JPH0423441B2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention is directed to an AGC
This invention relates to RF AGC circuits for performing automatic gain control (automatic gain control), and in particular to RF AGC circuits that have a remarkable effect on beat interference.

(b) Conventional technology AM radio receivers are
As shown in Publication No. 88318, an RF AGC circuit is provided so that broadcasts of weak electric fields to broadcasts of strong electric fields can be received without distortion. The RF AGC circuit is an RF
The level of the output signal of the amplifier circuit is detected and the RF
As mentioned above, the level of the output signal of the amplifier circuit is constant.
It controls the gain of the RF amplifier circuit.

By the way, an AM radio receiver equipped with the RF AGC circuit is shown as shown in FIG. In Figure 2, the RF signal received by antenna 1 is RF
AGC amplified by amplifier circuit 2 and RF AGC circuit 3,
The mixing circuit 4 mixes the signal with the output signal of the local oscillation circuit 5, and converts the frequency into an IF signal. The IF signal having a frequency of 450 KHz is amplified by the IF amplifier circuit 6, then detected by the detection circuit 7, and is transmitted to the subsequent stage as a low frequency signal. Furthermore, in the second circuit,
Description regarding AGC for the IF amplifier circuit is omitted because it is not directly related to the present invention.

Using the AM radio receiver shown in Figure 2, there is no problem in normal reception, and broadcasts from weak electric fields to strong electric fields can be received without distortion.

(c) Problems to be solved by the invention However, the AM radio receiver shown in Fig. 2 had a problem in that beat interference occurred when receiving broadcasts from a station on a particular frequency. For example, if a broadcast with a strong electric field of 900 KHz is received by the antenna 1 in FIG. 2, the output oscillation frequency of the local oscillation circuit 5 will be 1350 KHz, and the output signal (IF signal) frequency of the mixing circuit 4 will be 450 KHz.
On the other hand, when a large signal of 900KHz is applied to the tuning varactor diode in the antenna circuit or RF amplifier circuit 2, the frequency is 1800KHz (twice the frequency of the RF signal).
harmonics are generated, and this harmonic and local oscillation circuit 5
is mixed with the output signal of 450 (1800-1450) KHz
A pseudo IF signal is generated. Therefore, a beat occurs between the IF signal and the pseudo IF signal, and the distortion rate worsens.

In addition, between broadcasting stations whose frequencies are doubled, such as traffic information broadcasting (1620KHz) and FEN (Far East) method (810KHz), if the electric field strength of the 810KHz broadcast is large, the antenna circuit and RF Due to the selective characteristics of the amplifier circuit, the broadcast signal cannot be sufficiently attenuated, and a relatively large signal is applied to the varactor diode. Therefore, double harmonics (1620 KHz) are generated from the varactor diode, causing interference when receiving traffic information broadcasts.

To prevent beat disturbance and interference as described above, set the operation start level of the RF AGC circuit 2 low and use a varactor.
It is removed by lowering the voltage across the diode. However, lowering the operation start level of the RF AGC circuit 2 causes new problems such as deterioration of the SN ratio in the medium electric field for broadcasting without beat interference and deterioration of the desensitization characteristic in the two-signal characteristic. The operation start level cannot be lowered.

(d) Means for solving the problems The present invention has been made in view of the above points.
A first AGC circuit that generates an AGC signal when the output signal of the RF amplifier circuit reaches a first level, and a second AGC circuit that generates an AGC signal when the output signal reaches a second level that is lower than the first level. and,
a selection circuit that selects one of the output signals of the first and second AGC circuits and supplies it to the RF amplifier circuit;
The present invention is characterized in that it includes a comparison circuit that compares preset data with data corresponding to the frequency of a received signal and applies a control signal to the selection circuit.

(E) Effect According to the present invention, when normal broadcasting is received, since the preset data and the data corresponding to the frequency of the received signal do not match, the first
RF AGC can be performed using the output signal of the first AGC circuit that starts operation at the first level, and when a broadcast of a special frequency is received, the two data match, so the RF AGC can be performed using the output signal of the second AGC circuit that starts operating at level 2.

(F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention.
8 is an antenna for receiving broadcasting, 9 is an RF amplification circuit for amplifying the RF signal received by the antenna 8, and 10 is for mixing the output signal of the RF amplification circuit 9 with the output signal of the local oscillation circuit 11. a mixing circuit 12 that generates an IF signal; 12 amplifies the IF signal;
IF amplifier circuit, 13 is an AM detection circuit for AM detecting the output signal of the IF amplifier circuit 12, and 14 is the RF
a first AGC circuit that generates a first AGC signal when the output signal of the amplifier circuit reaches a first predetermined level _V1 ;
15, the output signal of the RF amplifier circuit 9 is the first
a second AGC circuit that generates a second AGC signal when the second predetermined level _V2 , which is lower than the predetermined level of circuit, 17
is a counter connected to the RF amplifier circuit 9 and counts the frequency of the RF signal, 18 is a memory in which predetermined data is set in advance, and 19 is a counter that stores data corresponding to the counted value of the counter 17 and the memory 18.
This is a comparison circuit that compares the data set in the selection circuit 16 and applies a first control signal to the selection circuit 16 when there is a mismatch, and applies a second control signal when there is a match.

Next, an explanation will be given of the operation for removing beat interference of broadcasting having a frequency of 900KHz. Prior to reception, data corresponding to 900KHz is set in the memory 18.
However, if a broadcast having a frequency other than 900KHz is received by the tuning operation, the reception
The RF signal is amplified by an RF amplifier circuit 9 and then sent to a mixing circuit 10.
is applied to At that time, the frequency of the RF signal is counted by a counter 17 and compared with data set in a memory 18 in a comparison circuit 19. In that case, since the frequency of the received signal is other than 900 KHz, the first control signal indicating a mismatch is generated from the comparison circuit 19, and the selection circuit 16 is sent to the first AGC circuit 1.
The first AGC signal from No. 4 is controlled to be selected. Therefore, the electric field strength of the received signal is large, and the output signal level of the RF amplifier circuit 9 is at the first predetermined level.
When V is ₁ or more, the first AGC circuit 14
1AGC signal is generated, and the gain of the RF amplifier circuit 9 is controlled by the first AGC signal. As a result, an RF signal of a substantially constant level is applied to the mixing circuit 10, and reception with good distortion and signal-to-noise ratio can be achieved.

Next, if a broadcast having a frequency of 900KHz is received, the frequency of the received signal is
The data is counted and compared with the data set in the memory 18. Since the data set in the memory 18 corresponds to 900KHz, a second control signal is generated from the comparison circuit 19 when there is a match.
In response, the selection circuit 16 is switched to select the second AGC signal from the second AGC circuit 15.
Therefore, the output signal level of the RF amplifier circuit 9 is
As soon as the predetermined level is exceeded, the second AGC circuit 1
A second AGC signal is generated from 5, and the gain of the RF amplifier circuit 9 is controlled by the second AGC signal. As a result, AGC is applied to the 900KHz received signal from a time when the level is relatively low, and the voltage applied to the varactor diode in the RF amplifier circuit 9 becomes small, causing beat interference. Removal is planned. In that case, although the SN ratio due to noise other than the beat noise deteriorates, the SN ratio is sufficiently improved because the beat noise is sufficiently removed.

Figure 3 shows the degree of improvement in the SN ratio when using the circuit shown in Figure 1. When normal broadcasting other than 900KHz is received, the antenna input is V _A1 (the output signal level of the RF amplifier circuit 9 is V ₁ ), when the
If RF AGC is performed using the 1AGC circuit 14, the noise output will be as shown by the dotted line A in FIG. Also, when receiving a 900KHz broadcast with beat noise shown by the dashed line (b) in Figure 3, the antenna input is V _A2.
(When the output signal level of the RF amplifier circuit 9 reaches V ₂ ), the second AGC circuit 15 is operated and the RF AGC
By doing this, it is possible to suppress beat noise as shown by the two-dot chain line C. Therefore, by using the present invention, the ratio of noise to signal (solid line d), that is, the SN ratio, can be sufficiently improved.

The same applies to the case of receiving signals from the traffic information bureau, and data corresponding to 1620 KHz may be set in the memory 18 shown in FIG. Then, when the frequency of the RF signal reaches 1620KHz, the comparator circuit 19
Since the output signal from the second AGC circuit 15 is selected, RF AGC starts from a low input level and is applied to the varactor diode.
Since the level of the 810KHz FEN signal is suppressed by the RF AGC, interference can be prevented.

In the embodiment shown in FIG. 1, a counter 17 is connected to the output terminal of the RF amplifier circuit 9 in order to detect the frequency of the received signal. 17 may be connected. In that case, data corresponding to the value obtained by adding the IF frequency to the frequency of the received signal may be set in advance in the memory 18. Also, the first
In the illustrated embodiment, in the comparator circuit 19,
Although it has been explained that the data in the memory 18 and the data corresponding to the count value of the counter are compared, the comparison of the data may be performed digitally, or the comparison may be performed using DC voltage after D-A conversion. . In addition, the present invention can be used with various modifications.

(G) Effects of the Invention As described above, according to the present invention, it is possible to provide an RF AGC circuit that can reliably eliminate beat interference and interference. At that time, RF AGC is performed by distinguishing between broadcasts that cause beat interference and other broadcasts, so RF AGC is performed that does not adversely affect the other broadcasts.
We can provide AGC circuit.

[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 RF AGC circuit, and FIG. 3 is a characteristic diagram showing the noise characteristics of FIG. 9...RF amplifier circuit, 14...1st AGC circuit,
15...Second AGC circuit, 16...Selection circuit, 17
... Counter, 18 ... Memory, 19 ... Comparison circuit.

Claims (1)

[Claims] 1. An RF AGC circuit for keeping the output signal level of the RF amplifier circuit substantially constant, the first AGC circuit generating an AGC signal when the output signal of the RF amplifier circuit reaches a first level, and the RF amplifier. A second AGC circuit that generates an AGC signal when the output signal of the circuit reaches a second level lower than the first level, and one of the output signals of the first and second AGC circuits is selected and sent to the RF amplifier circuit. It consists of a selection circuit to supply, and a comparison circuit that compares preset data with data corresponding to the frequency of the received signal and applies a control signal to the selection circuit. During normal reception, the output AGC of the first AGC circuit RF AGC is performed using the signal, and when a signal with a frequency corresponding to preset data is received, the selection circuit is controlled, and the output AGC of the second AGC circuit is
An RF AGC circuit characterized by performing RF AGC using signals.