JPS6219003Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an FM receiver, and in particular to an FM receiver that can reliably suppress multipath interference due to pulse noise peculiar to FM receivers even in medium to high electric fields. be.

FM receivers are most vulnerable to multipath interference caused by pulsed noise, which is a major problem especially in vehicle-mounted FM receivers where the received electric field changes significantly. For this reason, various circuits for suppressing multipath interference have been proposed in FM receivers.

FIG. 1 is a block diagram showing an example of an FM receiver having a function of suppressing multipath interference, which has been commonly used in the past. Two antennas are configured such that their directivity characteristics are different from each other, and this antenna 1
The outputs of a and 1b are sent to multipath detection circuit 1, which will be described later.
Either one of the signals is taken in by the antenna switching circuit 2 whose switching is controlled by the antenna 8. 3 is a front end circuit which receives the antenna signal selected by the antenna switching circuit 2; 4 is an intermediate frequency amplifier which receives the output of the front end circuit 3; and 5 detects the output of the intermediate frequency amplifier 4. A detection circuit 6 for extracting the demodulated output is a soft muting circuit connected to the output end of the detection circuit 5, and the output level is inversely proportional to the muting signal proportional to the received electric field strength output from the intermediate frequency amplifier 4. lowered. 7 is a pulse noise removal circuit that removes pulse noise contained in the output signal sent from the soft muting circuit 6;
8 is a multiplex decoder that separates the low frequency signal output from the pulse noise removal circuit 7 into L and R channel signals and sends them to output terminals 9 and 10;
11 is a high-pass filter connected to the output terminal of the pulse noise removal circuit 7; 12 is a variable gain amplifier that amplifies the output of the high-pass filter 11 and supplies it to the noise detector 13; and 14 inputs the output of the noise detector 13. The Schmitt circuit 15 is a bistable multi-circuit triggered by the output of the Schmitt circuit 14, and the switching control of the antenna switching circuit 2 is performed by the output thereof. 16 is an amplifier that amplifies the output of variable gain amplifier 12; 17 is amplifier 1;
This is a gain control circuit that detects the output level of variable gain amplifier 12 and controls the gain of variable gain amplifier 12. The part indicated by the dotted line constitutes a multipath detection circuit 18 that detects multipath noise included in the detection output.

In the FM receiver configured in this way,
One of the outputs of the antennas 1 a and 1 b is selected by an antenna switching circuit 2 and supplied to an intermediate frequency amplifier 4 via a front end circuit 3 . The intermediate frequency amplifier 4 amplifies the target intermediate frequency signal and supplies the amplified signal to the detection circuit 5, and also connects to the output side of the detection circuit 5 a soft mutating circuit which outputs a muting signal M having a value proportional to the received electric field. Supply to 6. Therefore, the soft muting circuit 6 will send out an output whose level is reduced as the S/N (signal-to-noise ratio) ratio of the detected output sent out from the detection circuit 5 deteriorates. . In this way, the detected output, which has been given uniformity with respect to the received electric field strength, is sent to output terminals 9 and 10 as stereo signals of the L and R channels via the pulse noise removal circuit 7 and the multiplex decoder 8. Ru.

On the other hand, the output signal of the pulse noise removal circuit 7 is
Only the high-frequency components contained therein are extracted through the high-pass filter 11, and the variable gain amplifier 12
After being amplified at , the signal is supplied to the noise detector 13 . Furthermore, since the output of the variable gain amplifier 12 is further amplified in the amplifier 16 and then subjected to gain control via the gain control circuit 17,
Automatic gain control is performed based on the input noise level, and the output level supplied from the variable gain amplifier 12 to the noise detector 13 is made constant. The noise detection circuit 13 detects a noise signal contained in the supplied signal, and sends its output to a Schmitt circuit 14 to trigger a bistable multicircuit 15. Therefore, the bistable multi-circuit 15 is triggered and inverted when the noise component included in the input signal increases, and this inverted signal causes the antenna switching circuit 2 to be inverted.
By controlling this, multipath interference is reduced by selecting and taking in the other antenna signal with different directivity characteristics.

However, in the FM receiver with the above configuration, the multipath detection circuit performs automatic gain control based on the noise level contained in the detected output that has been subjected to soft mute control, so it is frequently used under strong electric fields. Under conditions where multipath noise occurs, the level of the noise signal decreases, the multipath detection sensitivity decreases, and the multipath noise removal effect is lost. Therefore, the multipath suppression effect under medium and strong electric fields is particularly reduced. Also,
The above configuration has various drawbacks such as a special need for an amplifier and a gain control circuit because automatic gain control is performed using the noise signal detected by the multipath detection circuit.

Therefore, an object of this invention is to provide an FM receiver that has a simple configuration but has an improved multipath noise removal effect, especially under medium and strong electric fields.

In order to achieve this purpose, this idea
The detection sensitivity of the multipath detection circuit is controlled using a signal corresponding to the received electric field strength output from the intermediate frequency amplifier. The FM receiver according to this invention will be explained in detail below with reference to the drawings.

FIG. 2 is a block diagram showing an embodiment of the FM receiver according to this invention, in which the same parts as in FIG. 1 are designated by the same symbols and the details thereof are omitted.
In the figure, reference numeral 19 denotes an intermediate frequency amplifier, which is configured to send out a muting signal M and a received field strength signal P proportional to the received field strength. 20 receives the received field strength signal P as input, and the third
As shown in Figure a, this is a level detector that outputs a signal that shows a constant value in a weak electric field and whose voltage decreases as the electric field strength increases in medium and strong electric fields. Tekta 2
The gain of the variable gain amplifier 12 is set to 3 by the output of 0.
As shown in Figure b, control is performed to increase the amplification degree at medium and strong electric field strengths. The multipath detection circuit 21 includes a high pass filter 11, a variable gain amplifier 12, a noise detector 13, and a Schmitt circuit 14.
and a bistable multi-circuit 15.

In the FM receiver configured in this way,
When an FM broadcast is received under a strong electric field, the intermediate frequency amplifier 19 outputs a received electric field strength signal P with a high voltage value indicating that the input electric field is strong, and is supplied to the level detector 20. In response to the received field strength signal P having a high voltage value, the level detector 20 supplies a gain control signal having a low voltage value to the variable gain amplifier 12 as shown in FIG. 3a. As a result,
The variable gain amplifier 12 increases its gain as shown in FIG. 3b to increase the noise signal of a small value under a strong electric field, which is large in a weak electric field and small in a medium-strong electric field, as shown in FIG. 3c. This increases the multipath detection sensitivity in the multipath detection circuit 21. This operation is the same when receiving FM broadcasts in medium electric fields, thereby effectively eliminating noise caused by multipath interference and obtaining a reproduced signal with an excellent S/N ratio.

Next, under a weak electric field, the intermediate frequency amplifier 1
The received field strength signal P output from 9 has a small value. As a result, the output of the level detector 20 becomes a large value as shown in FIG. 3a, and the gain of the variable gain amplifier 12 controlled by the output of the level detector 20 becomes as shown in FIG. 3b. descend. Therefore, under this weak electric field, the detection sensitivity of the multipath detection circuit 18 is lowered to prevent malfunction due to noise other than multipath.

Therefore, in a device configured in this way, the external noise level under a strong electric field is extremely small, and control is performed to increase the multipath detection sensitivity under this strong electric field. .

FIG. 4 is a circuit diagram showing an example of the variable gain amplifier 12, and for reception under a weak electric field,
A control signal input terminal 21 is connected to the base of the transistor 20.
Since the gain control signal supplied through the resistor 24 has a high voltage as shown in FIG. 3a, the transistor 20 has a low impedance. As a result, the input signal supplied via the input terminal 23 and the coupling capacitor 24 is mostly grounded via the capacitor 25 and the low-impedance transistor 20, so that the input signal is input to the transistor 26. The signal level decreases and the amplified output sent to the output terminal 27 becomes small, resulting in a state where the gain is lowered.

Next, during reception under medium to strong electric fields, the control signal supplied to the control signal input terminal 21 has a small value as shown in FIG. 3a, and accordingly the transistor 20 has a high impedance. As a result, all the input signals supplied to the input terminal 23 are supplied to the base of the transistor 26, and accordingly, the output signal sent to the output terminal 27 becomes larger, increasing the gain. state. Note that 28 is a power supply terminal, 29 and 30 are base bias resistors, and 31 and 32 are collector and emitter resistors.

Note that in the above embodiment, an intermediate frequency amplifier was used to detect the input electric field level.
This idea is not limited to this. Furthermore, in current FM receivers, part of the intermediate frequency amplification stage is integrated into an integrated circuit, and this integrated part also includes a level detector, making assembly extremely easy. It is something that

As explained above, the FM receiver according to this invention uses a multipath detection circuit that detects pulsed multipath noise contained in the detection output and switches the receiving antenna to another antenna with different characteristics. Since the sensitivity is controlled to high sensitivity in medium to strong electric fields and low sensitivity in weak electric fields, it has an excellent effect of removing multipath noise when receiving FM broadcasts under medium to strong electric fields. have an effect.

[Brief explanation of the drawing]

Figure 1 shows a conventional system with a multipath detection circuit.
2 is a block diagram showing the FM receiving device according to this invention; FIGS. 3 a to 3 c are characteristic diagrams of each part of the block diagram shown in FIG. 2;
FIG. 4 is a circuit diagram showing an example of the variable gain amplifier shown in FIG. 2. 1a, 1b...Antenna, 2...Antenna switching circuit, 3...Front end circuit, 5...Detection circuit, 6...Soft muting circuit, 7...Pulse noise removal circuit, 8...Multiplex decoder , 11... High pass filter, 12... Variable gain amplifier, 13... Noise detector, 14... Schmitt circuit, 15... Bistable multi-circuit, 19
... intermediate frequency amplifier, 20 ... level detector, 21 ... multipath detection circuit.

Claims (1)

[Claims for Utility Model Registration] (1) An antenna signal selection circuit that selects and captures a plurality of antenna outputs with different directivity, and a pulse characteristic included in the detected output of the FM reception signal supplied from the antenna signal selection circuit. a pulse noise removal circuit that removes noise; and a multipath detection circuit that detects multipath noise included in the output of the pulse noise removal circuit and controls switching of input selection of the antenna signal selection circuit.
In the FM receiving device, a control signal generation section that generates a control signal corresponding to a received electric field level is provided, and a detection sensitivity control section is provided in the multipath detection circuit, and the detection sensitivity is controlled by the output of the control signal generation section. 1. An FM receiving device characterized in that the detection sensitivity of the multipath detection circuit is increased at medium to strong reception electric field levels by controlling the multipath detection circuit. (2) The detection sensitivity control section of the multipath detection circuit is constituted by a variable gain amplifier that amplifies multipath noise, and the detection sensitivity is controlled by the output of the control signal generation section. The FM receiving device according to claim 1 of the utility model registration claim, in which the adjustment is performed.