JPS596015Y2 - Receiving machine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a receiver, and more particularly to a receiver with an auto-search function capable of automatic channel selection.

Generally, in a receiver, when the level of the signal received by the antenna decreases, noise becomes noticeable and the S/N decreases, but in order to prevent this phenomenon, in the past, when there is a weak input, the IF (intermediate frequency) In order to improve the S/N ratio, the frequency band of the stage is controlled to be a narrow band.

However, such an improvement using a manual switch is undesirable because it naturally imposes a burden on the user.

On the other hand, it is possible to automatically control the bandwidth of such an IF stage.
Automatically detects signal level. Although there is a method to control the bandwidth, it is necessary to add a circuit for detecting the IF signal level and generating a switching control signal in order to switch the bandwidth, which is also one of the reasons for the complexity of the device. Become.

Particularly in in-vehicle equipment such as car radios, input signal levels fluctuate significantly, so a compact circuit for improving the S/N as described above is increasingly desired.

Therefore, an object of the present invention is to provide a receiver capable of automatically switching the IF bandwidth with a simple structure and improving the S/N ratio at the time of weak input.

The present invention is directed to a receiving system with an auto-search function, and such a receiver is equipped with a so-called search stop signal generation circuit that stops the search operation, detects the IF signal level, and detects the level when the level is lower than a predetermined value. Since it is designed to generate a safety stop signal when the level becomes too high, the IF signal level detection output is used to automatically switch the IF bandwidth. .

The present invention will be explained below with reference to the drawings.

The figure is a circuit diagram showing an embodiment of the present invention, in which an intermediate frequency signal is applied to bandpass filters 1 and 2 via a capacitor C1.

The bandpass filters 1 and 2 are designed to have different bandwidths, and the former has a characteristic that sufficiently satisfies the IF band during normal reception, and the latter has a characteristic that is narrower than Takagi. It is designed to have the following.

As shown in the figure, the selection switching circuit for these two-band filters 1 and 2 includes diodes D1 to D4 and resistors R1 to R8.
It becomes more.

That is, the IF signal is applied to bandpass filters 1 and 2 through diodes D1 and D3, respectively, and their outputs are connected in common through resistor R2, diode D2, resistor R5, and diode D4, and are applied to the IF signal through capacitor C2. Applied to amplifier 3.

The common cathode side of the switching diodes D1 and D3 is grounded via a resistor R7, and the switching control signals B and A are applied to each anode of the switching diodes D1 and D3 via resistors R1 and R4, respectively.

Further, the common cathode side of the switching diodes D2 and D4 is grounded via a resistor R8, and switching control signals B and A are applied to each of these anodes via resistors R3 and R6.

The output of the IF amplifier 3 is converted into an audio output by an AM detection circuit 5, and then becomes an audio output via a filter circuit 6 consisting of a resistor R9, a capacitor C3, and a transistor Q0.

Furthermore, when performing an auto search (automatic station selection) operation, it is necessary to generate an auto search stop (automatic station selection operation stop) signal, but as an example of such a circuit, A bandpass filter 7 having sharp IF band characteristics is provided, and a rectifying and smoothing circuit 8 is provided to convert the output signal level of this filter 7 to a DC level.

This circuit consists of capacitors C4, C5 and diode D5.
, D6, and generates a positive DC signal in response to the input IF signal level.

A circuit 9 is provided which generates a search stop signal when the DC signal level exceeds a predetermined value, and the search is automatically stopped by this stop signal.

In the present invention, this positive DC signal is adjusted by a volume VR and compared with a reference signal level E by a comparator 10, and the comparison output is used as the above-mentioned bandwidth switching control signals A and B.

That is, when the positive DC signal level is higher than the reference voltage level E, the output A of the comparator 10 is set to a low level, and the inverted output B of the inverter 11 is set to a high level.

As a result, diodes D1 and D2 are both forward biased and turned on, while the other diodes D3 and D4 are both reverse biased and turned off.

Therefore, the IF bandpass filter 1 having a normal bandwidth is
The signal will pass through, and it will be faithfully received and reproduced all the way to Takagi.

On the other hand, when the DC signal level is lower than the reference voltage level E, the output A of the comparator 10 is inverted and becomes a high level, and the inverter output B becomes a low level.

Therefore, the diodes D3 and D4 are forward biased, and the IF signal passes through the filter 2 having narrow band characteristics, thereby cutting off the high frequency signal.

As a result, noise can be reduced.

In addition, when the power is low, the high level output A of the comparator 10 is used to turn on the switching transistor Q1 of the filter 6, activating the low-pass filter consisting of the resistor R9 and the capacitor C3, and controlling the narrow band of the IF filter. The S/N ratio can be improved more effectively by setting the audio frequency characteristics at any time.

As described above, according to the present invention, when the RF input level is high, a wider band BPF can be used, and when the RF level decreases, a narrower band BPF can be used.
/N, automatic switching control of the above-mentioned BPF is possible, and the circuit structure is simplified by using the auto search stop circuit for detecting the level of the RF signal and even the IF signal. , especially suitable for car radios.

Note that although the BPF switching circuit is specifically shown using resistors and diodes in the above, it is not limited to the illustrated configuration and various modifications are possible.

Further, although the rectifying and smoothing circuit 8 is configured to generate a positive DC level, it may be configured to generate a negative DC level. In this case, the reference voltage E is also set to be negative and the absolute value levels are compared. Of course, you can do whatever you want.

Furthermore, although the low-pass filter 6, which is turned on and off by the output A of the comparator 10, is inserted into the audio line, it is obvious that it may be omitted.

[Brief explanation of the drawing]

The figure is a circuit diagram showing an embodiment of the present invention. Explanation of symbols of main parts 1, 2...Band filter, 8...Level detection circuit, 9...Search stop signal generation circuit, 10...Comparison vessel.

Claims (1)

[Scope of utility model registration request] The apparatus includes a DC signal generating means for generating a DC signal corresponding to the intermediate frequency signal level, and a stop signal generating means for generating an automatic tuning operation stop signal when the absolute value of the DC signal level exceeds a predetermined value. A receiver capable of automatic tuning, comprising control signal generating means for generating a control signal for controlling the frequency bandwidth of the intermediate frequency signal stage when the absolute value of the DC signal level is smaller than the reference level. The receiver is characterized in that the frequency bandwidth is switched to a smaller value during the existence period of the control signal.