JPH0215375Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an amplitude modulation circuit, and more particularly to an amplitude modulation circuit that prevents circuit components from being damaged even if the operation is stopped by a load abnormality detection circuit or the like.

[Technical background of the invention and its problems]

Conventionally, in order to improve the efficiency of an audio signal amplitude modulation circuit, the audio signal is subjected to single pulse width modulation.
It has been put into practical use to reproduce the original audio signal from the output and amplitude modulate the carrier wave using this signal. As usual, this modulation circuit is also provided with a protection circuit against lightning strikes to the transmitting antenna or short circuits in the antenna load. This protection circuit is implemented by stopping the excitation of carrier wave introduction into the modulation circuit by the load abnormality detector.

However, if this excitation is interrupted momentarily, the amplitude modulation circuit has a pulse width modulation circuit output.
Due to this output, a transient voltage generated in a choke coil such as a low-pass filter in the circuit is applied to the amplitude modulation circuit. This transient voltage damages the active elements or components of the amplitude modulation circuit. Similar damage also occurs when excitation is stopped due to some kind of failure.

[Purpose of invention]

The present invention has been devised in view of this point, and provides an amplitude modulation circuit that prevents damage to circuit components even when excitation is stopped.

[Summary of the idea]

The present invention includes a pulse width modulator that performs pulse width modulation using a modulation signal, a means for regenerating a modulation signal from the output of this modulator, an amplitude modulator that amplitude modulates a carrier wave using the regenerated modulation signal, and a means for regenerating a modulation signal from the output of this modulator. A detection circuit detects the stop and return of the carrier wave supply to the modulator, and generates a control signal that is at least at the stop level when stopped and reaches the operating level with a delay when returned; and means for controlling the operation of the width modulator.

[Example of idea]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the circuit configuration. A modulated signal such as an audio signal is introduced into a terminal 2, passes through an amplifier 4, and then
is introduced into the pulse width modulator 6. This pulse width modulator 6 is constructed of a normal circuit that generates a pulse width modulated output in accordance with a modulation signal, but is constructed so that its operation is stopped by the introduction of a control signal, which will be described later. The output of this pulse width modulator 6 is introduced into a low pass filter 10 via an amplifier 8, and the original modulated signal is reproduced.

This regenerated modulation signal is supplied to the amplitude modulator 12 and introduced into the terminal 14, and the controllable drive circuit 1
Amplitude modulation of the carrier wave via 5 is performed. The output of the amplitude modulator 12 is led out to a terminal 16 and supplied to a power amplifier and an antenna (not shown).

On the other hand, the carrier wave introduced into the terminal 14 is branched from the stage before the amplitude modulator 12 and supplied to the detection circuit 18. This detection circuit 18 detects the presence or absence of a carrier wave, and controls the carrier wave supply and the pulse width modulator accordingly, and is configured as shown in FIG. 2.

That is, the carrier wave is introduced from the terminal 20 to the excitation detection circuit 22, which has characteristics of instantaneous operation and time-limited recovery.
The output is introduced into the AND circuit 24. Also,
The terminal 26 is used to operate this amplitude modulation circuit from the outside.
An external control signal is introduced to control the stop, AND
It is supplied to circuit 24. The output of the AND circuit 24 is supplied to a drive circuit 28 to generate a control signal for controlling the operation of the pulse width modulator 6, which is led out from a terminal 30 to the pulse width modulator 6.

Further, the external control signal is introduced into a drive circuit 32 that generates a control signal for controlling the introduction of the carrier wave, and its output is delayed by a delay circuit 34 and then sent to a terminal 36.
is led out to the drive circuit 15.

The operation of the present invention constructed as described above will now be described with reference to FIG.

When the carrier wave is momentarily interrupted at time T ₀ as shown in Figure 3a due to load abnormality, etc., the excitation detection circuit 22 of the detection circuit 18 instantly detects it, and the output of the drive circuit 28 is stopped as shown in Figure 3c. (OFF) level,
The operation of the pulse width modulator 6 is stopped. When the carrier wave is restored from this state at time T ₁ , the excitation detection circuit 22 generates a return signal after time t ₁ from time T ₁ by time-limited operation, and operates the drive circuit 28 , as shown in FIG. 3c. The output is set to the operating (ON) level with a delay of time _t1 , and the pulse width modulator 6 is operated. Therefore, when the carrier wave is momentarily interrupted, the pulse width modulator 6 is stopped, and when the carrier wave is restored, the pulse width modulator 6 is activated after a delay of time _t1 , and the amplitude modulator 12 receives the carrier wave. The modulation signal will be introduced after the signal is supplied.

In addition, in order to forcibly stop the operation, the external control signal is stopped (OFF) at time T ₃ as shown in Figure 3b.
When the drive circuit 32 changes the excitation control signal shown in _FIG . be done. Furthermore, in response to a change in the stop (OFF) level of the external control signal, the output of the AND circuit 24 goes to zero level, and the output of the drive circuit 28 also goes to the stop (OFF) level as shown in FIG. 3c. This stops the operation of the pulse width modulator 6. Note that when the external control signal is restored at time _T4 , the excitation control signal becomes the operating (ON) level with a delay of time _t2 due to the delay circuit 34 as shown in FIG. The introduction will take place from 15 onwards. In response to the introduction of this carrier wave, this is detected by the excitation detection circuit 22,
After a delay of time _t1 , the output of the drive circuit 28 is brought to the operating (ON) level as shown in FIG. 3c, and the pulse width modulator 6 is operated.

Therefore, the pulse width modulator 6 is the same as the amplitude modulator 1.
2, the operation is stopped first and the operation is delayed.

[Effect of idea]

As explained above, according to the present invention, the operation of the pulse width modulator 6 is stopped when the carrier wave to the amplitude modulator 12 is stopped, and is started after the carrier wave is supplied to the amplitude modulator 12. No transient voltage occurs,
The circuit components of the amplitude modulator 12 will not be damaged.

In addition, although the control of the external control signal was explained above separately from the instantaneous interruption of the carrier wave, the same effect as described above may be applied even if the external control signal is also set to the stop (OFF) level due to load abnormality, etc. effect can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram illustrating an embodiment of the amplitude modulation circuit according to the present invention, FIG. 2 is a circuit diagram illustrating the detection circuit of FIG. 1, and FIG. 3 is a circuit diagram illustrating the operation of FIG. 1. FIG. 6...Pulse width modulator, 10...Low pass filter, 12...Amplitude modulator, 18...Detection circuit.

Claims (1)

[Scope of utility model registration request] A pulse width modulator that performs pulse width modulation using a modulation signal, a means for reproducing a modulation signal from the output of the modulator, an amplitude modulator that amplitude modulates a carrier wave using the regenerated modulation signal, and a detection circuit that detects the stop and return of the carrier wave supply and generates a control signal that is at least at the stop level when stopped and reaches the operating level with a delay when returned; and means for controlling the operation of the amplitude modulation circuit.