JPS62294316A - Am radio receiver - Google Patents

Abstract

PURPOSE:To attain the excellent reception of a station in a weak electric field by setting the sensitivity of a broad band RF AGC circuit higher in the automatic searching and lower at the normal reception. CONSTITUTION:In setting an AM radio reciever in a automatic searching state, a control signal is generated from an automatic search circuit to turn on a switch circuit 9 in the lst state. Then an RF signal generated at a connecting point between a turning coil 4 a load resistor 6 is supplied to the 1st amplifier circuit 10 via the switch circuit 9, the signal is amlified with a gain G1 and the result is fed to an AGC drive circuit 12. Thus, the sensitivity of the broad band RF AGC circuit is increased thereby sufficiently suppressing an undersired signal generated by inter-modulation, and preventing mis-stop of the automatic search at an undesired signal frequency. In receiving a desired station by the automatic search, a control signal from an automatic search circuit is lost and the switch circuit 9 is turned on to the 2nd state.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (a) Industrial Application Field The present invention has an auto search function and a broadband R
The present invention relates to an improvement of an AM radio receiver equipped with an AGC circuit, and particularly relates to an AM radio receiver that prevents erroneous stoppage during auto-search operation.

(b) Prior Art As disclosed in Japanese Unexamined Utility Model Publication No. 58-116314, an AM radio receiver equipped with a wideband RF AGC circuit is known. The AM radio receiver, as shown in FIG.
E T (2), an A G C+- transistor (3) cascade-connected to the drain of the FET (2),
R including a tuning coil (4) and a variable capacitance diode connected to the mini-2 rector of the AGCl-transistor (3).
An AGC drive circuit (5) that generates an AGC signal for gain control using an amplified RF multiplied signal obtained at the connection point between the F tuning circuit (5) and the tuning coil (4) and the load resistor (6). 7).However, when the RF multiplied signal electric field strength received by the antenna (1) is small, the output AGCt pressure of the AGC drive circuit (7) becomes high, and the AGC transistor (3) ) increases, and the conductivity of R
When the F-fold signal electric field strength is dog, the AGC drive circuit (7
) becomes low, the antenna damping circuit turns on and limits the input of FET (2), and the conductivity of AGC transistor (3) decreases, making it difficult to perform stable AGC operation. I can do it. Also,
As a signal indicating the RF multiplied signal electric field strength, the tuning coil (
4) and the load resistor (6), which is not subjected to band limitation, is used, so the AGC operation is performed over a wide band.

By the way, in order to receive a signal from a weak electric field station located near a strong electric field station, it is necessary to reduce the sensitivity of the wideband RF AGC circuit. If the sensitivity of the broadband RF AGC circuit is allowed to decrease and the start level of AGC operation is delayed until just before interference occurs, the signal from the weak electric field station can be clearly received. The sensitivity of the broadband RFAGC circuit is kept low to some extent.

(C) Problems to be Solved by the Invention However, when the sensitivity of the wideband RF AGC circuit is reduced, another problem arises: the intermodulation characteristics deteriorate. That is, FET (2) connected to antenna (1)
) tends to generate harmonics twice the input signal frequency, and the variable capacitance diode used in the RF tuning circuit (5) has a frequency mixing effect, so two signals with different frequencies are transmitted to the antenna (1). ), the signal having the sum and difference frequencies of each frequency or two of the input signal frequencies
A signal having a frequency that is the sum and difference of the double frequency and each frequency is generated. For example, if signals from two stations with frequencies fl and f are received by antenna (1), then rI+
rx, t, -r,, 2 f'l+ r*, 2fIf's
Signals having various frequencies are generated.

For example, when a signal 2 f', -f enters the reception band, if an AM radio receiver with a low-sensitivity wideband RF AGC circuit performs an auto search, the signal 2 f', -f, which originally does not have a station, will be detected. f'! This causes a malfunction in which the auto search stops at this frequency.

(d) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and in order to prevent erroneous termination of auto search, a wideband RF AGC circuit is installed in response to a signal notifying the start of auto search. It is characterized by providing means for increasing the sensitivity of the sensor.

(*) Effect According to the present invention, wideband RF during normal reception
It is possible to keep the sensitivity of the AGC circuit low and receive weak electric field stations well. Also, during auto search, the sensitivity of the wideband RFAGC circuit is set to -4'C! This prevents automatic search from stopping erroneously.

(f) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention, and (8) is an auto search signal that generates an auto search signal that varies the capacity of the variable mixer diode of the RF tuning circuit (5) during auto search. an auto search circuit (9) which generates a control signal indicating that the AM radio receiver is in the auto search operation;
) is switched according to the control signal from the auto search circuit (8), and the amplified RF multiplied signal obtained at the connection point between the tuning coil (4) and the load resistor (6) is switched to the first or second amplified RF signal.
The switch circuit generated at the output terminal, (lO) is the first RF multiplied signal generated at the first output terminal of the switch circuit (9).
A first amplifier circuit (11) amplifies the RF multiplied signal generated at the second output terminal of the switch circuit (9) with a second gain G2 that is smaller than the first gain. an amplifier circuit, and (12) an AG that applies an AGC signal according to the output signal of the first or second amplifier circuit (10) or (11) to the base of the AGC transistor (3);
This is a C drive circuit. In FIG. 1, circuit elements that are the same as those in FIG. 2 are designated by the same reference numerals, and explanations thereof will be omitted.

Now, if we put the AM radio receiver into the auto search state, a control signal is generated from the auto search circuit (8),
Switch the switch circuit (9) to the first state. Then,
The RF multiplied signal generated at the connection point between the tuning coil (4) and the load resistor (6) is applied to the first amplifier circuit (10) via the switch circuit (9), and is amplified with a gain of G. A
It is applied to the GC drive circuit (12). Therefore, wideband R
The sensitivity of the FAGC circuit increases, unnecessary signals generated by intermodulation are sufficiently suppressed, and erroneous termination of auto search at the unnecessary signal frequency is prevented. When the desired station is received by auto search, the control signal from the auto search circuit (8) disappears and the switch circuit (9
) switches to the second state. In that case, the control signal may be set in the auto-search circuit (8), for example, by providing a flip-flop circuit that is set in response to the suppression of the auto-search button (4'' shown in the figure) and reset in response to the stop signal. can be easily avoided from occurring.

In addition, in the normal reception state, there is no control signal indicating the 1/2 state, so the switch circuit (9
) maintains the second state, and the RF multiplied signal obtained at the connection point between the tuning coil (4) and the load resistor (6) is applied to the second amplifier circuit (11) via the switch circuit (9). , gain G, and then applied to the AGC drive circuit (12). Therefore, the sensitivity of the wideband RF AGC circuit decreases,
Good reception of weak electric field stations near strong electric field stations is 111
come.

Figure 3 shows the switch circuit (9) of Figure 1, the first and second
This shows a specific circuit example of the amplifier circuits (10) and (11). When the control signal 1H is applied to the control terminal (13) without indicating that auto-search is in progress, the first transistor (14) is turned on. The second transistor (15) is turned off. For that reason,
The gain of the differential amplifier circuit (↓9) is a value G determined according to the value of the first resistor (17), and the RF multiplied signal applied to the input terminal (18), the differential amplifier circuit (effective ) is amplified with a gain G1 and applied to the AGC drive circuit (12) in FIG. 1 from the emitter of the output l transistor (19). In addition, in the normal reception state, the control signal r H,
is not applied, the eleventh transistor (14) is turned off and the second transistor (15) is turned on, and the gain of the differential amplifier circuit is equal to the first and second resistors (17) and (2).
0) is determined by the value G, (where G>(,). Therefore, the RF multiplied signal applied to the input terminal (18) is amplified by the differential amplifier circuit (gain G at V), From the emitter of the output transistor (19) to the AGC drive circuit (1
2) is applied. Therefore, the value of the first resistor (17) is F
The ET (2) and the RF tuning circuit (5) are set so that no intermodulation occurs, and the first and second resistors (17) and (20
) is set so that cross-modulation does not occur in the FET (2), it is possible to prevent erroneous termination of auto search and achieve good reception of weak electric field stations.

The fourth example is a circuit diagram showing another embodiment of the present invention, in which the load resistance is divided into a first and second load resistance (21> and (22)), and the first and second load resistance ( A control transistor (23) is provided that connects the connection point of 21) and (22) to ground in an alternating current manner, and a control signal that becomes rH during auto search is applied to the base of the control transistor (23). In the case of Fig. 4, the control transistor (23) is turned off during auto search, so the load resistance value is the first
and the series resistance value of the second load resistors (21) and (22), and the RF multiplied signal level applied to the AGC drive circuit (24) becomes a dog, making it possible to increase the sensitivity of the wideband RF AGC circuit. Also, during normal reception, the control transistor (23) is turned on, and the first and second load resistors (23) are turned on.
Since the connection point of 1) and (22) is AC grounded and only the second load resistor (22) acts as a load resistor, the sensitivity of the broadband RF AGC circuit can be reduced.

(g) Effects of the Invention As described above, according to the present invention, the sensitivity of the wideband RFAGC circuit can be set high during auto-search operation and low during normal reception, thereby reducing intermodulation and cross-modulation. This makes it possible to prevent the erroneous stop of the O1 search and achieve good reception of weak electric field stations.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing an AM radio receiver having a conventional wideband RF AGC circuit, and FIG. 3 is a specific example of the circuit shown in FIG. The circuit diagram and FIG. 4 are circuit diagrams showing another embodiment of the present invention. (2)...FET, (3)...AGCl~Randisk, (8)...auto search circuit, (9)...
Switch circuit, (10)...first amplifier circuit, (11
)...Second amplifier circuit, (12)...AGC drive circuit. Applicant Tamayo Denki Co., Ltd. and one other agent Patent attorney Higashi Nishino and one other person No. 3rd rs Figure 4

Claims (1)

[Claims] (1) In an AM radio receiver capable of performing an auto-search operation and equipped with a wideband RFAGC circuit, means is provided for increasing the sensitivity of the wideband RFAGC circuit in response to a signal for performing the auto-search operation. An AM radio receiver characterized by: