JPS6046887B2 - AGC circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an AGC (automatic gain control) circuit for automatically controlling the gain of an amplifier according to the magnitude of an input signal, thereby keeping the magnitude of the output signal constant. The purpose of this improvement is particularly to improve characteristics when a strong input signal is present and when an interfering signal is present.

Conventionally, a radio receiver having an M°C circuit as shown in FIG. 1 has been known.

In FIG. 1, an input signal applied to an input terminal 1 is amplified by a radio frequency amplification circuit 2, converted to an intermediate frequency signal by a mixing circuit 3, amplified by an intermediate frequency amplification circuit 4, and then amplified by a detection circuit 5. The signal is detected and output to the output terminal 6. A part of the detection output signal obtained at the output terminal of the detection circuit 5 is applied to the M°C signal generation circuit 10 via the low-pass filter 1 consisting of a resistor 7 and a capacitor 8,
An M°C signal is obtained at the output terminal of the M°C signal generating circuit 10. The M° signal is used on the one hand to control the gain of the intermediate frequency amplification circuit 4 and on the other hand to control the gain of the radio frequency amplification circuit 2. In normal cases, a M°C circuit as shown in FIG. 1 can perform gain control to some extent. However, when there is strong input detuning or when a strong input signal is present in an adjacent station, the M℃ circuit does not operate and gain control cannot be performed, causing the radio frequency amplification circuit and mixing circuit to become saturated, causing the output to decrease. It has the disadvantage that large distortion and noise are generated at the terminal. The present invention has been made in view of the above points, and will be described below based on embodiments with reference to the drawings.

FIG. 2 shows an embodiment of the present invention, in which 11 is an antenna input circuit, 12 is a radio frequency amplification circuit, 13 is a mixing circuit, 14 is an intermediate frequency amplification circuit, and 15 is a detection circuit, which are conventionally commonly used. This is the same as the radio receiver used, and the explanation will be omitted. Thus, the M°C circuit includes a first low-pass filter 1■ consisting of a resistor 16 and a capacitor 17 that pass low-frequency components in the output signal of the detection circuit 15, and an AG to which the output signal of the low-pass filter ±■ is applied.
a C signal generation circuit 19; a delay circuit 20 that delays the first AGC signal that is the output signal of the N°C signal generation circuit 19;
The output signal of the radio frequency amplification circuit 12 is detected and the second AG
an AGC detection circuit 21 that generates the C signal; a synthesis circuit 22 that synthesizes the first AGC signal that has passed through the delay circuit 20 and the 　V℃ signal; and a second low-pass that passes the output signal of the synthesis circuit 22. It consists of a filter 23 and an AG
The output signal of the C signal generation circuit 19 controls the gain of the intermediate frequency amplification circuit 14, and also controls the gain of the second low-pass filter 2.
The gain of the antenna circuit 11 and the radio frequency amplification circuit 12 is controlled by the output signal No. 3. Note that 24 is a correction circuit for linearly controlling the gain change of the radio frequency amplification circuit 12. Next, the operation will be explained.

If the radio receiver is now in a tuned state, the signal applied to the input terminal 25 will reach the output terminal 26 according to a predetermined operation, and the AGC circuit will also operate normally. At that time,
For the first AGC signal applied to the combining circuit 22, the second
Since the circuit design is such that the AGC signal is sufficiently small, the output composite signal of the composite circuit 22 is mainly composed of the first AGC signal, and gain control similar to that of the AGC circuit shown in FIG. 1 is performed. This is performed for the antenna circuit 11 and radio frequency amplification circuit 12. However, when the synchronization shifts,
The intermediate frequency signal is attenuated due to the filter characteristics of the intermediate frequency amplifier circuit 14, and the output signal from the detection circuit 15 becomes small.

Therefore, the antenna circuit 11 and the radio frequency amplification circuit 1
The gain control for 2 becomes shallower, and the radio frequency amplification circuit 1
The output signal of No. 2 becomes large. Therefore, the proportion of the 々V°C signal included in the output signal of the combining circuit 22 becomes large. Then, when the tuning is further detuned, the middle circumference. The wave signal is further attenuated, and the value of the first AGC signal becomes smaller. Therefore, the output signal of the synthesis circuit 22 also becomes small, and the output signal of the radio frequency amplification circuit 12 becomes even larger. However, as the output signal of the radio frequency amplification circuit 12 increases, the second AGC signal becomes larger. ,
The output signal of the radio frequency amplification circuit 12 does not exceed a certain value, the antenna circuit 11 and the radio frequency amplification circuit 12 are not saturated, and deterioration of the distortion factor is prevented. Further, when there is a strong signal in an adjacent station, a large interference signal enters the tuned station, but this signal has no relation to the first AGC signal, so the first AGC signal does not increase.

However, since the second AGC signal becomes large and the gains of the antenna circuit 11 and the radio frequency amplification circuit 12 are controlled thereby, the radio frequency amplification circuit 12 and the mixing circuit 13 are not saturated. At the time of tuning, the delay circuit 20 first controls the AG signal for the intermediate frequency l-wave amplification circuit 14.
AGC. It is intended to operate.

Because of the presence of the delay circuit 20, the AGC acts on the front stage from the latter stage, so that the S/N (signal-to-noise ratio) can be improved. Figure 3 shows the first and second
A characteristic diagram showing the relationship between the AGC signal and the composite AGC signal,
The first AGC signal indicated by the dotted line A in the figure is greatly attenuated within a predetermined range centered around the center frequency FO, and the second AGC signal indicated by the dashed line in the figure is greatly attenuated outside the predetermined range. It is decreasing in width.

Therefore, the composite AGC signal shown by the solid line C in the figure depends on the first AGC signal within the predetermined range, and on the second AGC signal outside the predetermined range, which enables effective gain control. can be done. FIG. 4 shows a specific example of the N°C circuit according to the present invention, in which a first input terminal 27 to which a detection output signal is applied, a low-pass filter u consisting of a resistor 28 and a capacitor 29, and a terminal 31 located in the middle An intermediate frequency amplification circuit control transistor 32 connected to the frequency amplification circuit 14, a delay circuit U including resistors 33 and 34 and a transistor 35 for dividing the output signal of the low-pass filter 12, and the output of the radio frequency amplification circuit 12. a second input terminal 37 to which a signal is applied;
An output circuit 1 consisting of a detection circuit 38 that detects a signal applied to the detection circuit 38, and a transistor 39 and a diode 40 that supplies the output signal of the detection circuit 38 to a point A serving as a combining circuit.
±, a low-pass filter consisting of a resistor 42 and a capacitor 43 that passes the composite AGC signal obtained at point A;
Antenna circuit 11 and/or radio frequency amplification circuit 12
and an output terminal 44 connected to.

The operations are the same as those in FIG. 2, and the explanation will be omitted.
In the embodiment shown in FIG. 4, the AGC signal having two components is combined at one point.

Therefore, it has the advantage that the AGC target circuit can be easily configured. Furthermore, since only one low-pass filter can be used for the first and second V°C signals, the circuit configuration can be simplified. According to the present invention, AG with good characteristics
Since a C circuit can be provided, effects such as improvement of distortion rate and prevention of coloring due to abnormal phenomena due to non-linearity can be achieved. In particular, this has been inconvenient in the past. It is possible to prevent saturation of the radio frequency amplification circuit and mixing circuit due to interference signals caused by strong input detuning and adjacent strong signals. As described above, the present invention can provide an N°C circuit having excellent advantages not found in the prior art.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional AGC circuit, FIG. 2 is a block diagram showing an embodiment of the AGC circuit according to the present invention,
FIG. 3 is a characteristic diagram for explaining the present invention, and FIG. 4 is a circuit diagram showing a specific example of the N°C circuit according to the present invention.

Claims (1)

[Claims] 1. A detection circuit that detects an intermediate frequency signal, a first low-pass filter that passes the output signal of the detection circuit, an AGC signal generation circuit to which the signal that has passed through the first low-pass filter is applied, and an output signal of the AGC signal generation circuit. a synthesis circuit that synthesizes a first AGC signal obtained by delaying the first AGC signal and a second AGC signal obtained by detecting the output signal of the radio frequency amplification circuit; and a second low-pass through which the output signal of the synthesis circuit is passed. a filter, the output signal of the AGC signal generation circuit is applied to the intermediate frequency amplification circuit to perform first AGC, and the signal passing through the second low-pass filter is applied to the radio frequency amplification circuit and the radio frequency An AGC circuit characterized in that a second AGC is performed by applying voltage to at least one of the circuits preceding the amplifier circuit.