions thereof are omitted.

In FIG. 1, after a received RF signal is amplified by an RF amplifier circuit (1), it is converted to an IF signal by a local oscillation signal from a local oscillation circuit (3) in a mixing circuit (2). The IF signal is amplified by an IF amplification circuit (4) and then detected by an AM detection circuit (5). Further, the detection output signal generated from the AM detection circuit (5) is applied to the low frequency amplification circuit (8) and amplified. In the detection circuit (9), an audio signal component is detected from the detection output signal, and the detected audio signal component is inverted by the inversion circuit (10). A detection output signal generated from another output terminal of the AM detection circuit (5) is added to an output signal of the inversion circuit (10) by an addition circuit (11). In the adder circuit (11), the audio signal component in the detection output signal of the AM detection circuit (5) and the output signal of the inversion circuit (10), that is, the inverted audio signal component are added, and the addition is performed in the detection output signal. Are canceled out. The output signal of the adder circuit (11) is smoothed by a low-pass filter (6), and then applied to an AGC circuit (7). The AGC signal is supplied from the AGC circuit (7) to the RF amplifier circuit (1) and the IF amplifier circuit. This is applied to (4) to perform gain control of the RF amplifier circuit (1) and the IF amplifier circuit (4).

Therefore, the audio signal component is not included in the input signal of the low-pass filter (6), and the cut-off frequency of the low-pass filter (6) can be set in the audio signal band. The capacitance of the capacitor to be used can be reduced. FIG.
FIG. 3 is a diagram showing a specific circuit example of a main part of FIG.
And (13) are amplification transistors for amplifying the IF signal,
(14) is a detection transistor for detecting an IF signal, and (1)
5) is a current mirror circuit for inverting the output current of the detection transistor (14), and (16) is a detection transistor (14).
(17) is a resistor to which the output current of the current mirror circuit (15) is supplied, (18) is a coupling capacitor for cutting direct current, and (19) is a low-frequency amplifier circuit (9). An input resistor, (20) is an inverting amplifier for inverting the audio signal component, and (21) is a capacitor that constitutes a low-pass filter together with the resistor (17).

In FIG. 3, an IF signal applied to an input terminal (22) is supplied to amplification transistors (12) and (1).
After being amplified in 3), it is detected by the detection transistor (14). The output current of the detection transistor (14), that is,
The collector current is supplied to the collector of the input transistor (23) of the current mirror circuit (15) and is generated at the collector of the output transistor (24). The collector current of the output-side transistor (24) is supplied to a low-pass filter including a resistor (17) and a capacitor (21). Further, the detection output signal generated at the connection point between the detection transistor (14) and the resistor (16) has a DC component in the AM detection output signal cut by the coupling capacitor (18). The AM detection output signal from which the DC component has been cut is applied to the low-frequency amplifier circuit (9).

The audio signal component in the detection output signal generated at one end of the input resistor (19) includes a bias voltage,
The audio signal component is (−) of the inverting amplifier (20).
Applied to terminal. In the inverting amplifier (20),
Since the bias voltage is applied to the (+) terminal, a signal in which only the audio signal component is inverted is generated at the output terminal of the inverting amplifier (20). At connection point (a),
An audio signal generated from the collector of the output transistor (24) and a detection output signal including a DC component are added to an inverted audio signal component generated from the inverting amplifier (20). Thereby, only the audio signal component in the detection output signal is canceled, and the DC component is supplied to the low-pass filter including the resistor (17) and the capacitor (21). At this time, since there is no audio component, it is sufficient for the low-pass filter to remove the IF signal component, so that the value of the capacitor constituting the low-pass filter can be reduced.

FIGS. 4 and 5 are views showing another example of the removing means according to the present invention. In FIG. 4, only the audio component of the detection output signal of the AM detection circuit (5) is detected by the detection circuit (9). On the other hand, AM detection circuit (5)
The detection output signal generated from the other output terminal is applied to an inversion circuit (10) and inverted. The audio component detected by the detection circuit (9) and the output signal of the inversion circuit (10) are added in the addition circuit (11) to cancel the audio signal component in the output signal of the inversion circuit (10). be able to.

In FIG. 5, after an audio signal component in a detection output signal of an AM detection circuit (5) is detected by a detection circuit (9), another output of the AM detection circuit (5) is output by a subtraction circuit (25). It is subtracted from the detection output signal generated from the end. Therefore, it is possible to cancel the audio component in the detection output signal.

[0015]

Thus, according to the present invention, an audio signal component is detected from a detection output signal, and the audio signal in the output signal of the detection circuit is canceled by using the audio signal component. The cutoff frequency of the low-pass filter to which the low-pass filter is applied can be made several tens to several hundred times the conventional value, and the capacitance of the capacitor constituting the low-pass filter can be reduced. Therefore, it is suitable for downsizing the set.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional example.

FIG. 3 is a circuit diagram showing a main part of one embodiment of the present invention.

FIG. 4 is a block diagram showing another example of the removing means according to the present invention.

FIG. 5 is a block diagram showing still another example of the removing means according to the present invention.

[Explanation of symbols]

 9 detection circuit 10 inversion circuit 11 addition circuit 25 subtraction circuit

Claims (4)

(57) [Claims] 1. An AM radio receiver comprising: an AM detection circuit for detecting an IF signal; a low-pass filter; and an AGC circuit to which an output signal of the low-pass filter is applied. A detection circuit for detecting the component; and a means for mixing an output signal of the detection circuit and a detection output of the detection circuit to remove an audio signal component in the detection output. An AM radio receiver supplied to a low-pass filter. 2. The apparatus according to claim 1, wherein said removing means comprises an inverting circuit for inverting a detection output of said detection circuit, and an adding circuit for adding a detection output of said detection circuit and an output signal of said inversion circuit. Item 3. An AM radio receiver according to Item 1. 3. The apparatus according to claim 1, wherein said removing means comprises an inverting circuit for inverting a detection output of said detection circuit, and an adding circuit for adding output signals of said detection circuit and said detection circuit. AM radio receiver. 4. The AM radio receiver according to claim 1, wherein said removing means comprises a subtraction circuit for subtracting a detection output of said detection circuit and an output signal of said detection circuit.