A frequency converter including a mixer circuit which inputs a local oscillation signal and a radio-frequency input signal modulated for communication of information, and performs frequency conversion. A buffer amplifier is higher than a desired signal frequency band, and has a low-pass passage characteristic of cut-off frequency lower than an adjacent channel carrier frequency.

An optical receiver for responding to optical energy propagating in an optical fibre transmissions path (1) is detected by a reverse-biassed p-i-n diode (2) which is in series with a forward-biassed p-i-n diode (3). The junction between the two diodes is connected to the input of an amplifier (4), which feeds on auto-equalizer. The auto-equalizer includes another p-i-n diode (7) which is in series with the signal path, and is forward-biassed. Due to the associated components including resistors (9,10), the diode (7) passes the same average current as do the diodes (2,3) at the front end of the receiver. The overall effect of the third diode (7) is to provide the receiver with a substantial region in its characteristic whose frequency response is flat.

A method for equalizing power levels of channels of an optical frequency division multiplexed signal determines the power level of each channel of the optical frequency division multiplexed signal and introduces attenuation into the channels having high power levels corresponding to the difference between a level deduced from these high levels and a low reference level. The equalization system includes at least one optical equalizer module which measures the optical power levels of the channels of the signal to be monitored, a processor for implementing the method and variable attenuators controlled by the processor and operating separately on the power levels of the channels.

A repeater for PCM or digital transmission systems includes, in addition to the usual primary feedback control loop, a secondary feedback control loop having a low-pass filter and a peak detector to derive a control signal for a controllable impedance in the equalizer circuit. This control loop optimizes the repeater performance for different cable types and characteristics with regard to the low frequency energy content of the eye diagram.

A method and apparatus is provided for regulating transmission power of a signal on a line. The method includes determining characteristics of the line, determining the transmission power needed to transmit the signal in response to the characteristics of the line, and transmitting the signal on the line in response to determining the transmission power.