JPH0752851B2 - FM receiver - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM receiver, and in particular, to ensure a good reception state even in a strong electric field region where a large number of FM reception stations are likely to cause intermodulation and intermodulation. The present invention relates to an FM receiver.

2. Description of the Related Art A conventional FM receiver is a high frequency automatic gain adjustment (RF / AGC) output circuit (hereinafter,
A wide band AGC circuit), an intermediate frequency / automatic gain adjustment (IF / AGC) output circuit (hereinafter referred to as a narrow band AGC circuit) that detects electric field strength in a relatively narrow band, and both of these AGC
It has an RF (high frequency) attenuator and RF amplifier that are controlled by the output value of the circuit.
If the amplifier is controlled by the output values of the wide band and narrow band AGC circuits, various characteristics such as intermodulation and intermodulation can be improved to some extent.

Problems to be Solved by the Invention However, in the AGC circuit of the conventional FM receiver, since the link is formed by the closed loop, the RF attenuator and the RF amplifier cannot be controlled completely independently and in the optimum state. In addition, even if the characteristics such as intermodulation and cross modulation can be improved to some extent, the AGC
It could not be used at the device optimum bias point in the loop.

Therefore, the present invention makes it possible to control the RF attenuator and the RF amplifier completely independently and in an optimum state to perform intermodulation,
It is an object of the present invention to provide an FM receiving apparatus that improves various characteristics such as cross-modulation and enables good FM reception.

Means for Solving the Problems In order to achieve the above object, the present invention provides two types of AGC voltages in an AGC loop, which are determined according to the magnitudes of a wide band electric field strength level and a narrow band electric field strength level. Respectively R
A control circuit that applies independently to the F attenuator and the RF amplifier is provided, and the electric field strength of the desired receiving station and the interfering station is analogized from the wideband AGC circuit output and the narrowband AGC circuit output,
The optimum AGC voltage is independently supplied to the attenuator and RF amplifier.

Therefore, according to the present invention, it becomes possible to independently apply the optimum AGC voltage to the RF attenuator and the RF amplifier, and even if the characteristics of the element are the same as the conventional performance, the Various characteristics such as inter-modulation and cross-modulation under input can be improved, and better FM reception becomes possible.

Embodiment FIG. 1 is a block diagram showing a schematic configuration of an FM receiver which is an embodiment of the present invention.

In FIG. 1, an input RF (high frequency) signal from an antenna 1 passes through an RF attenuator 2 and then is amplified by an RF amplifier 3 having a 2-gate FET structure and further amplified by an FE (front end) unit 5. After that, it is converted into an IF (intermediate frequency) signal by the local oscillator signal of the OSC (local oscillator) unit 4. After that, it is input to the IF amplifier / detection unit 6 having a limiter function, and is converted into a composite signal by the detection means having a quadrature detection configuration, for example.

The composite signal is M which obtains the left and right channel signals L and R.
It is applied to the PX (multiplex) demodulation circuit 7. The left and right channel signals L and R are amplified by an AF (audio frequency) signal amplifier 8 and output as drive signals for an external speaker (not shown).

On the other hand, in order to detect the electric field strength level of a wide band (generally including an interfering signal), the detection signal A detected by the FE unit 5 as a voltage changing according to the input RF signal level is controlled by the RF attenuator and the RF amplifier. Input to the circuit 9.

On the other hand, in order to detect the electric field strength level of a narrow band (generally only the desired signal), the IF signal level of the IF amplifier / detection unit 6 is
The detection signal B, which is detected by the level detection means having the AM detection structure and is output according to the level, is also input to the control circuit 9.

In this way, the control circuit 9 to which the two detection signals A and B are input determines the optimum AGC voltage for various characteristics such as intermodulation and intermodulation based on these levels, and determines the voltage. An RF amplifier gain control signal C is output to the RF amplifier 3, and an RF attenuator drive control signal D is output to the RF attenuator 2.

The operation of the control circuit 9 will be further described with reference to FIGS.

FIG. 2 shows the electric field strength of the reception frequency (that is, the desired signal) and the output characteristics of the electric field strength level detection signals of the wide band A and the narrow band B corresponding thereto.

Fig. 3 shows the electric field strength levels of the wide band A and narrow band B when the frequency of the input RF signal is shifted with respect to the received frequency when the desired signal input electric field strength is sufficiently added (for example, about 110 dB). It shows the output characteristics of the detection signal. The values of the output voltage shown in FIGS. 2 and 3 are values inverted by an inverter (not shown).

FIG. 4 shows the output characteristic of the RF amplifier gain control signal C with respect to the desired signal input electric field strength of the control circuit 9 of FIG.

Here, for example, by setting the midpoint between the narrow band and wide band AGC voltage change regions to 50 dB, the narrow band electric field strength level detection signal B shows 50 dB or less, and the wide band electric field strength level detection signal A is 50 dB or more. In the case of, the control circuit 9 judges that there is an interference level input, and changes the output characteristic of the RF amplifier gain control signal C in an analog manner as shown by the broken line in FIG. RF when is weak
It has a logical function of increasing the gain of the amplifier.

Similarly, FIG. 5 is for the desired signal input electric field strength of the control circuit.
It shows the output characteristics of the RF attenuator drive control signal D.

As is clear from FIG. 5, the control circuit 9 has
Further, it has the following characteristic functions.

That is, in any of the output characteristics of the control signal C, D,
When the desired signal input field strength level is a weak input (for example, 20 dB or less) that cannot secure a sufficient S / N input,
The control circuit 9 controls both control signals so as not to attenuate the RF signal.
It functions to secure the output characteristics of C and D. That is, the gain of the RF amplifier 3 decreases as the control signal C increases, and the attenuation of the RF attenuator 2 increases as the control signal D increases. In addition, the numbers shown on the horizontal axis in FIGS. 4 and 5 indicate the desired signal input electric field strength in a narrow band, and do not indicate the wide band input electric field strength including an interfering signal.

FIG. 6 shows the intermodulation characteristics.
And the level of the interfering waves separated by 2Δf is plotted on the horizontal axis, and S /
It represents the minimum level of the desired signal that can secure N30.

FIG. 7 shows the intermodulation characteristics, in which the horizontal axis represents the level of one interfering wave that is .DELTA.f away from the desired signal, and shows the minimum level of the desired signal that can secure S / N30.

In FIGS. 6 and 7, a is the characteristic when there is no AGC, and b is the characteristic when the control signals C and D are controlled only by the broadband electric field strength level detection signal A shown in FIG. And c is a characteristic when the control signals C and D are controlled by both detection signals A and B. Further, Δf is a frequency within a wide band without entering the narrow band area. It should be noted that the numerical values used in the above embodiments are merely examples and are not limited to these.

EFFECTS OF THE INVENTION The present invention, as is clear from the above embodiment,
AGC circuit, narrow-band AGC circuit, and two different AGC voltages that are determined from the output values of both circuits depending on the magnitude of the electric field strength of the reception frequency (desired signal) and the frequencies around it (interference signal). Since each is equipped with a control circuit for independently applying to the RF attenuator and the RF amplifier,
The RF attenuator that controls the RF signal level to the next stage and the RF amplifier can be controlled independently and continuously in an optimum state. As a result, interference due to intermodulation and cross-modulation is less likely to occur than with conventional devices, and good FM reception is possible even in areas with many FM stations and strong electric field strength.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an FM receiver according to an embodiment of the present invention, FIG. 2 is an output characteristic diagram of a wide band and narrow band electric field strength level detection signal with respect to electric field strength of a receiving frequency, FIG. Is a wide band for changes in the input RF signal frequency,
Output characteristic diagram of narrow band electric field strength level detection signal, FIG. 4 is an output characteristic diagram of RF amplifier gain control signal with respect to input electric field strength, FIG. 5 is an output characteristic diagram of RF attenuator drive control signal with respect to input electric field strength, 6th The figure shows the intermodulation characteristic diagram, No. 7.
The figure is a cross-modulation characteristic diagram. 1 ... Antenna, 2 ... RF (high frequency) attenuator, 3
...... RF amplifier, 4 ... OSC (local oscillator) section, 5 ... Front end (FE) section, 6 ... IF (intermediate frequency) amplifier / detection section, 7 ... MPX (multiplex) demodulation circuit, 8 …… A
F (audio frequency) amplifier, 9 ... RF attenuator,
RF amplifier control circuit, A ... Wide band electric field strength level detection signal, B ... Narrow band electric field strength level detection signal, C ...
RF amplifier gain control signal, D ... RF attenuator drive control signal.

Claims (1)

[Claims] 1. An RF attenuator that controls the attenuation of the RF (high frequency) signal level to the next stage, an RF amplifier that amplifies the output from this RF attenuator, and a broadband electric field strength level that includes an interfering signal. Wideband AG that outputs detection signal
C circuit, a narrow band AGC circuit that outputs a detection signal according to the narrow band electric field strength level including only the desired signal, and is determined according to the magnitude of the wide band electric field strength level and the narrow band electric field strength level A control circuit that separately applies two different AGC voltages to the RF attenuator and the RF amplifier, and controls the attenuation of the RF attenuator and the amplification of the RF amplifier separately. And FM
Receiver.