JP2000031745A - Am detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect an original video signal from an IF signal at the time of over-modulation. SOLUTION: An attenuator 10 that decreases an amplitude of an input IF signal is connected between an output terminal of an IF signal amplifier 3 and an input terminal of a phase detector 6 of a PLL. The AM detector is provided with a comparator 1 that compares a level of a video detection signal outputted from a synchronization detector 4 with a threshold voltage to control the attenuator 10 based on the comparison result. The threshold voltage is a voltage equal to a zero barrier bias voltage or a little higher than it. In the case that the level of the video detection signal is higher than the threshold voltage, it is regarded that the input IF signal is over-modulated and the comparator 1 provides an output of an ON signal to the attenuator 10. Since the attenuator 10 attenuates the amplitude of the input IF signal and gives the resulting signal to a phase detector 6, the gain of the phase detector 6 is decreased. Then adverse effect due to the over-modulated input IF signal is reduced. In the case that the level of the video detection signal is lower than the threshold voltage, the input IF signal is not attenuated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AM detector, and more particularly to a detector for detecting a video signal from an image intermediate frequency signal (hereinafter, referred to as an IF signal) during overmodulation.

[0002]

2. Description of the Related Art Generally, in a video signal receiving apparatus for a television or the like, an RF signal input from an antenna is
It is converted into an IF signal that can be easily processed by the mixer circuit. This IF signal is, as shown in FIG.
(B)) is a signal (FIG. 6 (C)) obtained by AM-modulating a video signal (FIG. 6 (A)) with an IF signal having a frequency of 5 in Japan.
8.75 MHz. The video signal illustrated in FIG. 6A is a signal of 100% white.

The degree of modulation of this IF signal varies depending on the country and system, but is generally set to 87.5% to 90%.
In Japan, the modulation depth is usually 87.5%. Here, the meaning of the modulation degree of 87.5% will be described. If the leading end of the synchronization signal is 0% unmodulated and the point at which the carrier wave disappears is 100%, when the video signal is 100% white, it becomes 87.5%. Modulation degree.

FIG. 7 shows a conventional phase-locked loop (hereinafter, referred to as “loop”).
FIG. 3 is a block diagram of an AM detection device using a PLL. This AM detector includes an IF signal amplifier 3 for amplifying an externally input IF signal to an appropriate amplitude, a voltage controlled oscillator 8 for reproducing a demodulation carrier for performing synchronous detection of the IF signal, A + 45 ° phase shifter 5 that advances the phase of the carrier reproduced by the oscillator 8 by 45 °;
Using the carrier whose phase has been advanced by the 45 ° phase shifter 5, the IF signal output from the IF signal amplifier 3 (hereinafter, referred to as “IF signal”).
This is used as an input IF signal), a synchronous detector 4 for performing synchronous detection, a −45 ° phase shifter 7 for delaying the phase of the carrier reproduced by the voltage controlled oscillator 8 by 45 °, and a −45 ° phase shifter 7. A phase detector 6 that outputs a signal corresponding to the phase difference between the carrier whose input signal is delayed and the carrier whose phase is delayed by
And an LL loop filter 9.

[0005] The phase detector 6, the -45 ° phase shifter 7, the voltage controlled oscillator 8 and the loop filter 9 constitute a PLL. The voltage control oscillator 8 is controlled based on the voltage signal output from the phase detector 6 based on the phase difference between the carrier whose phase is delayed by 45 ° and the input IF signal.

The operation of the AM detector shown in FIG. 7 will be described. The IF signal input from the outside is supplied to the IF signal amplifier 3
Is amplified so as to have an appropriate amplitude, and input to the synchronous detector 4 and the phase detector 6. On the other hand, the voltage controlled oscillator 8 outputs a carrier having the same frequency of 58.75 MHz as the carrier signal used at the transmitting end (not shown).

The carrier is first input to the synchronous detector 4 via the + 45 ° phase shifter 5 and secondly to the phase detector 6 via the −45 ° phase shifter 7. . The phase detector 6 controls the oscillation of the voltage controlled oscillator 8 according to the phase difference between the input IF signal and the carrier whose phase is delayed by 45 °.
The synchronous detector 4 detects the carrier whose phase is advanced by 45 ° and the input I
Synchronous detection is performed using the F signal to obtain a video detection output.

FIG. 8 is a schematic diagram showing the relationship between the phase of the input IF signal, the output phase of the voltage-controlled oscillator 8, and the phases of the carriers sent to the synchronous detector 4 and the phase detector 6, respectively. The phase detector 6 does not output a control signal to the voltage controlled oscillator 8 when the phase difference between the two inputs to the phase detector 6 is 90 °, so that the PLL of the AM detector has a phase detection function. Is stabilized when the phase difference between the carrier sent to the device 6 and the input IF signal is 90 °.

The phase difference between the carriers sent from the voltage controlled oscillator 8 to the synchronous detector 4 and the phase detector 6 is 45 ° with respect to the output phase of the voltage controlled oscillator 8.
Since it advances and the other lags by 45 °, it becomes 90 ° as shown in FIG. Therefore, the phase difference between the IF signal input to the synchronous detector 4 and the demodulation carrier is 0 ° or 1 °.
80 °, and efficient synchronous detection is possible.

FIG. 9 is a circuit diagram showing an example of the synchronous detector 4. For example, the synchronous detector 4 is a general circuit called a multiplier or a multiplier.
N transistors Tr1, Tr2, Tr3, Tr4, Tr
5, Tr6 and four load resistors R1, R2, R3, R4
And a constant current source 41. The actual input is a double input of positive / negative phase or a single input of one DC bias. As in the signal waveform example shown in FIG.
AM detection is performed by bidirectionally rectifying the IF signal with a carrier.

[0011]

However, depending on a broadcasting station or a recording medium such as a video tape, the degree of modulation of a video signal exceeds 100% (this is called overmodulation).
There is. FIG. 10 shows an overmodulated IF signal. In FIG. 10, (A) shows a video signal before modulation,
(B) is a carrier signal, and (C) is an IF signal.

In FIG. 10A, a broken line indicates a zero carrier level at which the carrier wave level becomes zero, that is, 100 carrier levels.
% Modulation point (the same applies to other figures). From FIGS. 10B and 10C, it can be seen that in the overmodulated IF signal, the phase of the carrier signal is inverted when white is 100%. The conventional AM detector shown in FIG. 7 has a drawback that the original video signal cannot be demodulated when a signal in which the carrier signal is inverted in phase is received.

The reason why such a defect occurs will be described. As described above, in the synchronous detection method using the PLL, detection is performed by always reproducing the carrier synchronized with the phase of the IF signal by 0 ° or 180 °. Therefore, when the phase of the carrier of the IF signal is suddenly inverted and input to the AM detector and further input to the phase detector 6, the PL
L also inverts the phase of the output carrier of the voltage controlled oscillator 8. However, since the PLL includes the loop filter 9, the output carrier phase is not immediately inverted.

Therefore, in the initial stage of the overmodulation state, the phase of the output carrier of the voltage controlled oscillator 8 is held by the loop filter 9, so that the low-pass filter (FIG.
The video detection signal smoothed by (omitted in FIG. 11) is shown in FIG.
As shown in (C), similarly to the original signal, detection is performed so that the signal level is above the zero carrier. After a certain amount of time has passed, the PLL reverses the phase of the carrier (see FIG. 1).
1 (B)), the phase of the voltage controlled oscillator 8 is also inverted. As shown in FIG. 11 (C), the detection waveform is detected so as to be located on the opposite side of the zero carrier, that is, on the lower side. Will be done. Therefore, the original video signal is not demodulated in a part of the overmodulation section. FIG. 1
1 (A) shows a video signal before the AM modulation, and the signal before the modulation deviates from the standard, and the white level is considerably high.

In order to obtain a normal demodulated output from such an overmodulated IF signal, Japanese Patent Application Laid-Open No. 7-7686 discloses that the amplitude level of an IF signal is set to a level slightly higher than the amplitude at which overmodulation occurs. A signal is compared with a set threshold value, a signal less than the threshold value is regarded as overmodulation, the PLL loop is opened to stop the PLL operation, and the voltage-controlled oscillator is controlled by a pre-hold of the loop filter during that time.
An M demodulator is disclosed.

However, in the AM demodulator disclosed in Japanese Patent Laid-Open Publication No. 7-7686, an amplitude level slightly higher than the amplitude at which overmodulation actually occurs is used as a threshold value. After the operation is stopped, if the amplitude level of the IF signal changes between the threshold level and the amplitude level at which overmodulation actually occurs, the amplitude level is actually larger than the threshold level. Despite the over-modulation, there is a disadvantage that it is regarded as not over-modulation by comparison with the threshold, and the operation of the PLL is restarted.

The present invention has been made in view of the above,
It is an object of the present invention to provide an AM detector capable of detecting an original video signal from an IF signal at the time of overmodulation.

[0018]

According to the present invention, there is provided a phase locked loop for controlling the oscillation of a carrier based on an AM-modulated input signal and a carrier recovered by the phase locked loop. An attenuator for attenuating the input signal input to the phase locked loop;
A synchronous detector that performs synchronous detection of the input signal based on the carrier.

According to the present invention, since the input signal is attenuated by the attenuator, the amplitude of the input signal to the phase locked loop is reduced, and the gain of the phase detector of the phase locked loop is reduced. As a result, the adverse effect of the overmodulation input signal is reduced, so that the phase locked loop is not disturbed by the AM signal whose carrier phase is inverted in the overmodulation section.

In the present invention, there is further provided a comparator for comparing a detection output signal output from the synchronous detector with a reference potential equal to or slightly higher than zero carrier bias, and the attenuator includes: Controlled by the comparison result of the comparator, when the detection output signal is higher than the reference potential, the input signal is attenuated and supplied to the phase locked loop, while when the detection output signal is lower than the reference potential, the phase is reduced. The input signal may be supplied to the lock loop without attenuation.

According to the present invention, when the detection output signal is higher than the reference potential equal to or slightly higher than the zero carrier bias, that is, at the time of overmodulation, the input signal is attenuated and supplied to the phase locked loop, On the other hand, when the detection output signal is lower than the reference potential, that is, when the signal is not overmodulated, the input signal is supplied to the phase locked loop without being attenuated.
Since the signal does not disturb the phase locked loop and the input signal is not attenuated when not overmodulating,
It is effective when the electric field is weak.

Further, in order to achieve the above object, the present invention provides:
A phase-locked loop for controlling the oscillation of the carrier based on the AM-modulated input signal and the carrier reproduced by the phase-locked loop; a synchronous detector for performing synchronous detection of the input signal based on the carrier; A comparator for comparing a detection output signal output from the detector with a first reference potential equal to or slightly higher than the zero carrier bias and a second reference potential slightly lower than the zero carrier bias; Switch means for opening the phase locked loop when the signal is higher than the first reference potential, and closing the phase lock loop when the detection output signal is lower than the second reference potential.

According to the present invention, when the detection output signal is higher than the first reference potential equal to or slightly higher than the zero carrier bias, that is, at the time of overmodulation, the phase lock loop is opened, and the detection output signal is output. When the signal is lower than the second reference potential, which is slightly lower than the zero carrier bias, that is, when the phase is not overmodulated, the phase lock loop is closed. Since the operation of the lock loop is stopped and the voltage control oscillator is controlled by the pre-hold of the loop filter of the phase lock loop during the period, the phase lock loop is not disturbed by the AM signal whose carrier phase is inverted in the overmodulation section. I'm done.

In particular, according to the present invention, after the detection output signal becomes higher than the first reference potential and the phase lock loop is opened, the phase lock loop is continued until the detection output signal becomes lower than the second reference potential. Is maintained in an open state, which is disadvantageous as in the AM demodulator disclosed in Japanese Patent Laid-Open No. 7-7686, that is, after the operation of the PLL is temporarily stopped after being regarded as overmodulation, Even though is still overmodulation, it is possible to eliminate the inconvenience that the PLL operation is restarted by being regarded as not overmodulation due to slight level fluctuation.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of an AM detector according to the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIG. 1 shows an AM according to the present invention.
It is a block diagram showing an example of a detection device. A shown in FIG.
The difference between the M detector and the conventional AM detector of FIG.
First, an attenuator (ATT) 10 is connected between the output terminal of the IF signal amplifier 3 and the input terminal of the phase detector 6 of the PLL, and second, based on the output of the synchronous detector 4. Comparator 1 for outputting a control signal for controlling attenuator 10
Is provided. Other configurations and functions are the same as those of the apparatus of FIG. 7, and therefore, the same reference numerals as in FIG.

The comparator 1 is composed of a well-known circuit. The video detection signal output from the synchronous detector 4 is input to one input terminal, and the constant voltage source 18 is input to the other input terminal. A DC voltage (hereinafter referred to as a threshold voltage) that is connected and serves as a reference is applied. The threshold voltage is a voltage equal to or slightly higher than the zero carrier bias. The comparator 1 compares the voltage level of the video detection signal with the threshold voltage, and outputs an ON signal to the attenuator 10 when the potential of the video detection signal is higher, and outputs the ON signal to the attenuator 10 when the potential of the video detection signal is lower. An off signal is output to the attenuator 10.

When the input IF signal is overmodulated, the potential of the video detection signal is a threshold voltage (actually, zero carrier bias).
Higher than. Therefore, when the potential of the video detection signal is higher than the threshold voltage, the input IF signal is regarded as overmodulated. On the other hand, when the input IF signal is not overmodulated, the potential of the video detection signal becomes lower than the threshold voltage (actually, zero carrier bias). Therefore, when the potential of the video detection signal is lower than the threshold voltage, the input IF signal is regarded as not being overmodulated. That is, the comparator 1 outputs an ON signal to the attenuator 10 when it is regarded as overmodulation, and outputs the attenuator 1 when it is not considered as overmodulation.
An off signal is output to 0.

When the ON signal is input from the comparator 1, the attenuator 10 receives the input IF supplied from the IF signal amplifier 3.
The signal is attenuated and its amplitude is reduced to be supplied to the phase detector 6. On the other hand, when the OFF signal is input from the comparator 1, the attenuator 10 inputs the I signal supplied from the IF signal amplifier 3.
The F signal is supplied to the phase detector 6 without being attenuated. That is, when the off signal is input, the attenuator 10
The damping action is stopped.

In the phase detector 6, since the amplitude of the input IF signal at the time of overmodulation is attenuated by the attenuator 10 and input, the gain is reduced. As a result, adverse effects caused by overmodulation of the input IF signal are reduced.

The comparator 1, IF signal amplifier 3, synchronous detector 4, + 45 ° phase shifter 5, phase detector 6, -45 ° phase shifter 7, voltage controlled oscillator 8, loop filter 9, and attenuator 10 May be integrated and formed on the same semiconductor substrate.

According to the first embodiment, since the input IF signal is attenuated by the attenuator 10, the amplitude of the input IF signal to the phase detector 6 of the PLL is reduced, and the gain in the phase detector 6 is reduced. Because it falls, the input IF of overmodulation
The adverse effect of the signal is reduced, the PLL is not disturbed by the AM signal whose carrier phase is inverted in the overmodulation section, and the original video signal can be detected from the overmodulated IF signal.

In particular, according to the first embodiment, since the attenuating effect of the attenuator 10 is effective only in the section considered as overmodulation, the input IF signal is attenuated in the section considered as not overmodulation. Instead, the signal is input to the phase detector 6 and is effective when the electric field is weak. Therefore, even in the case of a weak electric field, the original video signal can be effectively detected from the overmodulated IF signal.

In the AM detector shown in FIG. 1, the comparator 1 is not provided, or the attenuator 10
May be invalidated and the attenuator 10 always attenuates the input IF signal.

Embodiment 2 FIG. 2 shows an AM according to the present invention.
FIG. 10 is a block diagram illustrating another example of the detection device. The AM detector shown in FIG. 2 differs from the conventional AM detector of FIG. 7 in that the first is a loop connected to a feedback loop from the output terminal of the phase detector 6 of the PLL to the input terminal of the voltage controlled oscillator 8. Between the filter 9 and the phase detector 6,
Secondly, a switch 100 for opening and closing the feedback loop is connected, and second, a comparator 100 for outputting a switch control signal for controlling the opening and closing of the switch 2 based on the output of the synchronous detector 4 is provided. It is a point. Other configurations and functions are the same as those of the device of FIG.
The same reference numerals as in FIG. 7 are assigned and the description is omitted.

The switch means 2 opens, for example, when the input switch control signal is at the "Hi" level with a relatively high potential (when the input IF signal is overmodulated), while the switch control signal is at a relatively high potential. When the input IF signal is not overmodulated, the signal is closed.

When the switch means 2 is closed, PL
L is a closed circuit, so the voltage controlled oscillator 8
Oscillation is controlled based on the phase difference signal output from the phase detector 6 so as to synchronize with the input IF signal. On the other hand, when the switch means 2 is opened, the PLL becomes an open circuit and the phase detector 6
Therefore, no phase difference signal is input to the voltage controlled oscillator 8, and the oscillation of the voltage controlled oscillator 8 is controlled by the previous value held by the loop filter 9 before the PLL is opened.

It should be noted that the comparator 100, the switching means 2, I
Part or all of F signal amplifier 3, synchronous detector 4, + 45 ° phase shifter 5, phase detector 6, -45 ° phase shifter 7, voltage controlled oscillator 8, and loop filter 9 are integrated on the same semiconductor substrate It may be formed by being formed.

FIG. 3 is a circuit diagram showing an embodiment of the comparator 100. The comparator 100 includes a constant current source 14 having one end connected to a power supply line to which a power supply voltage is applied, a differential pair including a pair of PNP transistors Tr10 and Tr11, and a pair of NPN transistors Tr12 and Tr13.
NPN transistor Tr15 serving as a switching element
, Two resistors R16 and R17, and a constant voltage source 18.

The PNP transistor Tr10 has an emitter connected to the constant current source 14, a collector connected to the collector of the NPN transistor Tr12, and a video detection signal input from the synchronous detector 4 to its base. It has become so.

The base and the collector of the NPN transistor Tr12 are short-circuited, and the emitter is grounded.

The PNP transistor Tr11 has an emitter connected to the constant current source 14, a collector connected to the collector of the NPN transistor Tr13, and a base connected to the constant voltage source 18 via the resistors R16 and R17. , A predetermined voltage is applied.

The base of the NPN transistor Tr13 is connected to the base of the NPN transistor Tr12, and the emitter is grounded.

One end of each of the resistors R16 and R17 is commonly connected to the base of the PNP transistor Tr11. The other end of the resistor R16 is connected to the constant voltage source 18. The other end of the resistor R17 is an NPN transistor Tr
It is connected to 15 collectors. The potential of the constant voltage source 18 is a first reference DC potential (hereinafter, referred to as a first threshold voltage) which is equal to or slightly higher than the zero carrier bias. Then, the resistors R16 and R17
Divides the first threshold voltage by the two resistors R16 and R17, and supplies a second reference DC potential (hereinafter referred to as a second threshold voltage) slightly lower than zero carrier bias to the base of the PNP transistor Tr11. ) Are applied.

The NPN transistor Tr15 has its emitter grounded and its base connected to the collector of the NPN transistor Tr13. The output of the comparator 100 is obtained from the collector of the NPN transistor Tr13.

The operation of the comparator 100 will be described with reference to FIGS. In the comparator 100, the NPN transistor Tr15 is normally off, so that the first threshold voltage is applied to the base of the PNP transistor Tr11. In this state, when the potential of the video detection signal input from the synchronous detector 4 to the base of the PNP transistor Tr10 is lower than the first threshold voltage (when there is no overmodulation), the constant current source 14 supplies the transistor Tr1.
0, a current flows to the ground point via Tr12.

Therefore, the NPN transistor Tr15 remains off. At this time, the output of the comparator 100 is “Lo”
Level, switch means 2 (see FIG. 2) is closed, and P
The operation of LL becomes effective. Therefore, the voltage controlled oscillator 8
Oscillation is controlled based on the phase difference between the input IF signal and the carrier generated by the voltage controlled oscillator 8.

At the time of overmodulation, the PNP transistor Tr1
Since a video detection signal having a potential higher than the first threshold voltage is input to the base of 0, a current flows from the constant current source 14 to the PNP transistor Tr11. As a result, the NPN transistor Tr15 is turned on, and serves as a saturation switch, and the resistor R17 is grounded. Therefore, the PNP transistor Tr
The potential applied to the base of the reference voltage 11 is the output potential of the constant voltage source 18, that is, the first threshold voltage,
The potential is divided at 17 (second threshold voltage). At this time, the output of the comparator 100 becomes "Hi" level, the switch means 2 (see FIG. 2) is opened, and the operation of the PLL is stopped. Therefore, the oscillation of the voltage controlled oscillator 8 is controlled by holding the previous value of the loop filter 9.

Then, while the potential of the video detection signal applied to the base of the PNP transistor Tr10 is higher than the second threshold voltage, the NPN transistor Tr15 remains on. Therefore, the output of the comparator 100 is “Hi”.
The level remains, the switch means 2 (see FIG. 2) opens, and the operation of the PLL stops. Therefore, the oscillation of the voltage controlled oscillator 8 is controlled by holding the previous value of the loop filter 9.

When the potential of the video detection signal applied to the base of the PNP transistor Tr10 becomes lower than the second threshold voltage, the constant current source 14 supplies the transistors Tr10, T
The current flows to the ground via r12, and NP
The N transistor Tr15 turns off. As a result, the output of the comparator 100 changes to the “Lo” level, the switch means 2 (see FIG. 2) is closed, and the operation of the PLL is enabled again. Therefore, the oscillation of the voltage controlled oscillator 8 is controlled again based on the phase difference between the input IF signal and the carrier generated by the voltage controlled oscillator 8. At this time, since the NPN transistor Tr15 is off, the PNP transistor T15 is turned off.
The potential applied to the base of r11 returns to the first threshold voltage.

According to the second embodiment, when the video detection signal is higher than the first threshold voltage to be regarded as overmodulation, the switch means 2 of the PLL is opened, and when the video detection signal is not overmodulation. When the voltage is lower than the second threshold voltage to be considered, the switching means 2 of the PLL is closed, so that the PLL operates effectively when there is no overmodulation, while the PL does not
Since the operation of L stops and the voltage control oscillator 8 is controlled by the pre-hold of the loop filter 9, the PLL is not disturbed by the AM signal whose carrier phase is inverted in the overmodulation section, and the overmodulation is not performed. The original video signal can be detected from the IF signal.

According to the above-described embodiment, after the video detection signal becomes higher than the first threshold voltage and the PLL is opened, the PLL is opened until the video detection signal becomes lower than the second threshold voltage. After the operation of the PLL is temporarily stopped because it is regarded as overmodulation, a slight level fluctuation of the input IF signal causes the overmodulation in spite of the fact that the input IF signal is still overmodulation. PLL
The disadvantage that the operation is restarted can be solved.

In the AM detector shown in FIG.
Instead of the comparator 100, the video detection signal output from the synchronous detector 4 is compared with a reference DC threshold voltage equal to or slightly higher than the zero carrier bias,
When the potential of the video detection signal is higher than the threshold voltage, the switch means 2 is opened, and when the potential of the video detection signal is lower than the threshold voltage, a switch control signal for closing the switch means 2 is output. The comparator may be configured as described above. FIG. 5 shows the waveforms of the video detection signal and the switch control signal in this case.

In the above, the present invention can be variously changed in design. The present invention can be applied not only to the case where the original video signal is detected from the IF signal, but also to the case where the original signal is detected from the AM-modulated signal.

[0055]

As described above, according to the present invention, since the input signal is attenuated by the attenuator, the amplitude of the input signal to the phase locked loop is reduced, and the phase detector in the phase locked loop is used. As the gain goes down,
The adverse effect of the overmodulation input signal is reduced, and the phase lock loop is not disturbed by the AM signal whose carrier phase is inverted in the overmodulation section. Therefore, the overmodulation IF
The original video signal can be detected from the signal.

According to the present invention, when the detected output signal is higher than the reference potential equal to or slightly higher than the zero carrier bias, that is, at the time of overmodulation, the input signal is attenuated and supplied to the phase locked loop. On the other hand, when the detection output signal is lower than the reference potential, that is, when the overmodulation is not performed, the input signal is supplied to the phase locked loop without being attenuated.
Since the signal does not disturb the phase locked loop and the input signal is not attenuated when not overmodulating,
It is effective when the electric field is weak. Therefore, even in the case of a weak electric field, the original video signal can be effectively detected from the overmodulated IF signal.

Further, according to the present invention, when the detection output signal is higher than the first reference potential which is equal to or slightly higher than the zero carrier bias, that is, at the time of overmodulation, the phase lock loop is opened, and the detection is performed. When the output signal is lower than the second reference potential, which is slightly lower than the zero carrier bias, that is, when the phase is not overmodulated, the phase lock loop is closed. Since the operation of the phase locked loop is stopped and the voltage controlled oscillator is controlled by the pre-hold of the loop filter of the phase locked loop, the phase locked loop is not disturbed by the AM signal whose carrier phase is inverted in the overmodulation section. Only Therefore, the original video signal can be detected from the overmodulated IF signal.

In particular, according to the present invention, after the detection output signal becomes higher than the first reference potential and the phase lock loop is opened, the phase lock loop is continued until the detection output signal becomes lower than the second reference potential. Is maintained in an open state, which is disadvantageous as in the AM demodulator disclosed in Japanese Patent Laid-Open No. 7-7686, that is, after the operation of the PLL is temporarily stopped after being regarded as overmodulation, Even though is still overmodulation, it is possible to eliminate the inconvenience that the PLL operation is restarted by being regarded as not overmodulation due to slight level fluctuation.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of an AM detection device according to the present invention.

FIG. 2 is a block diagram showing another example of the AM detection device according to the present invention.

FIG. 3 is a circuit diagram showing an embodiment of a comparator in the AM detector.

FIG. 4 is a waveform chart for explaining the operation of the comparator.

FIG. 5 is a waveform chart for explaining an operation of another example of the comparator.

FIG. 6 is a schematic diagram for explaining an IF signal.

FIG. 7 is a block diagram showing a conventional AM detector.

FIG. 8 is a schematic diagram for explaining a phase relationship between an input IF signal and a carrier.

FIG. 9 is a circuit diagram illustrating an example of a synchronous detector.

FIG. 10 is a schematic diagram for explaining an overmodulated IF signal.

FIG. 11 is a schematic diagram for explaining an overmodulated IF signal.

[Explanation of symbols]

1,100 comparator, 2 switch means, 3 IF signal amplifier, 4 synchronous detector, 5 + 45 ° phase shifter, 6 phase detector, 7-45 ° phase shifter, 8 voltage controlled oscillator,
9 Loop filter, 10 attenuator, 18 constant voltage source.

Claims (3)

[Claims] 1. A phase-locked loop for controlling oscillation of a carrier based on an AM-modulated input signal and a carrier reproduced by the phase-locked loop, and an attenuation for attenuating the input signal input to the phase-locked loop. An AM detector comprising: a detector; and a synchronous detector that performs synchronous detection of the input signal based on the carrier. 2. A comparator for comparing a detection output signal output from the synchronous detector with a reference potential equal to or slightly higher than a zero carrier bias, wherein the attenuator includes a comparator. Controlled by the result of the
If the detection output signal is higher than the reference potential, the input signal is attenuated and supplied to the phase locked loop, while if the detection output signal is lower than the reference potential, the input signal is not attenuated to the phase lock loop. The AM detector according to claim 1, wherein the signal is supplied to the AM detector. 3. A phase-locked loop for controlling oscillation of a carrier based on an AM-modulated input signal and a carrier reproduced by a phase-locked loop, and synchronous detection for performing synchronous detection of the input signal based on the carrier. And a comparator for comparing a detection output signal output from the synchronous detector with a first reference potential equal to or slightly higher than the zero carrier bias and a second reference potential slightly lower than the zero carrier bias. And a detector, when the detection output signal is higher than the first reference potential, opening the phase locked loop, and when the detection output signal is higher than the second reference potential.
Switch means for closing the phase locked loop when the potential is lower than the reference potential of the AM detection device.