US 4,225,822 AGranted

Amplitude modulation circuit for a transmitter

AssigneeTokyo Shibaura Electric Co., Ltd.

InventorKeiichiro Kariatsumari

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Abstract

An amplitude modulation circuit for a transmitter has an automatic level controlled circuit (ALC circuit), including in its negative feedback loop a series circuit of a high-pass filter, a rectifier circuit and an eliminator circuit, for modulation degree suppression, thereby avoiding overmodulation. The high-pass filter is so designed that the negative feedback signal level for modulation degree suppression of the ALC circuit is raised as the audio-signal frequency supplied to the transmitter increases. In the modulation circuit having such ALC circuit, a sufficiently high mean modulation degree may be obtained without encountering overmodulation if the main spectrum components of the input audio-frequency signal are distributed in the intermediate range of the audio-signal frequency band. On the other hand, if the main spectrum components of the audio-frequency signal are distributed in the higher (high-pass) range, the upper limit of the modulation degree is restricted to a moderately lower value so as to reduce production of spurious signals. Here the ALC circuit operates so as not to change the frequency characteristic of the audio-frequency input signal but to suppress the modulation degree uniformly throughout the frequency band of the audio-frequency input signal. Therefore, the modulation circuit does not cause variation in a tone quality of the input signal.

Claims7

Dependent claims

An amplitude modulation circuit for a transmitter, comprising:

(a) gain controlled amplifier means for amplifying an input audio-frequency signal to produce a modulation signal, the amplification gain of said gain controlled amplifier means being negative-feedback-controlled by an automatic level control signal;
(b) filter means for producing a filtered modulation signal from said modulation signal, the relative gain of said filter means in response to a signal of approximately 2.5 kHz being approximately 3 dB greater than the relative gain of said filter means in response to a signal of approximately 400 Hz;
(c) rectifier means for rectifying the output signal of said filter circuit in order to provide a DC component proportional to the amplitude of said filtered modulation signal;
(d) eliminator means for eliminating ripples from the output signal of said rectifier means in order to provide said automatic level control signal converted into a direct current;
(e) modulation means for amplitude-modulating an input carrier in order to produce a modulated radio-frequency signal for transmission, the modulation degree of said modulation means being controlled by said modulation signal;
and
(f) frequency synthesizer means for synthesizing a radio-frequency signal equivalent to the transmission frequency in order to provide said carrier;
whereby uniform modulation throughout the frequency band of said modulation signal is achieved when the modulation signal level is relatively low;
overmodulation is avoided while a sufficiently high mean amplitude modulated signal is maintained when the modulation signal amplitude is raised and the main frequency components of said modulation signal are distributed in an intermediate frequency range below 2.5 kHz;
and the upper limit of the amplitude of said modulation signal is restricted to a moderately lower level when the main frequency components of said modulation signal are distributed in a higher frequency range over 2.5 kHz so that the level of produced spurious signals is reduced.

7 more elements

An amplitude modulation circuit for a transmitter, comprising:

a plurality of automatic level control circuits connected in series, each of said automatic level control circuits including:
(a) gain controlled amplifier means for amplifying an input audio-frequency signal to produce a modulation signal, the amplification gain of said gain controlled amplifier means being negative-feedback-controlled by an automatic level control signal, (b) filter means for producing a filtered modulation signal from said modulation signal, the relative gain of said filter means in response to a signal having a first frequency being larger than the relative gain of said filter means in response to a signal having a second frequency lower than said first frequency, (c) rectifier means for rectifying the output signal of said filter circuit to provide a DC component proportional to the amplitude of said filtered modulation signal, and (d) eliminator means for eliminating ripples from the output signal of said rectifier means to provide said automatic level control signal converted into a direct current;
modulation means for amplitude modulating an input carrier to produce a modulated radio-frequency signal for transmission, the modulation degree of said modulation means being controlled by said modulation signal;
and
frequency synthesizer means for synthesizing a radio-frequency signal equivalent to the transmission frequency to provide said carrier;
whereby uniform modulation throughout the frequency band of said modulation signal is achieved when the modulation signal level is relatively low;
overmodulation is avoided while a sufficiently high means amplitude modulated signal is maintained when the modulation signal amplitude is raised and the main frequency components of said modulation signal are distributed in lower frequency range below said first frequency;
and the upper limit of the amplitude of said modulation signal is restricted to a moderately lower level when the main frequency components of said modulation signal are distributed in higher frequency range over said first frequency so that the level of produced spurious signals is reduced.

5 more elements

Dependencies are inferred from the claim text.