JP3071138B2 - 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 AM radio receiver capable of tuning reception or non-tuning reception.

[0002]

2. Description of the Related Art Conventionally, an RF tuning circuit for receiving an RF signal in a narrow band using a tank circuit comprising a varicap diode and a coil in an RF tuning stage, and an RF signal in a wide band without using a tank circuit. There has been a radio receiver in which an RF non-tuning circuit is provided, and two circuits switch output signals as appropriate according to an operation state such as a reception frequency or a search. A radio receiver including an RF tuning circuit composed of such two circuits is known as shown in FIG.

In FIG. 2, after a received RF signal is amplified by an RF amplifier circuit (1), an RF tuning circuit (2) and an RF
Applied to the non-tuning circuit (3). In the RF tuning circuit (2), a received RF signal is tuned at a resonance frequency of a tank circuit including a coil (2a), a varicap diode (2b) and a diode (2c), and only a signal having a reception frequency near the resonance frequency is used. Is the subsequent switching circuit (4)
Is applied to Further, since the RF non-tuning circuit (3) does not have a tank circuit, a signal having a wide frequency band is applied from the RF tuning circuit (3) to the switching circuit (4). The output signal of the switching circuit (4) is mixed with a local oscillation signal generated from the local oscillation circuit (6) by a mixing circuit (5) and converted into an IF signal. The IF signal is amplified by an IF amplifier circuit (7) and then detected by an AM detector (8).

The frequency of the local oscillation signal is changed according to a first control signal of a frequency control circuit, and the frequency control circuit transmits a second control signal to the tank circuit in response to the first control signal. Is applied. Therefore, the RF tuning circuit (2)
Is substantially equal to the reception frequency, the receiving broadcast station can be cut off by the RF tuning circuit (2), and good reception can be performed.

Incidentally, a varicap diode (2)
b) has a characteristic that harmonics are easily generated.
Therefore, in a state where the output signal of the RF tuning circuit (2) is selected by the switching circuit (4), when the harmonic is equal to the frequency of a broadcasting station existing in the AM frequency band, interference occurs due to the harmonic. I was Therefore, in the circuit of FIG.
At the receiving frequency where the harmonics are equal to the broadcasting station frequency in the AM frequency band, the switching circuit (4) selects the output signal of the RF non-tuning circuit (3) without using the varicap diode. Therefore, no harmonic was applied to the mixing circuit (5), and interference was reduced.

[0006]

However, in the radio receiver shown in FIG. 2, when the RF tuning circuit (2) and the RF non-tuning circuit (3) are switched, the total gain changes. Here, the total gain refers to the sum of the gains of the RF amplifier circuit (1), the RF tuning circuit (2) or the RF non-tuning circuit (3), the mixing circuit (5), and the IF amplifier circuit (7), The received input signal level and the noise level change as shown in FIG. 3 by changing the total gain at the time of a weak electric field. Then, due to the presence of the coil and the resistance of the tank circuit, the total gain when switching to the tank circuit during a weak electric field becomes larger than the total gain when the tank circuit is not selected, so that the input signal level and the noise level are reduced. This causes a problem that the sense of incongruity is caused by the change and the switching.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has an RF tuning circuit for performing a tuning operation or a non-tuning operation to tune a received RF signal, and an output signal of the RF tuning circuit. A frequency conversion circuit for converting the signal into an IF signal;
An F amplifier circuit; and a switching signal generating circuit for generating a switching signal for causing the RF tuning circuit to perform a tuning operation, wherein the gain of the IF amplifier circuit changes greatly according to the switching signal. .

An IF AGC circuit for generating an AGC signal for controlling a gain of the IF amplifier circuit in accordance with a received electric field intensity, the IF amplifier circuit includes: an input resistor for converting an input signal into a current signal; A current variable circuit that varies the magnitude of the current signal converted according to the AGC signal; and an output that determines a gain of the IF amplifier circuit together with the input resistance and converts an output current signal of the current variable circuit into an output signal. And a resistance, wherein the magnitude of the input resistance is reduced according to the switching signal.

[0009]

FIG. 1 is a diagram showing an embodiment of the present invention. (1) is an RF amplifier circuit including an FET (1a) having an amplifying function, (2) is a coil (2a), a burr RF tuning circuit including a tank circuit composed of a cap diode (2b) and a capacitor (2c), (3) RF non-tuning circuit including a coil (3a), (4) RF tuning circuit (2) or RF non-tuning circuit A switching circuit for switching the output signal of (3), and (5) outputs the output signal of the switching circuit (4) to I
A mixing circuit for converting to an F signal, (6) a local oscillation circuit for generating a local oscillation signal, (7 ') an IF amplification circuit for amplifying the IF signal, and (8) an IF amplification circuit (7'). An AM detection circuit for AM detection of an output signal, (9) an RF tuning circuit (2) and a frequency control circuit for controlling a local oscillation frequency, and (10) an AGC signal according to an output signal of the AM detection circuit (8). The generated IFAGC circuit (11) is a control circuit for switching the gains of the switching circuit (4) and the IF amplifier circuit (7 ').

In FIG. 1, a received RF signal is applied to the gate of an FET (1a) of an RF amplifier circuit (1) and amplified. The amplified RF signal is generated from the drain of the FET (1a), and is connected to the coil (2a) of the RF tuning circuit (2) and R
Applied to the coil (3a) of the F non-tuning circuit (3). R
In the F tuning circuit (2), the RF signal is supplied to the coil (2
a) tuning to the resonant frequency of the tank circuit consisting of the varicap diode (2b) and the diode (2c);
After being limited to a frequency band near the resonance frequency, the voltage is applied to the switching circuit (4). The second control signal of the frequency control circuit (9) is applied to the varicap diode (2b), the capacitance of the varicap diode (2b) changes according to the second control signal, and the resonance frequency of the tank circuit changes. , The tuning frequency of the RF tuning circuit (2) changes. Further, since the RF non-tuning circuit (3) does not have a tank circuit, the RF signal applied to the RF non-tuning circuit (3) is not band-limited and is switched as a signal having a wide frequency band.
Is applied to The switching circuit (4) selects one of the two input signals according to the switching signal and applies it to the mixing circuit (5). The output signal of the switching circuit (4) is mixed with a local oscillation signal generated from a local oscillation circuit (6) by a mixing circuit (5),
For example, it is converted into a 450 KHz IF signal. The frequency of the local oscillation signal changes according to the first control signal of the frequency control circuit (9). When the local oscillation frequency changes, the frequency of the RF signal converted to the 450 kHz IF signal changes, so that the frequency of the received RF signal is determined by the first control signal. By generating a second control signal in accordance with the first control signal, the tuning frequency of the RF tuning circuit (2) is made to match the frequency of the received RF signal. The IF
After the signal is amplified by the IF amplifier circuit (7 '), it is detected by the AM detection circuit (8).

The output signal of the AM detection circuit (8) is I
Applied to the FAGC circuit (10). In the IFAGC circuit (10), the received electric field strength is detected by smoothing the AM detection signal.
A signal is generated. In accordance with the AGC signal, the gain of the IF amplifier circuit (7 ') is reduced so that the output signal level of the IF amplifier circuit (7') is controlled so as not to exceed a predetermined level. Therefore, the AM detection circuit (8) is prevented from being saturated.

By the way, during the normal receiving operation, the control circuit (1)
1) generates an "L" level switching signal, for example, and the switching circuit (4) selects the output signal of the RF amplifier circuit (2) and the gain of the IF amplifier circuit (7 ') according to the switching signal. Is set to small. Also, when receiving an RF signal whose harmonics are equal to those of a broadcasting station in the AM frequency band,
The control circuit (11) generates an "H" level switching signal,
The switching circuit (4) selects the output signal of the RF non-tuning circuit (3), and the gain of the IF amplifier circuit (7 ') changes greatly. Since the RF non-tuning circuit (3) does not include a varicap diode and harmonics generated from the RF tuning circuit (2) are cut off by the switching circuit (4), it is possible to reduce adjacent interference due to the harmonics. it can. When the RF tuning circuit (2) is switched to the RF non-tuning circuit (3), RF
The gain of the amplification stage decreases, and conversely, the IF amplification circuit (7 ')
Gain increases. If the amounts of change in the gains of the RF amplification stage and the IF amplification circuit (7 ') are matched, the total gain of the radio receiver does not change. In particular, since the total gain does not change when the electric field is weak, the input received signal level and the noise level do not change at the time of switching, so that uncomfortable feeling can be prevented.

The present invention may be applied not only to a single conversion type AM receiver as shown in FIG. 2 but also to a double conversion type AM receiver. Figure 4 shows the IF
FIG. 9 is a diagram illustrating a specific circuit example of an amplifier circuit (7 ′), wherein (1)
2) and (13) are transistors whose emitters are commonly connected, (14) and (15) are transistors whose emitters are commonly connected, (16) is a current transistor to which an input signal is applied to the base, and (17) is Current transistors with a bias applied to their bases, (18) and (19) are input resistors connected to the emitters of transistors (16) and (17), respectively, and (20) and (21) are transistors (12) and ( 15) a current mirror circuit for reversing the collector current, (22) a current mirror circuit for reversing the output current of the current mirror circuit (20), (2)
3) is an output resistor for converting the difference current between the current mirror circuits (21) and (22) into a voltage, (24) and (25) are resistors connected in parallel with the input resistor, and (26) and (27) are It is a control transistor that performs a switching operation in response to a switching signal.

In FIG. 4, first, at the time of normal reception, the switching signal is at "L" level, and the control transistor (2
6) and (27) are turned off. IF signal is input terminal IN
To the base of the transistor (16).
The emitter voltage of the transistor (16) is calculated based on the voltage of the IF signal from the base-emitter voltage V of the transistor (16).
The voltage is reduced by be, and a current is generated according to the emitter voltage of the transistor (16) and the resistance of the input resistor (18). The current varies according to the amplitude of the IF signal and flows through the common emitter of transistors (12) and (13). The emitter voltage of the transistor (17) is a voltage obtained by subtracting the base-emitter voltage of the transistor (17) from the bias.
A current corresponding to the emitter voltage of (7) and the resistance value of the input resistor (19) flows through the resistor. The current is a constant current and flows through the common emitter of transistors (14) and (15).

When the IFAGC operation is not performed, A
The GC signal is at a higher level than Vref2, and the transistors (12) and (14) are turned on. The collector current of the transistor (12) is inverted by the current mirror circuits (20) and (22), and the difference current between the current mirror circuits (21) and (22) is supplied to the resistor (23). A voltage is generated at one end of the resistor (23), and is applied from the output terminal OUT to the subsequent AM detection circuit (8). Here, assuming that the resistance values of the input resistance (18), the output resistance (23), and the resistance (24) are Rin and Rout, respectively, and the input signal of the IF amplifier (7 ') is Vin, the output signal Vou
t is

[0016]

(Equation 1)

The gain of the IF amplifier circuit (1) is determined by the resistance ratio between the input resistance (18) and the output resistance (23). When the AGC signal is applied and the base voltages of the transistors (12) and (14) decrease, the transistor (1)
5) starts operating, and a current is generated at the collector of the transistor (15). The collector current of the transistor (15) is inverted by the current mirror circuit (21) and mixed with the output current of the current mirror circuit (22), and then the output resistance (23)
Supplied to Since the collector current of the transistor (12) decreases, the current corresponding to the IF signal supplied to the output resistor (23) decreases, and the signal level generated at the output terminal OUT decreases. Therefore, the transistors (12) and (14) and the transistor (1) are
By changing the inter-base voltage in 3) and (15), the level of the IF signal generated at the output terminal OUT can be changed. Since the reference voltage Vref1 is higher than the reference voltage Vref2 so that the transistors (12) and (14) do not turn off at least, the IF
The signal is not interrupted.

When the switching signal becomes "H" level,
The control transistors (26) and (27) are turned on, and the resistors (24) and (25) are connected in parallel to the input resistors (24) and (25), respectively. Therefore, the input signal is Vin
Then, the output signal Vout becomes

[0019]

(Equation 2)

As a result, the gain of the IF amplifier circuit (7 ') can be increased. Switching of the gain of the IF amplifier circuit (7 ') is also performed by changing the resistance value of the output resistor (23). In FIG. 3B, when the gain of the IF amplifier circuit (7 ') is small, the IF amplifier circuit (7')
Output signal level changes as indicated by the solid line according to the received electric field strength. When the gain of the IF amplifier circuit (7 ') is switched to a large value by the input resistance, the output signal of the IF amplifier circuit (7') changes as indicated by a dotted line, and when the output resistance is switched, the output signal is cut as indicated by a chain line. Be replaced. Looking at the reception strength at which the IFAGC is no longer effective, the output signal level of the IF amplifier circuit (7 ') becomes the same as the change in the solid line when the output resistance is switched.
In the switching of the input resistance, the electric field intensity at which the IFAGC does not work becomes stronger than the electric field strength when the gain of the amplifier circuit (7 ') is small, and the range of the electric field strength at which the IFAGC works can be shifted as it is.

[0021]

As described above, according to the present invention, the RF
In the tuning stage, when switching between the RF tuning circuit and the RF non-tuning circuit, the input signal level and the noise signal level change because the gain of the IF amplifier circuit is switched to keep the total gain of the radio receiver constant. Can be prevented. In particular, when the electric field is weak, there is a large adverse effect due to the change in the total gain, but this adverse effect can be prevented.

In an IF amplifier circuit comprising an input resistor, a current variable circuit for varying current according to an AGC signal, and an output resistor, wherein the gain is determined by the ratio of input / output resistance, the gain is switched by the value of the input resistor. Even after switching
FAGC can be effectively used.

[Brief description of the drawings]

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

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

FIG. 3 is a characteristic diagram for explaining the present invention.

FIG. 4 is a circuit diagram showing a specific example of a main part of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 RF amplification circuit 2 RF tuning circuit 3 RF non-tuning circuit 4 switching circuit 5 mixing circuit 6 local oscillation circuit 7 ′ IF amplification circuit 8 AM detection circuit 9 frequency control circuit 10 IFAGC circuit 11 control circuit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-47112 (JP, A) JP-A-9-186618 (JP, A) JP-A-9-55674 (JP, A) JP-A-1- 135127 (JP, A) Japanese Utility Model 5-25845 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 1/18 H04B 1/16

Claims (2)

(57) [Claims] An RF tuning circuit for performing a tuning operation or a non-tuning operation to tune a received RF signal; a frequency conversion circuit for converting an output signal of the RF tuning circuit into an IF signal; An IF amplification circuit for amplifying the IF signal; and a switching signal generation circuit for generating a switching signal for causing the RF tuning circuit to perform a tuning operation. The gain of the IF amplification circuit is increased in accordance with the switching signal. AM radio receiver characterized by the following: 2. An IFAGC circuit for generating an AGC signal for controlling a gain of an IF amplifier circuit according to a received electric field strength, wherein the IF amplifier circuit converts an input signal into a current signal. A current variable circuit that varies the magnitude of the current signal converted according to the AGC signal; and an output that determines a gain of the IF amplifier circuit together with the input resistance and converts an output current signal of the current variable circuit into an output signal. 2. The AM radio receiver according to claim 1, wherein the AM radio receiver comprises a resistor, and the magnitude of the input resistor is reduced according to the switching signal.