The present invention pertains to wideband amplifiers of the type used in cable television transmission systems, and more particularly to circuits for controlling the gain and slope characteristics of such amplifiers.
A cable television (CATV) system generally has a "cable plant" that distributes broadband television signals originating in a "headend", or transmission station, to downstream subscribers. The cable plant has a number of "trunk" lines to which the headend is connected, and a number of feeder lines connected to the trunk lines fanning out to subscribers over the service area. The cable plant generally employs wideband amplifiers to regenerate the television signals periodically along the length of the trunk and feeder cables, in order to overcome losses in the cables and thereby provide a signal of adequate amplitude and quality to each subscriber.
The transmitted television signals are attenuated, or reduced in amplitude, as they travel through the cable plant. One function of the amplifiers, then, is to restore the signal amplitude to a known desired value. Because the signal attenuation is unknown a priori, the gain of the amplifiers must be adjusted to achieve this known signal amplitude, and thus require a function referred to as "gain control". The attenuation of the signals is also non-linear, i.e., higher-frequency signals tend to be attenuated more than lower-frequency signals. This nonlinear attenuation with frequency is termed "slope". Another function of the amplifiers is to restore the slope of the signals to a known desired value; this function is referred to as "slope control".
One particular cause of gain and slope variations in the cable plant is temperature variations. The attenuation and slope characteristics of transmission cable and amplifier components vary with temperature. However, because it is desirable that CATV systems have predictable operation across a wide operating temperature, compensation circuits are generally used in CATV amplifiers to compensate for such temperature-caused variations, i.e., to keep gain and slope within a much narrower range than would be obtained in an uncompensated system. CATV amplifier compensation circuits have generally been realized with analog components, including at least one component whose temperature-varying characteristics are exploited to counteract the temperature-caused variations in the amplifier slope and gain.
One specific class of CATV amplifier components requiring compensation includes components such as attenuators and equalizers that use devices known as "PIN diodes" (where "PIN" refers to a positive-intrinsic-negative doping profile used in manufacturing the diode). PIN diodes are specially-constructed diodes that operate as current-controlled variable resistors at high frequencies such as that of the television signals encountered in the CATV cable plant. PIN diodes have a non-linear current-versus-forward-resistance characteristic, so that circuits that provide PIN diode control current generally must operate in a corresponding non-linear fashion in order to achieve the desirable linearization of either slope or gain over an operating temperature range. Prior circuits have been limited in their ability to fully compensate for slope and gain across a broad temperature range. Also, prior circuits have generally been specially tailored for use in specific cable/amplifier configurations, and thus have not generally been usable for different cable/amplifier configurations.