1. Field of the Invention
The present invention relates to a waveform shaping device for obtaining low-voltage and high-frequency clock pulses having a duty factor of 50%.
2. Description of the Related Art
A microprocessor or a communication encoder/decoder driven in synchronism with a clock requires clock pulses of 50% duty factor from the view points of high speed operation and easiness of logic design.
As conventional methods of obtaining clock pulses of 50% duty factor, there are known a method that a clock pulse output from a clock oscillator is input to a flip-flop circuit to divide the frequency into xc2xd, and a method that a level of a clock pulse output from a clock oscillator is converted by integrating the duty factor and a threshold value of the clock oscillator is negatively fed back using a differential amplifier.
In addition to the above, methods of obtaining clock pulses of 50% duty factor are also disclosed, for example, in Japanese Patent Application Laid-Open No.10-75461 and Japanese Patent Application Laid-Open No.7-30422.
In the method of dividing the frequency using the flip-flop circuit, the power consumption becomes large because both of the clock oscillator and the flip-flop circuit require an operation frequency twice as high as that of the clock pulse, and when the frequency of the clock pulse becomes 10 MHz to 1 GHz, the necessary pulses having the double oscillation frequency themselves are difficult to be obtained even if the power consumption is increased.
On the other hand, in the method of negatively feeding back the threshold value of the clock oscillator, the differential amplifier is difficult to be construct so as to accurately operate under a condition of a low voltage of nearly 1 V and a high frequency of 1 GHz.
The conventional technologies have a problem in that the duty factor of clock pulses of a low voltage and a high frequency can not be set to 50% with high accuracy.
In order to solve the above problem, an object of the present invention is to provide a waveform shaping circuit by which the duty factor of clock pulses can set to 50% with high accuracy even if the clock pulses are of a low voltage and a high frequency.
The present invention is characterized by that an inverter amplifier for receiving clock pulses through an alternating current coupling capacitor is provided with a non-linear limiter element for limiting an amplitude of an output symmetrically in a positive and a negative sides, and a first current-limiting impedance and a second current-limiting impedance are connected between a power supply side terminal of the inverter amplifier and a power supply bus and between a grounding side terminal of the inverter amplifier and a grounding bus, respectively.
In the present invention, it is preferable that an inverter amplifier having a high input impedance is used as the inverter amplifier, diodes in anti-parallel connection is used for the non-linear element, and a constant-current circuits are used for the current-limiting impedances.
According to the present invention, since the inverter amplifier does not allow a direct-current component pass through and the amplitude of the output of the inverter amplifier is limited so as to be symmetrical in a positive and a negative sides, a threshold value of the output waveform (clock pulse waveform) of the inverter amplifier is shifted so that the threshold value of the positive half-cycle and the threshold value of the negative half-cycle always becomes equal to each other. In addition to this, since the output waveform of the inverter amplifier has a rising and a falling gradients due to the current-limiting impedances, there exist a rise time and a fall time. As a result, the output waveform of the clock pulse of the inverter amplifier by the threshold value shifted so that the threshold values of positive the half-cycle and the negative half-cycle becomes equal to each other is shaped in a waveform nearer to 50% duty factor than the input clock pulse waveform.