JPS6314505Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention provides automatic intermediate frequency (IF) filter switching that automatically switches the IF filter in an AM receiver that has an intermediate frequency (IF) filter that can be switched to wideband and narrowband. It is related to circuits.

Currently, in Japan, the channel spacing of AM broadcasting stations is 9KHz, so if you try to reproduce a wide band on an AM receiver, you will not be able to select the adjacent stations. On the other hand, there is a mutually contradictory relationship in that increasing selectivity comes at the expense of sound quality.

Therefore, conventionally, as shown in FIG.
The output of the f _IF ±9KHz detection circuit 2 detects the presence of an adjacent station, and upon this detection, the IF filter 3 in the latter stage of the mixing circuit 1 is automatically switched from wideband to narrowband. I was doing it. However, in the AM band, the audio component has a spectrum of about f _IF ±15 KHz, so the detection circuit 2 is likely to malfunction. Furthermore, due to fading that occurs at night, when a station with f _IF ±9KHz exists or disappears, the IF filter 3 is switched each time, making it difficult to reproduce sound.

This invention was made in view of the above points,
The purpose is to provide an automatic IF filter switching circuit for an AM receiver that is unlikely to malfunction and has a good switching feeling.

The present invention will be explained below with reference to FIG. 2 and subsequent figures, in which the same parts as in FIG. 1 are given the same reference numerals.

Figure 2 shows _the basic configuration of the automatic IF filter switching circuit according to the present invention.
Based on the signals from the Hz detection circuit 2 and the mute signal generation circuit 4 which generates the mute signal during the channel selection operation.
A control circuit 5 for controlling switching of the IF filter 3 is provided. This control circuit 5 includes the detection circuit 2
or the mute signal from the mute signal generation circuit 4 to narrow the IF filter 3,
Even after the outputs of both circuits 2 and 4 disappear, control is performed to maintain the IF filter 3 in a narrow band state for a certain period of time. The specific circuit configuration is explained in the third section.
In the figure, Q ₁ to Q ₁₇ are transistors, R ₁ to R ₁₅ are resistors, C ₁ and C ₂ are capacitors, D is a Zener diode, and I ₀ is a constant current source. The operation of this circuit will be described in detail below with reference to FIG. 4 showing waveforms at various locations.

Now, the output a of the f _IF ±9KHz detection circuit 2, the output b of the mute signal generation circuit 4, and the output c of the control circuit 5 are at the L level, and the output of the control circuit 5 is at the H level (Vcc (R ₁₂ /(R ₁₂ + R ₁₁ ))) and IF
Assume that the filter 3 is in a broadband state. At this time, transistors Q ₂ to Q ₄ , Q ₆ to Q ₁₂ , Q ₁₄ and
Q ₁₆ is non-conducting.

When the f _IF ±9KHz detection circuit 2 detects an adjacent station and its output a becomes H level at time t ₁ , capacitor C ₁ is charged through resistor R ₃ , and at time t ₂ the first fixed time τ ₁ has elapsed. The potential is Vcc (R ₂ / (R ₁
+R ₂ )) is reached. As a result, when transistor Q ₂ becomes conductive and transistor Q ₁ becomes non-conductive, transistors Q ₃ and Q ₄ become conductive, and accordingly transistor Q ₁₅ becomes conductive. this transistor
Due to the conduction of _Q15 , the output c which was at H level becomes L.
The transistor _Q17 becomes non-conductive, the transistor _Q16 becomes conductive, the output c becomes H level, and the IF filter 3 is switched from wide band to narrow band. Also, at this time, transistor _Q14 is conductive and transistor _Q12 is conductive.

By the way, the above-mentioned fixed time τ ₁ means that even if the audio signal such as music contains a frequency component of f _IF ±9KHz and the detection circuit 2 detects this, the IF filter 3 will be erroneously switched due to this. is set to ensure that there is no

Next, at time _t3 , when the output a of the detection circuit 2 becomes L level, the transistors _Q2 to _Q4 become non-conductive, and the transistor _Q15 also becomes non-conductive. However, at this time, the states of transistors Q ₁₆ and Q ₁₇ remain unchanged,
Although the outputs c and are at L level and H level, respectively, transistors Q ₁₃ and Q ₁₄ become non-conductive, and transistor Q ₁₂ also becomes non-conductive. This causes capacitor C ₂ to be charged via resistor R ₇ . As this charging progresses, at time t ₄ after the second fixed time τ ₂ has elapsed, the charging potential becomes the sum of the Zener potential V _z of the Zener diode D and the conduction voltage V _BE of the transistor, that is, V _z + V _BE . , transistor Q ₁₀ is conductive, which causes transistor Q ₁₆ to be non-conductive, transistor Q ₁₇
becomes conductive, the outputs c and become L level and H level, respectively, and the IF filter 3 is switched from narrow band to wide band.

The above fixed time τ ₂ is the time for charging the capacitor C ₂ to V _z + V _BE , and is determined by the values of the resistor R ₇ and the capacitor C _2. Actually, assuming R ₇ = 300K and C ₂ = 22 μ, τ ₂ Set to ≒10 seconds. Note that the resistance _R8 is
It is for discharging capacitor C ₂ and is a transistor
It is set to a low value of about 100Ω so that Q ₁₂ is quickly discharged when it becomes conductive.

Next, at time _t5 , when the output a of the detection circuit 2 is at the L level, the output b of the mute signal generating circuit 4 becomes the H level, transistors _Q7 to _Q10 become conductive, and as a result, the transistor _Q17 becomes conductive. non-conducting,
Transistor _Q16 becomes conductive, outputs c and become H level and L level, respectively, and IF filter 3 is switched from wide band to narrow band.

At this time, due to the conduction of the transistor _Q7 ,
Transistor _Q5 , which had been conductive until then, becomes non-conductive and transistor _Q6 becomes conductive.
Further, due to the conduction of the transistor _Q10 , the transistor _Q13 , which had been conductive until then, becomes non-conductive, and the transistor _Q12 becomes non-conductive. Therefore, the resistance R ₆
By setting the value of C to be sufficiently smaller than the resistor R ₇ and R ₆ << R ₇ , the capacitor C ₂ becomes a transistor.
When Q ₆ becomes conductive and transistor Q ₁₂ becomes conductive, it is immediately charged to the Zener voltage V _z and holds that potential.

When the output b of the circuit 4 becomes L level at time _t6 , the transistors _Q7 to _Q10 become non-conductive.
In response, transistor _Q5 becomes conductive and transistor _Q6 becomes non-conductive, and capacitor _C2 becomes a resistor
_R7 is further charged. At time _t7 , which is the third fixed time _τ3 after time _t6 , when the charging voltage reaches _Vz + _VBE , transistor _Q11 becomes conductive, and transistors _Q16 and _Q17
become non-conductive and conductive, respectively, and outputs c and c become L level and H level, respectively, and IF
Filter 3 is switchable from narrowband to wideband.

The above-mentioned fixed time τ ₃ is a fraction of the second fixed time τ ₂
By setting the relationship _t2 > _t3 > _{t1 among t1, t2, and t3 , the IF} filter 3 can be smoothly switched. Even if the output b goes to the H level at time _t8 , and the output c goes to the H level accordingly and is switched to the narrow band, and then the output b goes to the L level at time _t9 , if the output a goes to the H level at time _t10 within a time shorter than time _τ3 , the output c will not go to the L level but will maintain the H level, and the IF filter 3 will be maintained in the narrow band state without being switched to the wide band.

As described above, the present invention switches the IF filter to the narrow band after a first fixed time delay in response to the detection of an adjacent station, and also switches the IF filter to the narrow band for a second fixed time even after the neighboring station is detected. Since the IF filter is held in the same state, even if the detection circuit responds to the frequency components of audio signals such as music, the IF filter is less likely to be switched erroneously, and furthermore, due to phasing, existing adjacent stations are eliminated. However, the IF filter is less likely to be switched by mistake, and there are fewer IF filter switching malfunctions.
Automatic switching with good feeling is possible.

In addition, since the IF filter is switched to a narrow band by the mute signal generated during the tuning operation, more accurate tuning can be achieved, and even if the mute signal disappears, it will take a relatively long time. Since the narrow band is maintained for a certain period of time, if there is a station adjacent to the tuned station, the narrow band state is continuously maintained without being switched to the wide band.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional example, Fig. 2 is a block diagram showing the basic configuration of the present invention, and Fig. 3 is a block diagram showing a conventional example.
FIG. 4 is a circuit diagram showing a specific example of a part of the figure, and a waveform diagram showing waveforms at various locations in FIGS. 2 and 3. 2...Detection circuit (f _IF ±9KHz detection circuit), 3...
IF filter, 4... means (mute signal generation circuit), 5... control circuit.

Claims (1)

[Scope of utility model registration request] In an AM receiver having an intermediate frequency filter that can be switched to a wide band and a narrow band, and means for generating a mute signal during a channel selection operation, a detection circuit that generates an output signal in response to detection of an adjacent station;
The intermediate frequency filter is switched to a narrow band after a first fixed period of time in accordance with the output signal of the detection circuit, and this state is maintained for a second fixed period of time even after the output signal disappears, and and a control circuit that performs control to switch the intermediate frequency filter to a narrow band in response and maintain this state for a third fixed period of time longer than the first fixed period of time even after the mute signal disappears. Intermediate frequency filter automatic switching circuit.