Fujitsu Corporation

Fugitsu Limited

Yuji Segawa

Kunihiko Gotoh

Atsushi Matsuda

Kazuhiro Kitani

Yukinori Abe

Asao Kokubo

Masaru Otsuka

Akira Haga

Yoshinori Yoshikawa

Noboru Inanami

Osamu Kikuchi

The switched capacitor filter circuit comprises an amplifier whose feedback capacitance Cf is variable. This feedback capacitance is changed based on the control signals received from the outside. Two switches control charging/discharging of a first capacitor. Another Two switches control charging/discharging of a second capacitor. Frequency f of a signal for switching the four switches is changed so that a beat frequency varies within a range having no influence with respect to a cutoff frequency while always keeping a value of the f /Cf constant.

Switched capacitor filter circuit

The inverter functioning as a comparator, dummy inverter having the same electric characteristics as the inverter, and control circuit are provided. Vth detecting input voltage output from the control circuit is input into the dummy inverter, Vth detecting output voltage output from the dummy inverter is input into the control circuit, and the threshold voltage of the dummy inverter is detected. The threshold voltage of the inverter is controlled by controlling the back gate voltages of the MOS transistors of the dummy inverter and the inverter in such a manner that the threshold voltage of the dummy inverter coincide with an external reference voltage.

Comparator and voltage controlled oscillator circuit

A cut-off state of a carrier signal or a carrier signal outside an effective range of a frequency is detected by a carrier detector, and a signal switching circuit inputs a clock from an external device into a phase comparator in place of the carrier signal, with which a locked state is maintained in a PLL comprising the phase comparator, a charge pump, a loop filter, a voltage control oscillator, and a 1/N divider, so that a high-speed locking operation is realized to another appropriate carrier signal.

PLL having switching circuit for maintaining lock during loss of input signal

Disclosed are an integration circuit capable of substantially raising the ratio of a current to a capacitance, I/C, and voltage-controlled oscillator and frequency-voltage converter which employ the integration circuit. The integration circuit comprises an integrating capacitor, a current source, a switch, a detection circuit, and a control circuit. The current source supplies a current to the integrating capacitor. The switch is installed on a path along which a current is supplied from the current source to the integrating capacitor. The detection circuit detects a voltage developed at the integrating capacitor. The control circuit controls the switch so that a current will be supplied from the current source to the integrating capacitor during an integration period. The integration period falls into a current supply period and a stop period. The control circuit controls the switch so that a current will be supplied from the current source to the integrating capacitor during the current supply period and no current will be supplied from the current source to the integrating capacitor during the stop period.

Integration method and integration circuit offering improved signal-to-noise ratio, and voltage-controlled oscillator and frequency-voltage converter employing integration circuit

A correction circuit for controlling a correction required circuit includes an oscillator circuit, and a logic circuit which counts an oscillation frequency of the oscillator circuit and thus produces a control signal which causes the oscillator circuit to oscillate at a constant frequency. The control signal changes element values of elements of the oscillator circuit and the correction required circuit so that characteristics of the oscillator circuit and the correction required circuit can be controlled.

Correction circuit controlling sensitivities of an oscillator circuit

The present invention is a current switch, which supplies a constant current to a load circuit, and comprises: a transistor, the gate of which is supplied with a predetermined constant voltage, and the drain of which is connected to the above-mentioned load circuit; and a constant voltage supply circuit, which, in response to a control signal, supplies to the source of the above-mentioned transistor a first constant voltage, or a second constant voltage that differs from the first constant voltage, wherein when the above-mentioned first constant voltage is supplied to the above-mentioned transistor source by the above-mentioned constant voltage supply circuit, the transistor is turned ON and supplies the above-mentioned constant current to the above-mentioned load circuit, and when the above-mentioned secondary constant voltage is supplied to the above-mentioned transistor source, the transistor is turned OFF and interrupts the above-mentioned constant current. With a current switch configured as described above, high-precision current can be supplied and interrupted at the start and interruption of the current supply.

Current switch and PLL in which it is used

An AGC circuit according to the present invention comprises: a gain controller, having a variable gain, for amplifying an input signal at a predetermined gain and generating an output signal, and for varying the gain; a comparison circuit for comparing an amplitude of the input signal or of the output signal with a predetermined reference level, and for generating a control signal that goes to a first level when the amplitude is smaller than the reference level, and that goes to a second level when the amplitude is greater than the reference level; and a duty ratio detector for supplying the control signal generated by the comparison circuit, and for generating control code in accordance with a duty ratio of the control signal, wherein the gain of the gain controller is varied by using the control code, and so adjusted that the duty ratio of the control signal is maintained to be a predetermined value. When the amplitude of the input signal or the output signal is increased, a period during which the amplitude exceeds the reference level is also extended. By utilizing this phenomenon, a gain is varied so as to maintain a control signal duty ratio of 50%, for example, so that the amplitude of the output signal is amplified to a desired value, which is maintained.

Digital AGC circuit

An oscillation circuit for producing an output whose frequency accords with an input voltage value, comprises a first constant current source whose current has a value according to the input voltage value; a charge capacitor to be charged by the first constant current source; a comparator having one input terminal connected to a charge terminal of the charge capacitor to be charged and an other input terminal supplied with a first reference voltage, for comparing inputs to both input terminals with each other and outputting an output signal having a high level or a low level; and first switch means for pulling down a potential of the charge terminal of the charge capacitor to a second reference voltage lower than the first reference voltage under control by an output of the comparator.

Oscillation circuit and PLL circuit using same

A voltage-controlled oscillator has first and second switched capacitor filters, an operational amplifier, and a comparator. The first switched capacitor filter is controlled by a control clock signal to supply a positive current depending on an input voltage, and the second switched capacitor filter is controlled by the control clock signal to supply a negative current depending on the input voltage. The operational amplifier has an input terminal alternately connectable to the first and second switched capacitor filters by a first switch which can be rendered conductive in synchronism with an output clock signal, and another input terminal for being supplied with a constant voltage. A potential at an output terminal of the operational amplifier is lowered when the operational amplifier is supplied with the positive current through a feedback circuit which is provided between the input terminal and the output terminal, and is increased when the operational amplifier is supplied with the negative current through the feedback circuit. The comparator compares the potential at the output terminal of the operational amplifier alternately with a first constant voltage and a second constant voltage, lower than the first constant voltage, which can be switched in synchronism with the output clock signal, thereby to produce the output clock signal.

Oscillator having switching capacitors and phase-locked loop employing

A compander circuit includes first and second volume controllers, a comparator, a counter and a control part. The comparator compares a compared voltage with a rectified and smoothed signal of an input signal of the compander circuit or an output signal of the first volume controller receiving the input signal, the compared voltage being an output signal of the second volume controller. The counter executes a count operation on an output signal of the comparator in synchronism with a clock signal and controls gains of the first and second volume controllers on the basis of a counter value of the counter. The control part controls a frequency of the clock signal applied to the counter by a predetermined control signal.

Compander circuit

In a half-wave rectifier circuit for obtaining an DC voltage output depending on the level of an input signal through half-wave rectification of the input signal, an input voltage from which a DC component is removed by a capacitor is input to a switch circuit. This input voltage includes an offset voltage which is generated by use of the capacitor. The switch circuit outputs a half-wave rectified voltage to a smoothing circuit by switching the input voltage ON and OFF depending on the polarity of the input signal. Under the condition that an input voltage is controlled to OFF state, the switch circuit shows a high impedance viewed from the input side of the smoothing circuit. Therefore, an output voltage of the smoothing circuit is maintained when the switch circuit is in the OFF state. Thereby, the offset voltage generated by the capacitor is eliminated from the output voltage from the smoothing circuit.

Half-wave rectifier circuit

A digitally controlled variable gain circuit which receives an analog input voltage and produces an analog output voltage. The digitally controlled variable gain circuit includes a first resistor string having first and second ends and a plurality of resistors connected in series between the first and second ends, the second end of the first resistor string being grounded and the analog input voltage being applied across the first and second ends of the first resistor string. A first selection mechanism, coupled to the first resistor string and responsive to a first digital control signal, provides a selectively determinable voltage divided output from the first resistor string. A second resistor string has first and second ends and a plurality of resistors connected in series between the first and second ends, the second end of the second resistor string being grounded and the voltage divided output provided by the first selection mechanism being applied across the first and second ends of the second resistor string. A second selection mechanism, coupled to the second resistor string and responsive to a second digital control signal, provides a selectively determinable voltage divided output from the second resistor string as the analog output voltage.

Digitally controlled variable gain circuit

A variable resistor is provided with a series resistor network including first, second and third resistor parts which are connected in series, where the second resistor part is connected to the first and third resistor parts via first and second nodes, respectively, and a fourth resistor part, coupled in parallel to the second resistor part via the first and second nodes. The fourth resistor part includes a plurality of resistors which are connected in series via a plurality of third nodes. The first resistor part has a terminal opposite the first node for receiving an input signal, and an output signal of the variable resistor is obtained via an arbitrary one of the third nodes of the fourth resistor part.

Growing community of inventors

Kawasaki, Japan

Yuji Segawa