JPH0983304A - Automatic channel selection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically select all regular broadcast waves at practical levels with no influence of an AGC function. SOLUTION: An automatic channel selection device is provided with a switching circuit 26 which switches the AGC loop time constant against an IF amplifier circuit 15 in an automatic channel selection mode, a detection circuit which generates the DC voltage accordant with the receiving level of the broadcast wave from the output of an RF amplifier circuit 12, a 1st decision circuit which compares the DC voltage level with a 1st reference level, a control circuit which reduces the changed variable of frequency of the local oscillation signal based on the decision result of the 1st decision circuit, a gain control circuit which controls the gain of the circuit 12 based on the DC voltage level of the detection circuit in the automatic channel selection mode, a 2nd decision circuit which compares the the DC voltage level of the detection circuit with a 2nd reference level, a gain limiting circuit which reduces the gain of the circuit 12 down to a prescribed level based on the decision result of the 2nd decision circuit and regardless of the DC voltage level of the detection circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to, for example, AM (Am
plitude Modulation) and FM (Frequency Modulatio)
The present invention relates to a receiving system for receiving radio broadcasts or television broadcasts by the n) method, and particularly to improvement of an automatic channel selecting device for automatically selecting a channel for the broadcast.

[0002]

2. Description of the Related Art As is well known, in a receiving system such as the one mentioned above, in order to facilitate the tuning operation for a plurality of broadcasting channels, the tuning of broadcast radio waves, that is, broadcasting waves is automatically performed. The automatic channel selection device is widely adopted. This automatic channel selecting device automatically performs an operation of searching for a broadcast wave from the received radio wave and an operation of storing the frequency of the searched broadcast wave.

FIG. 3 shows such a conventional automatic channel selecting apparatus. That is, the broadcast wave received by the antenna 11 is supplied to the RF (Radio Frequency) amplifier circuit 12, amplified, and then mixed with the local oscillation signal output from the local oscillation circuit 14 by the mixing circuit 13. , Is converted to an intermediate frequency signal. The intermediate frequency signal output from the mixing circuit 13 is IF (Intermediate F
requency) is supplied to the amplification circuit 15 and amplified.

The intermediate frequency signal amplified by the IF amplifying circuit 15 is supplied to the detecting circuit 16 to be demodulated and then supplied to the microcomputer 17. The intermediate frequency signal amplified by the IF amplifier circuit 15 is supplied to an AFT (Automatic Fine Tuning) circuit 18, converted into a voltage according to the received broadcast wave frequency, and then supplied to the microcomputer 17. It

The microcomputer 17 determines the presence / absence of a broadcast wave based on whether the level of the demodulated signal output from the detection circuit 16 exceeds a preset reference level. In addition, this microcomputer 1
When it is determined that the broadcast wave exists, the AFT circuit 1
Fine tuning control is performed on the basis of the voltage level output from 8 to determine the regular tuning frequency for the broadcast wave.

Here, the microcomputer 17
Is the built-in R when the automatic tuning key 19 is operated.
The automatic tuning operation is executed based on the automatic tuning program stored in the OM (Read Only Memory) 20.
That is, the microcomputer 17 controls the tuning voltage generating circuit 21 to sequentially change the tuning voltage VT generated from the low level to the high level.

The tuning voltage VT output from the tuning voltage generating circuit 21 is supplied to the RF amplifying circuit 12 to drive it, and to the local oscillating circuit 14 to supply the frequency of the local oscillating signal. Be used for control. In other words, the tuning voltage VT is sequentially changed from a low level to a high level, so that the frequency of the local oscillation signal is sequentially changed. As a result, in the reception band, the reception frequency is sequentially changed from low to high. It will be changed and a search for broadcast waves will be performed.

In such a broadcast wave search state, when a demodulation signal having a level exceeding the reference level is obtained from the detection circuit 16, the microcomputer 17 determines that a broadcast wave exists, and the AFT circuit 18 The receiving frequency is finely adjusted by finely increasing or decreasing the tuning voltage VT based on the output voltage level, and the regular receiving frequency for the broadcast wave is determined.

Then, the microcomputer 17 stores the tuning voltage VT corresponding to the determined reception frequency in the memory 2.
2, the automatic tuning operation is realized by repeating the series of processes described above within the reception band.

By the way, in the receiving system as described above, when the received radio wave is strong or weak, the level of the demodulated signal output from the detection circuit 16 varies, and the demodulated signal is distorted particularly when a large input signal is generated. .

Therefore, conventionally, a gain control signal corresponding to the strength of the received radio wave or a gain control signal corresponding to a large input signal is generated based on the demodulated signal output from the detection circuit 16, and this gain control signal is generated. The gain of the RF amplifier circuit 12 and the IF amplifier circuit 15 is controlled by the AGC (Automatic), which controls the level fluctuation and distortion of the demodulated signal.
It is generally performed to add a Gain Control function.

However, when the automatic tuning operation is performed in the receiving system having the AGC function, for example, when the broadcast waves are searched in order from the lower receiving frequency to the higher receiving frequency, the tuning voltage VT is set to the low level. However, if the amount of change per unit time is large, broadcast waves will be received rapidly one after another, and the AGC function cannot follow because of its time constant. Then, since the RF amplifier circuit 12 and the IF amplifier circuit 15 are saturated and distortion occurs in the signal, many unnecessary (pseudo) signals that are not regular broadcast waves are generated, and the tuning voltage VT corresponding to the unnecessary signal is also stored in the memory 22. There is a problem of being remembered by.

When such a problem occurs, the memory 22
Since the number of broadcast channels that can be stored in is limited, the inconvenience of not being able to automatically select a desired broadcast wave occurs. This problem becomes more remarkable as the level of the received signal is higher and the number of broadcast waves is higher.

On the other hand, if the change amount of the tuning voltage VT per unit time is reduced and the broadcast wave is searched slowly, the AGC function can follow up, but the above problem is solved. However, it takes a long time from the start to the end of the automatic channel selection operation, which causes a problem that the practicality is impaired.

[0015]

As described above, in the conventional automatic channel selection device, when the automatic channel selection operation is performed quickly, the AGC function does not follow up and many unnecessary signals are generated, and these unnecessary signals are also broadcast. It has the problem of being remembered as waves. Further, if the automatic channel selection operation is performed slowly in order to solve this problem, a long time is required, which impairs practicality.

Therefore, the present invention has been made in consideration of the above circumstances, and is extremely excellent in that all regular broadcast waves can be automatically selected at a practical level without being affected by the AGC function. An object is to provide an automatic channel selection device.

[0017]

An automatic channel selection device according to the present invention comprises a high frequency amplification means for amplifying a received broadcast wave,
Frequency conversion means for mixing the high frequency signal output from the high frequency amplification means with the local oscillation signal to convert it into an intermediate frequency signal, intermediate frequency amplification means for amplifying the intermediate frequency signal output from the frequency conversion means, and Detection means for demodulating the intermediate frequency signal output from the intermediate frequency amplification means, and automatic gain control means for controlling the gains of the high frequency amplification means and the intermediate frequency amplification means based on the demodulated signal output from the detection means. , A tuning control means for sequentially changing the frequency of a local oscillation signal given to the frequency conversion means in a required state of an automatic tuning operation by a constant change amount.

Then, in the non-demanding state and the demanding state of the automatic tuning operation, the time constant switching means for switching the loop time constants for determining the response characteristics of the automatic gain control to the intermediate frequency amplifying means, and the high frequency amplification means. Signal strength detecting means for generating a DC voltage corresponding to the reception level of the broadcast wave based on the output signal of the means or the intermediate frequency amplifying means, and the DC voltage level output from the signal strength detecting means has a predetermined first value. Based on the determination result of the first signal strength determination means and the first signal strength determination means, and the amount of change in the frequency of the local oscillation signal by the tuning control means. In the state of demand for automatic tuning operation, the gain control for the high frequency amplification means is replaced with the demodulation signal output from the detection means instead of the signal strength detection means. The gain control switching means for switching the operation based on the DC voltage level output from the means and the DC voltage level output from the signal strength detection means are determined to be higher or lower than a predetermined second reference level. Based on the determination result of the second signal strength determination means and the second signal strength determination means, the gain of the high frequency amplification means is reduced to a predetermined value regardless of the DC voltage level output from the signal strength detection means. A gain limiting means is provided.

According to the above configuration, when the automatic tuning operation is requested by the time constant switching means, the loop time constant for determining the response characteristic of the automatic gain control with respect to the intermediate frequency amplifying means is shortened. If the broadcast wave input level is determined to be high based on the output level of the high frequency amplifying means or the intermediate frequency amplifying means, the frequency of the local oscillation signal by the tuning control means is changed. By reducing the amount of change in the signal to prevent the demodulated signal level from changing suddenly, it is possible to follow the automatic gain control means at the time of automatic channel selection and prevent the occurrence of distortion in the intermediate frequency amplification means. Will be able to.

Further, during the automatic tuning operation, the automatic gain control by the original demodulation signal is cut off for the high frequency amplifying means, and the automatic gain control is performed based on the DC voltage level output from the signal strength detecting means. Is performed, that is, the gain is controlled by another loop in order to quickly reduce the gain when the broadcast wave input level becomes high, so the saturation of the high frequency amplification means is prevented and the distortion is prevented. Therefore, all regular broadcast waves can be automatically tuned at a practical speed without being affected by the automatic gain control function.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. In FIG. 1, the same parts as those in FIG. 3 are denoted by the same reference numerals. That is, the output signal of the RF amplifier circuit 12 is the mixed circuit 13
To the frequency conversion, capacitors C1, C2, resistors R1 to R3, diode D and FE.
It is supplied to a signal strength detection circuit 23 composed of a T (Field Effect Transistor) Q1 and the signal strength is detected as a DC voltage level.

The source voltage of the FET Q1, which is the detection output of the signal strength detection circuit 23, is the transistors Q2 to Q6.
And a resistor R4 to R11 to supply the signal strength determination circuit 24 to determine whether the voltage level is higher or lower than a preset reference level. Then, the collector voltage of the transistor Q6, which is the determination output of the signal strength determination circuit 24, is supplied to the microcomputer 17 as the determination signal Vh.

At the AGC gain control signal input terminal of the RF amplification circuit 12, the gain control signal output from the detection circuit 16 is supplied with the AGC signal changeover switch circuit 25.
And the gain control signal generated by the signal strength determination circuit 24 is supplied via the switch SW2 forming the AGC signal changeover switch circuit 25. .

Further, the IF amplifier circuit 15 includes capacitors C3 and C4 for determining the AGC time constant.
And a time constant switching circuit 26 including a switch SW3. Then, the switch SW2 controls the automatic tuning signal Va output from the microcomputer 17.
ON / OFF control is performed based on
W3 is on / off controlled based on a signal obtained by inverting the automatic tuning signal Va by the inverter INV.

The automatic tuning signal Va becomes H (high) level in a state where the automatic tuning key 19 is operated and the microcomputer 17 is performing the automatic tuning operation.
It goes to L (low) level when the automatic tuning operation is not performed. The switches SW1 to SW3 are controlled to be in the on state when the H level is supplied to the on / off control terminals, and are controlled to be in the off state when the L level is supplied to the on / off control terminals.

The operation of the above arrangement will be described below. First, when the automatic tuning key 19 is not operated, since the L-level automatic tuning signal Va is output from the microcomputer 17, the switch SW2 is controlled to the off state and the switch S
W1 and SW3 are controlled to be on.

Therefore, the RF amplifier circuit 12 is supplied with the gain control signal output from the detection circuit 16, and
Gain control processing for C is performed. Further, the IF amplifier circuit 15 is subjected to gain control processing for AGC based on the gain control signal output from the detection circuit 16 with a time constant based on the parallel combined capacitance of the capacitors C3 and C4. Stable reception is performed by the AGC function.

On the other hand, when the automatic tuning key 19 is operated,
Since the H-level automatic tuning signal Va is output from the microcomputer 17, the switch SW2 is controlled to be in the ON state and the switches SW1 and SW3 are controlled to be in the OFF state. In this case, first, since the capacitor C3 is disconnected, the AGC time constant of the IF amplifier circuit 15 is changed to a short time based only on the capacitance of the capacitor C4, so that the response characteristic is improved.

The output signal of the RF amplifier circuit 12 is input to the signal strength detection circuit 23, and the capacitor C
After passing through 1, the component of one polarity is taken out by the diode D and smoothed by the capacitor C2, the intensity is detected as a DC voltage level across the terminals of the capacitor C2. This DC voltage is transmitted to the signal strength determination circuit 2 through the source follower circuit by the FET Q1.
4 are supplied to the bases of the transistors Q2 and Q4, respectively.

Of these, the transistor Q4 is differentially connected to another transistor Q5 having the same polarity, and the base of the transistor Q5 has a reference level V1 obtained by dividing the power supply voltage + B by the resistors R6, R8 and R11. Is being applied. This reference level V1 is, as shown in FIG.
When a broadcast wave of a level (60 to 70 dB) for starting the AGC operation is input to the RF amplification circuit 12, F
Corresponds to the voltage level generated at the source of ETQ1.

Here, the broadcast wave input level becomes high, and F
When the source voltage level of ETQ1, that is, the base voltage level of the transistor Q4 becomes higher than the reference level V1, the transistor Q4 is turned on, the transistor Q6 is also turned on, and the H-level determination signal Vh is supplied to the microcomputer 17. To be done. When the H-level determination signal Vh is supplied, the microcomputer 17 controls so that the tuning voltage VT generated from the tuning voltage generation circuit 21 is reduced in the amount of change per unit time.

As a result, the level change of the demodulated signal output from the detection circuit 16 also becomes gentle, and the IF amplification circuit 1
Coupled with the shortened AGC time constant of No. 5 and improved response characteristics, it is possible to follow the AGC operation and prevent the IF amplifier circuit 15 from being distorted.

Further, in this automatic channel selection state, since the switches SW1 and SW2 are controlled to the off state and the on state, respectively, the gain control signal for the RF amplification circuit 12 constitutes the signal strength determination circuit 24. It is provided from the collector of the transistor Q2.

This transistor Q2 is differentially connected to another transistor Q3 having the same polarity, and a reference level V2 obtained by dividing the power supply voltage + B by resistors R6, R8 and R11 is applied to the base of the transistor Q3. Has been done. The reference level V2 is set higher than the reference level V1 by the voltage drop due to the resistor R8 (1 to 2 dB in terms of the input level).

Here, when the broadcast wave input level is low,
When the source voltage level of Q1, that is, the base voltage level of the transistor Q2 is lower than the reference level V2, the transistor Q2 is in the off state, and therefore the resistors R4 and R5.
The AGC voltage (maximum sensitivity voltage) obtained by dividing the power supply voltage + B is supplied to the RF amplifier circuit 12, and the gain thereof is determined.

On the other hand, when the broadcast wave input level becomes higher and the base voltage level of the transistor Q2 becomes higher than the reference level V2, a collector current flows through the transistor Q2 and the collector voltage thereof, that is, the AGC voltage is lowered. In this case, the RF amplification circuit 12 has a reverse AGC characteristic, and it is possible to prevent the output from saturating there because the gain is reduced due to the reduction of the AGC voltage. In addition, the AGC for the RF amplifier circuit 12 at this time has a short loop and therefore has a quick response, and practically causes no problem.

Therefore, according to the above-mentioned embodiment, when the automatic channel selection operation is requested, the IF amplifier circuit 15
When the AGC time constant is switched to be short to improve the response characteristic and it is determined that the broadcast wave input level is increased based on the output level of the RF amplifier circuit 12,
By reducing the amount of change in tuning voltage VT per unit time,
Since the level of the demodulated signal is prevented from changing suddenly, it is possible to follow the AGC operation and prevent the IF amplifier circuit 15 from being distorted.

Further, in the automatic channel selection operation, the gain control signal by the original AGC function is cut off to the RF amplifier circuit 12, and the gain is rapidly lowered when the broadcast wave input level becomes high. Since the gain control is performed by the separate loop, it is possible to prevent saturation of the RF amplifier circuit 12 and prevent distortion. Therefore, all regular broadcast waves can be automatically selected at a practical speed without being affected by the AGC function.

Here, in the above-mentioned embodiment,
The output signal of the RF amplifier circuit 12 is used to determine the magnitude of the broadcast wave input level.
The determination may be made using the output signal of the amplifier circuit 15. It should be noted that the present invention is not limited to the above-described embodiment, and can be variously modified and implemented without departing from the scope of the invention.

[0040]

As described above in detail, according to the present invention,
It is possible to provide an extremely good automatic tuning device capable of automatically tuning all regular broadcast waves at a practical level without being affected by the AGC function.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing an embodiment of an automatic channel selection device according to the present invention.

FIG. 2 is a diagram for explaining a relationship between a broadcast wave input level and a reference level in the same embodiment.

FIG. 3 is a block configuration diagram showing a conventional automatic channel selection device.

[Explanation of symbols]

 11 ... Antenna, 12 ... RF amplification circuit, 13 ... Mixing circuit, 14 ... Local oscillation circuit, 15 ... IF amplification circuit, 16 ... Detection circuit, 17 ... Microcomputer, 18 ... AFT circuit, 19 ... Automatic tuning key, 20 ... ROM, 21 ... Tuning voltage generation circuit, 22 ... Memory, 23 ... Signal strength detection circuit, 24 ... Signal strength determination circuit, 25 ... AGC signal changeover switch circuit, 26 ... Time constant changeover circuit.

Claims (4)

[Claims] 1. A high frequency amplification means for amplifying a received broadcast wave, a frequency conversion means for mixing a high frequency signal output from the high frequency amplification means with a local oscillation signal and converting the same to an intermediate frequency signal, and the frequency conversion means. An intermediate frequency amplifying means for amplifying the intermediate frequency signal outputted from the detecting means, a detecting means for performing a demodulating process on the intermediate frequency signal outputted from the intermediate frequency amplifying means, and the aforesaid based on the demodulated signal outputted from the detecting means. Automatic gain control means for controlling the gains of the high frequency amplification means and the intermediate frequency amplification means, and tuning control for sequentially changing the frequency of the local oscillation signal given to the frequency conversion means with a constant change amount in the required state of the automatic tuning operation. And a response characteristic of automatic gain control with respect to the intermediate frequency amplifying means in a non-requested state and a required state of the automatic channel selection operation. The time constant switching means for switching the loop time constants for determining the values to different values, and a DC voltage corresponding to the reception level of the broadcast wave is generated based on the output signal of the high frequency amplification means or the intermediate frequency amplification means. A signal strength detecting means, a first signal strength judging means for judging whether the DC voltage level output from the signal strength detecting means is higher or lower than a predetermined first reference level, and the first signal strength judging means. Based on the determination result of the signal strength determination means, a change amount control means for reducing the change amount of the frequency of the local oscillation signal by the tuning control means, and in the request state of the automatic tuning operation, to the high frequency amplification means The gain control is switched so as to be performed based on the DC voltage level output from the signal strength detection means instead of the demodulated signal output from the detection means. A gain control switching means, a second signal strength judging means for judging whether the DC voltage level outputted from the signal strength detecting means is higher or lower than a predetermined second reference level, and the second signal strength judging means. Gain limiting means for reducing the gain of the high frequency amplifying means to a predetermined value based on the determination result of the signal strength determining means, irrespective of the DC voltage level output from the signal strength detecting means. Automatic tuning device. 2. The unidirectional element for extracting the component of one polarity from the output signal of the high frequency amplifying means or the intermediate frequency amplifying means, and the capacitance for smoothing the output of the unidirectional element. 2. The automatic channel selection device according to claim 1, further comprising a flexible element. 3. The first signal strength determination means includes a first transistor to which a DC voltage output from the signal strength detection means is applied to the base, and a common connection of the first transistor and the emitter. The automatic channel selection device according to claim 1, wherein the automatic channel selection device comprises a differential circuit including a second transistor to which the first reference level is applied. 4. The third signal strength determination means includes a third transistor to which the DC voltage output from the signal strength detection means is applied to the base, and a common emitter connection of the third transistor and the base. The automatic channel selection device according to claim 1, wherein the automatic channel selection device comprises a differential circuit including a fourth transistor to which the second reference level is applied.