JPS6338586Y2 - - Google Patents

Description

[Detailed description of the invention] This invention is suitable for being placed on moving objects such as automobiles.
Regarding FM receiver.

From the perspective of traffic safety, an important issue for in-vehicle FM receivers installed in automobiles and other vehicles is to simplify operation, and one way to do this is to automatically search channels using electronic tuning. There is something that made it possible. This means that when one operation button is pressed, a channel is automatically searched, and when channel synchronization is obtained, the search is automatically stopped and the reception status of that channel is set. This allows you to search for the next channel. Therefore, the channel selection operation is extremely simple and can be carried out without any particular hindrance to driving operations.

In order to enable such automatic search, it is necessary to detect that the channel is tuned and obtain control to stop the search.For this purpose, conventionally, quadrature detection was used as an FM detection circuit. A detection circuit was used to obtain the above-mentioned control signal.

By the way, when a car moves, the surrounding environment naturally changes, but this change in the environment changes the reception condition, and as a result, the received electric field strength changes and is supplied to the detection circuit. The level of the intermediate frequency FM signal also changes. Also,
The quadrature detection circuit has the disadvantage that it tends to generate noise and the S/N ratio decreases for weak input signals.

Therefore, conventional in-vehicle FM receivers equipped with quadrature detection circuits and capable of automatic search had the disadvantage that noise was generated as the telegraph field strength decreased, and the S/N of the reproduced audio decreased. This reduction in S/N is due to the performance peculiar to the quadrature detection circuit, and is naturally audible if the above noise does not occur, but it is within the range of received field strength that is naturally audible. will be restricted.

Furthermore, when a channel is searched by pressing an operation button, the device becomes out of tune and generates noise by the time it is tuned to the next channel. In this case, since the quadrature detection circuit generates noise as the input signal drops, noise will be generated as soon as a search is started, and indeterminate noise will be noticeably generated from the speaker every time a search is performed.

The object of the present invention is to eliminate the shortcomings of the prior art.
It is possible to obtain reproduced audio with a good S/N ratio even for received radio waves with low received electric field strength, expand the range of audible received electric field strength, and suppress the generation of noise due to detuning during channel search.
To provide an FM receiver.

In order to achieve this purpose, the present invention provides a quadrature detection circuit and a ratio detection circuit as an FM detection circuit, and when the received signal strength is high, obtains an audio signal detected by the quadrature detection circuit, and When the received electric field strength is low, the ratio detection circuit obtains the detected audio signal, and the quadrature detection circuit enables automatic channel search, and the ratio detection circuit detects fluctuations in the level of the input signal. Also S/N
Focusing on the fact that a good detection output can be obtained, we have made it possible to obtain reproduced audio with a good S/N even when the receiving field strength is low.
A feature of the present invention is that by detecting an out-of-tune state, a detection output signal of the ratio detection circuit can be obtained even in the out-of-tune state.

Embodiments of the present invention will be described below with reference to the drawings.

The figure is a block diagram showing an embodiment of the FM receiver according to the present invention, in which 1 is an antenna, 2 is a tuner, 3 is an intermediate frequency stage, 4 is a ratio detection circuit, 5 is a quadrature detection circuit, and 6 is a switching circuit. , 7 is a stereo demodulation circuit, 8 and 9 are low frequency amplification circuits, 10,
11 is a speaker, 12 is a level detection circuit, 13 is a drive circuit, 14 is an inverter, 15 is a Hall detection circuit, 16 is a detuning detection circuit, 17 is an OR circuit, 1
8 is a variable resistor, 19, 20, 21 are transistors, 22, 23, 24 are resistors, 25 is a diode, and 26 is a capacitor.

In the figure, an antenna 1, a tuner 2, an intermediate frequency stage 3, a quadrature detection circuit 5, a stereo demodulation circuit 7, a low frequency amplification circuit 8, 9, a speaker 1
0 and 11 have the same configuration as a conventional FM receiver capable of automatic channel search. In this embodiment, a ratio detection circuit 4 is provided, and the intermediate frequency FM signal detected by the level detection circuit 12 is The switching circuit 6 is operated in accordance with the level of the output signal of the ratio detection circuit 4 and the quadrature detection circuit 5, and the output signals of the ratio detection circuit 4 and the quadrature detection circuit 5 are selectively switched and supplied to the stereo demodulation circuit 7.

Next, the operation of this embodiment will be explained.

The received signal frequency-converted by the tuner 2 is processed as an intermediate frequency FM signal by the intermediate frequency stage 3.
The signal is supplied to a ratio detection circuit 4 and a quadrature detection circuit 5, and detected stereo audio signals are obtained from each of them.

On the other hand, an intermediate frequency FM signal is supplied from the intermediate frequency stage 3 to the level detection circuit 12, and this intermediate frequency FM
The level of the envelope of the signal is detected. Therefore, the signal according to this envelope is sent to the drive circuit 1.
3, and is supplied to the base of a transistor 19 via a variable resistor 18. Therefore, the base potential of the transistor 19 is set in accordance with the level of the intermediate frequency FM signal.

On the other hand, the signal level-adjusted by the drive circuit 13 is inverted by the inverter 14 and supplied to the OR circuit 17, and the respective output signals of the Hall detection circuit 15 and the detuning detection circuit 16 are also supplied to the OR circuit 17. Ru. The output signal of the OR circuit 17 is supplied to the base of the transistor 21 via the diode 25, and therefore, the base potential of the transistor 21 is set at a level corresponding to the output signal of the OR circuit 17. The Hall detection circuit 15 is supplied with the intermediate frequency FM signal from the intermediate frequency stage 3, detects the C/N (carrier-to-noise ratio) of this intermediate frequency FM signal, and outputs a level corresponding to the C/N value. Generate a signal. In addition, the detuning detection circuit 16
It discriminates the frequency of the FM signal and generates a signal that becomes high level when detuned. The output signal of the detuning detection circuit 6 is also used as a control signal for automatic frequency control of the tuner 2.

Variable resistor 18, transistors 19, 20, 2
1, resistors 22, 23, 24, diode 25,
The capacitor 26 constitutes a drive signal generation circuit that generates a drive signal for operating the switching circuit 6.
The switching circuit 6 supplies the detection output signal of the quadrature detection circuit 5 to the stereo demodulation circuit 7 when the drive signal is at a high level (hereinafter referred to as "H"), and conversely, when the drive signal is at a low level (hereinafter referred to as "H") (referred to as “L”), the detection output signal of the ratio detection circuit 4 is supplied to the stereo demodulation circuit 7.

Transistors 19, 20, and 21 are switching transistors, and transistor 19 is turned on when its base potential is above its cutoff level, so that transistor 2
0 becomes off state as its base potential becomes low; conversely, when transistor 19 becomes off state as its base potential becomes below the cut-off level, transistor 20 becomes on state as its base potential becomes high. . In addition, the transistor 21
turns on when its base potential goes above the cutoff level, and turns off when its base potential goes below the cutoff level.
When both transistors 20 and 21 are in the off state, the level of the drive signal supplied to the switching circuit 6 becomes "H", and the transistor 2
When at least one of 0 and 21 is in the on state, the level of the drive signal is "L".

The variable resistor 18 is used to set the base potential of the transistor 19, and when the variable resistor 18 is adjusted so that the base voltage applied to the transistor 19 becomes smaller with respect to the current value from the drive circuit 13, the intermediate frequency Even if the FM signal drops slightly from its maximum level, transistor 19 is turned off and transistor 20 is turned on. Conversely, if the variable resistor 18 is adjusted so that the base voltage applied to the transistor 19 is increased, the transistor 19 will be turned off only when the intermediate frequency FM signal has sufficiently decreased from the maximum level.

On the other hand, the output level of the OR circuit 17 in which the transistor 21 is switched from the off state to the on state is constant, and therefore, depending on the adjustment of the variable resistor 18, the output level of the OR circuit 17, in which the transistor 21 is switched from the off state to the on state, is The switching circuit 6 can switch between the output signal of the ratio detection circuit 4 and the output signal of the quadrature detection circuit 5 depending only on the level of the frequency FM signal (the base voltage applied to the transistor 19 is small). ), or conversely, the switching circuit 6 switches to a ratio detection circuit depending on the level of the intermediate frequency FM signal, the magnitude of the C/N, and the degree of detuning by turning on and off the transistor 21. 4 and the output signal of the quadrature detection circuit 5 (if the voltage applied to the transistor 19 is increased).

However, if the received electric field strength becomes weaker by adjusting the variable resistor 18, or if the C/N decreases due to external noise, or
In the case of detuning during channel auto search, the detection output signal of the ratio detection circuit 4 is supplied to the stereo demodulation circuit 7, and the speaker 1
From 0.0 to 11, it is possible to obtain reproduced audio with a good S/N ratio. Also, intermediate frequency
Ratio detection circuit 4 responds only to the level of the FM signal.
When switching the output signals of the quadrature detection circuit 5 and the quadrature detection circuit 5,
The level of the FM signal can also be adjusted using the variable resistor 18.

The resistor 24 and the capacitor 26 form a time constant circuit, and the drive signal to the switching circuit 6 changes from "L" to "H".
It delays the change to That is, since the output signal from the OR circuit 17 is supplied to the base of the transistor 21 via the diode 25, during the process in which this output signal changes from "L" to "H", it becomes higher than the cutoff level of the transistor 21. Then, the transistor 21 immediately changes from the off state to the on state, and the drive signal changes from "H" to "L". Therefore, the switching circuit 6 immediately switches from the output signal of the quadrature detection circuit 5 to the output signal of the ratio detection circuit 4.

However, the output signal of the OR circuit 17 is “H”
In the process of switching from "L" to "L", even if the level of this output signal becomes below the cut-off level of the transistor 21, the base potential of the transistor 21 is kept above the cut-off level by the charging voltage of the capacitor 26. , the transistor 21 continues to be on. The capacitor 26 is discharged via the resistor 24 and the OR circuit 17 with a time constant determined by the capacitance of the capacitor 26 and the resistance value of the resistor 24, and when the discharge voltage of the capacitor 26 becomes below the cut-off level. First time with transistor 2
1 is in the off state.

Due to this delay in the change of the drive signal from "L" to "H", even if the level or C/N of the intermediate frequency FM signal exceeds a predetermined value, the synchronization in auto search will not be achieved. However, the switching from the output signal of the ratio detection circuit 4 to the output signal of the quadrature detection circuit 5 is not performed immediately;
Charging is performed for the first time after the time determined by the charging time constant described above has elapsed. For this reason, by appropriately setting the above charging time constant, even if there is fluctuation in the level or C/N of the intermediate frequency FM signal, or even if there is fluctuation at the moment when synchronization is achieved, During this time, the output signal of the ratio detection circuit 4 is selected, and it is possible to prevent difficulty in hearing due to frequent switching between the output signal of the ratio detection circuit 4 and the output signal of the quadrature detection circuit 5.

Note that the output signal of the drive circuit 13 is an intermediate frequency FM
Since it represents the signal level, it can be used to drive a signal level indicator meter.

As explained above, according to the present invention, it is possible to obtain reproduced audio with good S/N even in a weak received electric field, expand the range of audible received electric field strength, and To provide an FM receiver that has good S/N of reproduced audio even when external noise is mixed in or when detuned during auto search, and has excellent functions except for the drawbacks of the above-mentioned conventional technology. be able to.

[Brief explanation of the drawing]

The figure is a block diagram showing an embodiment of the FM receiver according to the present invention. 3...Intermediate frequency stage, 4...Ratio detection circuit, 5
...Quadrature detection circuit, 6...Switching circuit, 1
2... Level detection circuit, 13... Drive circuit, 14
...Inverter, 15 ... Hall detection circuit, 16
...Detune detection circuit, 17...OR circuit.

Claims (1)

[Scope of utility model registration request] A ratio detection circuit, a quadrature detection circuit,
a switching circuit to which respective output signals of the ratio detection circuit and the quadrature detection circuit are supplied; a level detection circuit for detecting the level of the intermediate frequency FM signal; and an inverter for inverting the output signal of the level detection circuit; A Hall detection circuit that generates an output signal as the carrier-to-noise ratio decreases when the intermediate frequency FM signal is at a low level, a detuning detection circuit that generates an output signal when the intermediate frequency FM signal is detuned, the inverter, the Hall detection circuit, and the detuning circuit. an OR circuit to which respective output signals of the tone detection circuit are supplied, and a drive to which respective output signals of the level detection circuit and the OR circuit are supplied and generate a drive signal for the switching circuit according to the level of the output signal. A signal generation circuit is provided, and when the received electric field strength is weak, the carrier-to-noise ratio decreases, and the signal is detuned,
An FM receiver characterized in that the FM receiver is configured to be able to switch from the output signal of the quadrature detection circuit to the output signal of the ratio detection circuit.