JP3015413B2 - Amplitude modulator - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial application field) The present invention relates to an amplitude modulation device suitable for a modulation output section of a medium-wave or short-wave broadcaster, for example.

(Prior art) In shortwave or mediumwave broadcasters, etc., simply AM (amplitude modulation) of audio
AM stereo broadcasts have been attempted in which not only signals but also carrier waves are phase-modulated and new information is added for transmission.

However, in the case of a normal AM signal, there is a time when the carrier wave becomes zero when the degree of modulation exceeds 100% near a negative peak of a voice or the like, so that there is a problem that information on the phase modulation side is interrupted.

In order to avoid this discontinuity of information on the phase modulation side,
In a conventional medium-wave broadcaster, as shown in FIG. 4, an amplitude limiting (clip) circuit 2 is inserted after the audio signal source 1 so that the modulation degree does not reach 100%, and the transmitter 3 4.

As a result, the audio AM signal transmitted from the antenna 4 can maintain at least a constant amplitude level p in the AM signal α and leave the carrier β as shown in FIG.

However, in the above-mentioned conventional medium wave broadcaster, the transmitter 3
Since the clipping circuit 2 is inserted in the preceding stage, not only the circuit becomes complicated, but also the stability of the clip level is not sufficiently obtained, so that the amplitude level p of the residual carrier wave is not constant. In a medium-wave broadcaster, the DC level fluctuates depending on the 125% modulation of the input signal waveform due to the modulation sensitivity and frequency characteristics of the transmitter 3 or the characteristics of a denaturer connected to an amplification stage or the like. Since a phenomenon occurs, it is difficult to always stably keep the amplitude level p of the residual carrier constant even if the clip circuit 2 is connected. This is the same for shortwave broadcasters.

Therefore, in a conventional medium wave broadcaster, it is conceivable that the limit level of the clipping circuit 2 is set smaller to increase the amplitude level p of the residual carrier.
Since the average degree of modulation of the signal is reduced and the broadcast service area is narrowed, improvement has been demanded.

(Problems to be Solved by the Invention) A conventional medium-wave broadcaster or the like has a disadvantage that a stable residual carrier cannot be formed even if the amplitude of a modulated wave such as an audio signal is limited by a clipping circuit.

Accordingly, an object of the present invention is to provide an amplitude modulation device that can be used in a medium-wave broadcaster or the like and can form a residual carrier wave having a stable level in an AM wave.

[Means for Solving the Problems] An amplitude modulation apparatus according to the present invention comprises a first signal source for generating a pulse width modulation signal, and a pulse width corresponding to an amplitude level of a carrier to be retained. A second signal source for generating a pulse signal comprising: a pulse signal from the second signal source; and a pulse width modulation signal from the first signal source. And a demodulation circuit for demodulating a signal.

(Operation) The amplitude modulation device according to the present invention is configured as described above, and adds a pulse signal having a pulse width corresponding to a constant amplitude level of a carrier to be retained to a pulse width modulation signal by an OR circuit. Therefore, a stable and constant level of residual amplitude can always be formed in the demodulated wave.

(Embodiment) Hereinafter, an embodiment of an amplitude modulation device according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of an amplitude modulation device according to the present invention. The device of the present invention comprises a first signal source 5,
It comprises a second signal source 6, an OR circuit 7 and a demodulation circuit 8. That is, the first signal source 5 obtains a pulse width modulation signal, and a voice modulation wave a as shown in FIG. 2A is output from a voice signal source 51 to a pulse width modulation (PMW) comparator. 2), a triangular waveform sawtooth wave b as shown in FIG. 2 (b) from the triangular wave generator 53 is supplied to the other negative terminal, and is synthesized. A PWM signal c as shown in FIG.

On the other hand, in the second signal source 6, the triangular wave b from the triangular wave generator 53 of the first signal source 51 is branched and supplied to the-terminal of the constant PW comparator 61, and to the other + terminal. A stabilized DC power supply 62 for supplying a reference direct current (DC) voltage corresponding to the amplitude level p of the carrier to be retained is connected.

Therefore, a signal d having a pulse width w corresponding to the amplitude level p of the carrier to be retained is derived from the second signal source 6 as shown in FIG. It is supplied to the circuit 7. Therefore, as shown in FIG. 2 (e), the addition signal e is not affected by the amplitude level of the audio signal a, and at least a pulse signal having a pulse width w corresponding to the amplitude level p of the carrier to be retained is provided. The PWM signal is always present.

The addition output signal e, which is a PWM signal, is supplied to the demodulation circuit 8.

The demodulation circuit 8 includes an amplifier 82 driven by a DC power supply 81 and a low-pass filter (LPF) 83. The added output signal e, which is a PWM signal, is amplified by the amplifier 82 and then demodulated by the LPF 83. As shown in FIG. 2 (f), a demodulated signal f in which the amplitude level p corresponding to the pulse width w stably remains can be obtained.

Therefore, this demodulated signal f is supplied to the modulation stage of the transmitter,
By performing AM modulation on the transmission carrier, an AM signal having a stable residual carrier amplitude level p as shown in FIG. 5 can be transmitted.

The output voltage of the stabilized DC power supply 62 includes the residual carrier level p
Can have a sufficient degree of stability. Although a normal DC power supply is sufficient as the DC power supply 81 of the demodulation circuit 8, even if a voltage fluctuation occurs, it can be easily corrected by incorporating a feedforward control circuit, for example.

When switching between normal AM modulation and an AM modulated wave having a residual carrier according to the present invention at the time of connecting and using the succeeding AM modulation stage, the circuit configuration shown in FIG.
It can be easily configured by incorporating an on / off switching circuit between the comparator 61 for use and the adding circuit 7.

Further, as a specific example of the circuit of the triangular wave generator 53, a configuration may be adopted in which a triangular wave b is obtained after a rectangular wave pulse signal is generated once.

FIG. 3 is a circuit diagram showing a second embodiment of the amplitude modulation device according to the present invention. First and second embodiments differ from those of the first embodiment.
And only the signal sources 5 and 6 of FIG.

That is, the triangular wave generator 53 of the first signal source 5 does not require detailed description of the circuit configuration, but the rectangular pulse signal wave generation circuit 53
1 and a cascade connection of the waveform conversion circuit 532.The rectangular pulse signal from the rectangular pulse signal wave generation circuit 531 is converted into a triangular wave by the waveform conversion circuit 532, supplied to the PWM comparator 52, and supplied from the audio signal source 51. It is configured to combine with a modulated wave.

On the other hand, the second signal source 6 derives a signal d having a pulse width w corresponding to the amplitude level p of the carrier to be retained. Since the rectangular pulse signal b has already been formed, the amplitude is obtained by supplying the rectangular pulse signal b to the monostable multivibrator circuit 63 without forming the constant PW comparator 61 as in the first embodiment. The constant pulse width w signal d corresponding to the level p can be directly derived.

Incidentally, even if the monostable multivibrator circuit 63 is replaced with a differentiating circuit, the constant pulse width w signal d can be similarly generated.

As described above, the amplitude modulation device according to the present invention is not easily affected by aging of the circuit configuration, and is always stable without lowering the average degree of modulation even if the modulated wave is 125% modulated. Because it is possible to derive a modulated wave having a residual carrier at a constant amplitude level, for example, in AM stereo broadcasting, such as a pilot signal for stereo display,
In addition to the audio signal, other information can be transmitted with high quality.

[Effect of the Invention] The amplitude modulation device according to the present invention can form and derive a modulated wave capable of stably incorporating the second information into the AM wave with a relatively simple configuration. When applied to an audio amplitude modulation device of a medium wave broadcaster, a great effect can be obtained practically.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of an amplitude modulation device according to the present invention, FIG. 2 is a signal waveform diagram showing signal waveforms of various parts of the device shown in FIG. 1, and FIG. FIG. 4 is a circuit configuration diagram showing a second embodiment of the amplitude modulator, FIG. 4 is a configuration diagram of a conventional medium-wave broadcaster, and FIG. 5 is a signal waveform diagram showing an output audio AM wave of the broadcaster shown in FIG. It is. 1,51 audio signal source, 2 ... clip circuit, 3 ... transmitter, 4 ... antenna, 5 ... first signal source, 52 ... comparator for PWM, 53 ... triangular wave generator, 6 ... Second signal source, 61... Constant PW comparator, 62... Stabilized DC power supply, 7... LPF.

Continuation of the front page (56) References JP-A-2-125504 (JP, A) JP-A-61-224705 (JP, A) JP-A-56-35505 (JP, A) JP-A-52-113668 (JP) JP-A-2-135905 (JP, A) JP-A-64-36133 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03C 1/00-1/06 H03C 5/00

Claims (1)

(57) [Claims] 1. An amplitude modulation apparatus for generating an amplitude-modulated wave which is amplitude-modulated by an audio signal and in which a carrier remains even when the level of the audio signal is near a negative peak, the pulse width-modulated by the audio signal A first signal source for generating a width-modulated signal; and a second signal source for generating a pulse signal having a pulse width corresponding to the amplitude level of a carrier remaining in the amplitude-modulated wave near a negative peak of the level of the audio signal. A signal source, an OR circuit for adding the pulse signal generated by the second signal source and the pulse width modulation signal generated by the first signal source, and an output signal added by the OR circuit And a demodulation circuit for demodulating the signal.