JPS63260308A - Radio receiver - Google Patents

Abstract

PURPOSE:To reduce the noise at the time of no signal and a weak electric field of a prescribed level or below by decreasing the gain of a 2nd intermediate frequency IF amplifier circuit when the reception signal reaches the prescribed level or below. CONSTITUTION:When the electric field strength is lower than a level (y), a 1st automatic gain control AGC circuit 13 is inactive and a 1st IF amplifier circuit 9 applies amplification by a prescribed gain. On the other hand, an output signal of a 2nd smoothing circuit 15 is impressed to a gain reduction circuit 16 and in this case the gain reduction circuit 16 reduces the gain of the 2nd IF amplifier circuit 10 in response to the output signal. Thus, the total gain to the IF signal from a mixer circuit 4 is decreased thereby reducing noise.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention relates to a radio receiver having an IF (intermediate frequency) AGC (automatic gain control) circuit, and is designed to reduce noise particularly in the case of a weak electric field. related to radio receivers.

(b) Conventional technology AM radio receivers use RF (radio frequency) and I
A FAGC circuit is provided.

An AM radio receiver including the RF and I FAGC circuits is shown in FIG. In Figure 2, the RF multiplied signal received by the antenna (1), the RF amplification circuit (2)
The signal is amplified by the mixing circuit (4), mixed with the output signal of the local oscillation circuit (5), and subjected to IF times signal frequency conversion. 45
The IF multiplied signal having a frequency of 0 KHz is amplified by the IF amplifier circuit (6), then detected by the detection circuit (8), and transmitted to the subsequent stage as a low frequency signal. At that time, depending on the electric field strength of the received signal, the RFAGC circuit (3) and the IFAG circuit
The C circuit (7) operates to perform gain control.

The AM radio receiver as shown in Figure 2 was installed on March 20, 1985.
"'85 Sanyo Semiconductor Handbook Monolithic Bibolar Integrated Circuits" edition No. 82, published by CQ Publishing on the date
It is written on the page.

(c) Problems to be solved by the invention By the way, in recent AM radio receivers, AGCF
A characteristic called OM is considered important. AGCFOM
indicates the width between the standard input of the antenna input of 74 dBμ and the input level at which an output that is 10 dB lower than the output level at the standard input is obtained, and the wider the width, the lower the output when receiving a weak electric field. The signal level can be kept close to the AGC level, and a stable output signal can be obtained. In order to widen the width of the ACCFOM, the total gain of the tuner may be set high. However, if this is done, there is a problem that noise when there is no signal and noise when the signal is detuned increases.

Therefore, in general, a compromise between the noise level and the width of the AGCFOM is found and its characteristics are determined. The situation is shown in Figure 3. In FIG. 3, the horizontal axis indicates the input signal level to the antenna, and the vertical axis indicates the output signal level. A solid line a in FIG. 3 shows input/output characteristics commonly used in the past. As is clear from the figure, in solid line a, AGCF
The width of the OM is A, which is relatively narrow, and a stable output signal cannot be obtained in a weak electric field. Therefore, if the total gain of the tuner is set high, its characteristics will become as shown by the solid line in FIG. 3, and the output signal level can be maintained near the AGC level even in a weak electric field.

By the way, in FIG. 3, the dashed dots 1aa and b indicate the noise levels corresponding to the output signals of the solid lines a and b, respectively.If the characteristics are as shown in the solid line b, the noise level becomes as shown by the dashed dot line. However, there is a drawback that the noise level increases when there is a weak electric field or no signal.

(d) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and includes an 11th F amplifier circuit for amplifying the output IF signal of the mixing circuit;
a second IF amplifier circuit that further amplifies the output IF multiplied signal of the amplifier circuit; a detection circuit that detects the output IF multiplied signal of the second IF amplifier circuit; a first smoothing circuit that smoothes the output signal of the detection circuit; The first IF according to the output signal of the smoothing circuit.
A radio receiver including an AGC circuit that controls the gain of an amplifier circuit, a received signal level detection circuit that detects the level of a received signal, a second smoothing circuit that smoothes an output signal of the received signal level detection circuit, and a second smoothing circuit that smoothes an output signal of the received signal level detection circuit. The present invention is characterized by further comprising a gain reduction circuit that reduces the gain of the second IF amplifier circuit in accordance with the output signal of the second smoothing circuit.

(E) Effect According to the present invention, when the received signal falls below a predetermined level, the second
Since the gain of the IF amplifier circuit is lowered, it is possible to reduce noise in a weak electric field below the predetermined level and when there is no signal.

(v) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (9) is a first IF amplification circuit that amplifies the IF multiplied signal from the mixing circuit (4), and (10) is a first IF amplification circuit. A 21st F amplifier circuit further amplifies the IF multiplied signal amplified by the circuit (9), (11) a detection circuit that detects the output IF multiplied signal of the second IF amplification circuit (10), and (12) the detection circuit (11). (13) is a first smoothing circuit that smoothes the output signal of the first smoothing circuit (13).
12) according to the output signal of the first IF amplifier circuit (9).
(14) is a received signal level detection circuit that detects the IF multiplied signal level from the mixing circuit (4); (15) is the received signal level detection circuit (
a second smoothing circuit for smoothing the output signal of (14); and (16)
) is a gain reduction circuit that reduces the gain of the second IF amplifier circuit (10) according to the output signal of the second smoothing circuit (15). In FIG. 1, the circuit configuration before the mixing circuit (4) is the same as that in FIG. 2, and will therefore be omitted, and circuit elements that are the same as those in FIG. Incidentally, the total gain in FIG. 1 is set high as in the case of the solid line in FIG. 3.

Next, the operation will be explained. IF from mixed circuit (4)
Double signal, first and second IF amplifier circuits (9) and (10)
After being amplified, the signal is detected by a detection circuit (11) and output to an output terminal (17). On the other hand, I from the mixing circuit (4)
The F-fold signal is applied to the received signal level detection circuit (14), and its level is detected. Then, the output signal corresponding to the level is smoothed by a second smoothing circuit (15), and the signal meter (31) is driven by the output signal. In this case, if the input signal level is higher than that at point X in FIG.
) is applied to Then, the first AGC circuit (13)
The first IF amplifier circuit (9) responds to the smoothed output signal.
) controls the gain. On the other hand, the output signal of the second smoothing circuit (15) can be applied to the gain reduction circuit (16). Then,
The gain reduction circuit (16) becomes inactive, and the 21F amplifier circuit (10) performs amplification with a predetermined gain.

Therefore, the circuit of FIG. 1 can perform the same operation as that of FIG. 2.

Next, a case where the electric field strength becomes lower than point X in FIG. 3 will be explained. In this case as well, the output signal of the detection circuit (11) is smoothed by the first smoothing circuit (12) and then applied to the first AGC circuit (13). However, the smoothing circuit (12
) since the level of the output voltage is low, the first AGC circuit (13)
does not operate, and the first IF amplifier circuit (9) performs amplification with a predetermined gain. On the other hand, although the output signal of the second smoothing circuit (15) is completely applied to the gain reduction circuit (16), the gain reduction circuit (
16) is inoperative as in the above case, and the 21F amplifier circuit (10) performs amplification with a predetermined gain.

Therefore, the IF multiplied signal from the mixing circuit (4) can be sufficiently amplified, and the AGCFOM can be expanded.

Further, a case where the electric field strength decreases from point y in FIG. 3 will be explained. In this case as well, the first AGC circuit (13) is inoperative and the 11F amplifier circuit (9) performs amplification with a predetermined gain, as in the case described above. On the other hand, the second smoothing circuit (15
) is applied to the gain reduction circuit (16),
At this time, the gain reduction circuit (16) reduces the gain of the second IF amplifier circuit (10) according to the output signal.

Therefore, the total gain for the IF multiplied signal from the mixing circuit (4) is reduced, thereby making it possible to reduce noise. The above-mentioned characteristics are shown as solid lines in FIG. 3C. That is, in the electric field strength from point X to point y, the IF amplification circuit performs amplification with a predetermined gain, so A
A for GCFOM.

When the electric field strength becomes below point y, the gain of the IF amplifier circuit is reduced, so that the noise can be reduced as shown by the dashed line C.

FIG. 4 is a circuit diagram showing a specific example of the circuit shown in FIG.
IF multiplied signal from F amplifier circuit (9), differential type 21st F
It is amplified by the amplifier circuit (10) and sent from the collector of the transistor (18) to the detection circuit (
11) is applied and detected. Here, the output signal is smoothed by a first smoothing circuit (ear) consisting of a resistor (20) and a capacitor (21), and the output signal is applied to the first AGC circuit (13). Then, the first AGC circuit (13
) controls the gain of the 11th F amplifier circuit (9) according to the output signal. In this state, the IF multiplied signal generated from the mixing circuit (4) and the received signal level detection circuit (14)
is applied and its level is detected. Then, the output signal corresponding to the level is transmitted to the resistor (22) and the capacitor (
The voltage is smoothed by a second smoothing circuit (23), which is applied to the base of the transistor (24) in the gain reduction circuit (16), and compared with the reference voltage of the reference power supply (25). Now, assuming that the electric field strength is greater than point y in Figure 3,
The output voltage of the second smoothing circuit (long) becomes higher than the reference voltage, and the transistor (26) is turned off. For that reason,
The transistor (28) is also turned off, and the second IF amplifier circuit (10) performs amplification with a predetermined gain.

Moreover, when the electric field strength becomes smaller than point y in FIG. The current flows through the diode (27) and the same current flows through the transistor (28). Then, the Q of the resonator (Ki) in the second IF amplifier circuit (1o) decreases, and its gain decreases.

Therefore, by appropriately setting the reference voltage of the reference power source (25), it is possible to reduce noise and widen the AGCFOM, thereby obtaining the characteristics shown in FIG. 3C.

(G) Effects of the Invention As described above, according to the present invention, when the received signal falls below a predetermined level, the gain of the 21F amplifier circuit is reduced, so that when a weak electric field is below the predetermined level and there is no signal, It is possible to measure noise reduction. Further, according to the present invention, when a signal of the predetermined level or higher is being received, amplification is performed using the gain of the second IF amplification circuit itself, so that the IF signal can be sufficiently amplified, and the AGCFO
It is possible to measure the expansion of M. Furthermore, as in the embodiment, the S meter output does not change even if the gain reduction circuit operates, so it does not affect the reception sensitivity during automatic tuning and has no effect on practical sensitivity.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional radio receiver, FIG. 3 is a characteristic diagram for explaining the present invention, and FIG. 4 is a circuit diagram showing a conventional radio receiver. FIG. 2 is a circuit diagram showing a specific example of the hitting path shown in FIG. 1; (9)...First IF amplifier circuit, (10)...Second
IF amplifier circuit, (11)...detection circuit, (12)
...first smoothing circuit, (13) ...first AGC circuit, (14) ...received signal level detection circuit, (15
)...second smoothing circuit, (16)...gain reduction circuit.

Claims (1)

[Claims] (1) A first IF amplifier circuit that amplifies the output IF signal of the mixing circuit, a second IF amplifier circuit that further amplifies the output IF signal of the first IF amplifier circuit, and an output I of the second IF amplifier circuit.
A detection circuit that detects the F signal, a first smoothing circuit that smoothes the output signal of the detection circuit, and an AGC circuit that controls the gain of the first IF amplifier circuit according to the output signal of the first smoothing circuit. In a radio receiver, a received signal level detection circuit detects the level of a received signal, a second smoothing circuit smoothes an output signal of the received signal level detection circuit, and a second smoothing circuit that smoothes an output signal of the second smoothing circuit. A radio receiver comprising a gain reduction circuit that reduces the gain of a 2IF amplifier circuit.