Fujitsu Corporation

Tohru Ogawa

Takashi Kaku

Hideo Miyazawa

Hiroyasu Murata

Ryoji Okita

Noboru Kawada

Takeshi Asahina

Michiko Kawada

A limiter is connected with the output of a PLL to limit the amplitude of the output from the PLL. The output from limiter is supplied to a voltage-controlled oscillator (VCXO). The limiter determines the difference between the input signal and the output signal. If the difference exceeds a predetermined value, the change in the output from the limiter is limited to the predetermined value. If the difference does not exceed the predetermined value, the change in the output is considered to be a difference value.

PLL controller, method of PLL control, and limiter

A data transmission system having, in addition to a normal system operation mode, a test mode wherein a plurality of stations confirm each other through sending and receiving check data for security confirmation and wherein the actual line switching operation is not performed. In the test mode, when notifying test data for confirmation of another station, the call terminating station side notifies the accurate data, while the call originating station side activating the test mode notifies deliberately changed check data thereby enabling a test for confirmation of the continuance of connection with a backup line without suspending communication with the working line.

Data transmission system having a backup testing facility

An automatic gain control circuit which is provided in a demodulation section of a modem to adjust the loop gain so that the level of a band-limited receive signal may have a predetermined reference value and to output the receive signal to an automatic equalization section of the modem. The automatic gain control circuit is improved in that, even when a disturbance of the circuit such as a gain hit or a drop-out occurs, regular communication can be continued without requiring a data mode leading-in operation. The automatic gain control circuit includes a reference value setting unit for generating a reference value, an error calculation unit for determining an error signal between the receive signal and the reference value, a receive level variation correcting controlling force setting unit for setting a receive level variation correcting controlling force to be added to the error signal, an error signal correction unit for correcting the error signal, an error signal averaging unit for averaging the corrected error signal and a feedback unit for feeding back, as the receive signal, the averaged corrected error signal. The receive level variation correcting controlling force to be set by the receive level variation correcting controlling force setting unit is varied in response to the error signal.

Automatic gain control circuit for a demodulation section of a modem

A frequency multiplexing control system includes a first modem unit connected to a transmission line, and a second modem unit connected to the transmission line. The first and second modem units communicate with each other via a plurality of primary channels and a secondary channel. The primary and secondary channels have different frequency bands. A plurality of signal detection units are respectively provided in the first and second modem units, to detect an abnormal primary channel and send to the second and first modem units, information concerning the abnormal primary channel via the secondary channel. A plurality of disabling units are respectively provided in the first and second modem units, to disable the abnormal primary channel in response to the received information and to send a training signal for recovering the abnormal primary channel to the transmission line.

System for controlling frequency multiplexing modems

A communication system in which a plurality of multipoint connected satellite stations are connected via down line and an up line to a master station. Each of the satellite stations monitors presence or absence of occurrence of an abnormality in the down line, judges whether the abnormality is present or not and, where the abnormality is present, informs the master station of the nature of the abnormality via the up line. In turn, the master station monitors presence or absence of an alarm indicating abnormality from each of the satellite stations, judges whether the alarm is detected or not and, where the alarm is detected, reads past record information concerning abnormality detection in the satellite station concerned and information on retraining execution time, then judges whether the down line is normal or not and, where the down line is normal, sends a training signal again to the down line to retrain a MODEM corresponding to the satellite station concerned. Accordingly, it is possible to quickly restore demodulation processing by each of the satellite stations to a respective normal status after a recovery of trouble in a down line.

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Kawasaki, Japan

Tohru Ogawa