JPH0321091Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an FM-TV automatic switching circuit.

Previously, it was possible to receive both FM broadcast and TV broadcast channels 1 to 3 (76 to 108 MHz).
There is an FM wideband receiver. However, while FM broadcasting has a maximum modulation deviation of 75KHz and de-emphasis 50μsec, TV broadcasting has a maximum modulation deviation of 25KHz and
There is a difference in de-emphasis of 75 μsec, which causes a difference of about 10 dB in detection output and about 3 dB in frequency characteristics between FM and TV. In order to eliminate this difference, conventionally the characteristics of the de-emphasis circuit 3 provided between the IF and detection stage 1 and the demodulation stage (MPX) 2 have been changed as shown in FIG.
Switching was performed using a switch SW operated using the rotation angle of the variable capacitor or the movement of the dial pointer.

That is, an output level adjustment resistor 3c is inserted between the IF and the detection stage 1 and the demodulation stage 2 via bypass capacitors 3a and 3b, and a series circuit of switching diodes 3d and 3e is connected in parallel with the resistor 3c. , the cathode of diode 3d and diode 3e
The connection point between the anode of the diode 3e and one end of the resistor 3c is connected to the ground via the frequency characteristic adjustment capacitor 3f, and the connection point between the cathode of the diode 3e and one end of the resistor 3c is connected to the ground via the bias resistor 3g, and the anode of the diode 3d and the resistor are connected to the ground through the bias resistor 3g. The connection point with the other end of 3c is connected to the positive power supply via bias resistor 3h and switch SW.

With the above configuration, by tuning the receiver,
When receiving TV band broadcasting, turn on the switch SW and connect the resistor 3 from the positive power supply.
h, a current is caused to flow through the diodes 3d, 3e and the resistor 3g to the ground to turn on the diodes 3d, 3e. Therefore, the output level adjustment resistor 3
c is short-circuited, and a frequency characteristic adjustment capacitor 3f is inserted between the IF and the output of the detection stage 1 and the ground. However, when receiving FM broadcasting and the switch SW is off, capacitor 3f is not inserted and the IF and detection stage 1
A level adjustment resistor 3c is inserted between MPX2 and the input level of MPX2 is lowered.

In the conventional circuit described above, when a TV broadcast band is selected, the output level adjustment resistor is unconditionally short-circuited. For this reason, there is no problem when receiving TV broadcasts, that is, when the TV broadcasts are synchronized, but if noise is generated between stations that are not synchronized, this noise will be passed through without being attenuated in any way. This has the disadvantage of causing discomfort to broadcast receivers.

Therefore, the present invention was devised to eliminate the above-mentioned drawbacks of the conventional system, and it adjusts the difference in level and frequency characteristics when receiving FM and TV broadcasts, and also enables the above-mentioned circuit switching only when tuning to TV. The object of the present invention is to provide an FM-TV automatic switching circuit which eliminates the problem of inter-office noise.

Hereinafter, an embodiment of the present invention will be described with reference to FIG. 2, in which the same components as those in FIG. 1 are given the same reference numerals.

In FIG. 2, reference numeral 4 denotes a tuning detection and display circuit, to the input of which a part of the IF signal from the IF and detection stage 1 is applied. When the IF signal is out of tune, its level is below a predetermined level, so the IF signal
Transistor _Q1 , to which the signal is applied to the base via the resistor, is off. At this time, transistor Q ₂ is turned on, so transistors Q ₃ and Q ₄ are turned off, and transistor Q 2 is turned on.
The tuning indicator LED connected to the collector of Q ₄ is not lit. On the other hand, the IF signal level during tuning becomes a predetermined value or higher, and the transistor _Q1 is turned on in response to this. transistor
When Q ₁ turns on, transistor Q ₂ , which had been on, turns off, and accordingly, both transistors Q ₃ and Q ₄ turn on. As a result, the tuning indicator LED lights up and the collector potential of transistor _Q3 , which is normally at L level, goes to L.
It rises to H level.

5 is a switching control circuit, and its input 5a has approximately 2~
A voltage that changes depending on the 22V tuning voltage is applied. The voltage applied to the input 5a is applied to the resistors R ₁ ,
The voltage is divided by _R2 and applied to the base of transistor _Q5 via resistor _R3 . Transistor Q ₅
The collector is connected to +B via resistor _R4 , and the emitter is connected to ground, and a voltage greater than or equal to the tuning voltage Vx, which corresponds to approximately 90MHz on the border of the FM and TV reception bands, is applied to the input 5a. When input, transistor _Q5 turns on in response. The collector of the transistor _Q5 is connected to the base of the first switching transistor _Q6 via a resistor _R5 , and the emitter is connected to ground via the collector-emitter path of the second switching transistor _Q7 . The collector of transistor Q ₆ is connected to +B via resistor R ₆ and also connected to resistor 3
It is connected to the cathode of diode 3d via h. And the base of transistor Q ₇ is a resistor
It is connected to the collector of the above transistor _Q2 via _R7 .

With the above configuration, transistor Q ₆ has input 5
When the voltage of a is lower than Vx, that is, when receiving FM band and transistor _Q5 is off, it turns on and its collector goes to L level, so diodes 3d and 3
e are both turned off, IF and detection stage 1 and MPX2
A level adjustment resistor 3c is inserted in series between the two.

However, if the voltage at input 5a is higher than Vx, i.e.
During TV band reception, transistor Q ₅ is in a state where it can be turned on, and at this time, when transistor Q ₇ is also in a state where it can be turned on, both transistors Q ₅ and Q ₇ are turned on. When transistors Q ₅ and Q ₇ turn on,
Transistor _Q6 , which had been on until then, turns off, and its collector becomes H level. Therefore,
Diodes 3d and 3e are both turned on, resistor 3c is short-circuited, and capacitor 3f is inserted between IF and the output of detection stage 1, thereby performing circuit switching.

As mentioned above, transistor Q ₇ is controlled by the collector potential of transistor Q ₂ , and when out of tune, transistor Q ₂ is turned on and its collector is at L level. Even though transistor _{Q 5} is in a state where it can be turned on, transistor Q ₇ is in an off state.
At this time, transistor _Q5 is also off, so diodes 3d and 3e are both off,
A resistor 3c is inserted in series between the IF and the output of the detection stage 1. Therefore, even in the TV reception band, when out of tune, the output of the IF and detection stage 1 will be attenuated by the resistor 3c, and the unpleasant inter-office noise when out of tune will be suppressed to a low level. Become.

As explained above, according to the present invention, the fact that a TV broadcast band is selected and the fact that tuning is detected are ANDed, and only when the TV broadcast band is selected and tuning is detected, Since the output level adjustment resistor is short-circuited and a frequency characteristic adjustment capacitor is inserted between the output of the IF and detection stage and the ground, tuning is not achieved even if the TV broadcast band is selected. sometimes,
The output level adjustment resistor is inserted in series between the IF, the detection stage, and the demodulation stage, and even if the TV broadcast band is selected, the inter-station noise generated when the tuning is not achieved will be reduced to the output level. The adjustment resistor ensures sufficient attenuation. Therefore, the broadcast receiver can receive broadcasts with appropriate fidelity and level simply by receiving FM and TV broadcasts without causing discomfort due to increased inter-station noise.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional circuit, and FIG. 2 is a circuit diagram showing an embodiment of a circuit according to the present invention. 1... IF and detection stage, 3... De-emphasis circuit, 4... Tuning detection display circuit, 5... Switching control circuit, 5a... Input of voltage corresponding to tuning voltage, Q ₅ , Q ₇ ... Transistor.

Claims (1)

[Claim for Utility Model Registration] It is possible to continuously receive from the FM broadcast band to the TV broadcast band, and when the FM broadcast band is selected in conjunction with the tuning operation, the output level between the IF and the detection stage and the demodulation stage. A de-emphasis circuit that inserts an adjustment resistor in series, shorts the output level adjustment resistor when a TV broadcast band is selected, and inserts a frequency characteristic adjustment capacitor between the output of the IF and detection stage and the ground. In an FM wideband receiver equipped with a TV broadcast band, the fact that the TV broadcast band is selected and the fact that tuning is detected is ANDed, and only when the TV broadcast band is selected and tuning is detected, the above-mentioned Short-circuit the output level adjustment resistor and
An FM-TV automatic switching circuit characterized by comprising a circuit for inserting a frequency characteristic adjustment capacitor between the output of the IF and detection stage and the ground.