A high frequency amplifier includes a constant voltage driven amplifier 1 using as its amplifying element a bipolar transistor 7 with its base biased by a constant voltage, and a constant current driven amplifier 2 using as its amplifying element a bipolar transistor 8 with its base biased by a constant current. The serial communications device also comprises a signal controller configured to encode the digital diagnostic data onto a serial data signal for transmission via the serial cable by adjusting signal levels of the serial data signal while preserving original data in the serial data signal. This method allows the locality principle to be maintained, for example on experimental hardware. The perceptual constraint is a gain andor shape constraint. The same principals apply to the more popular max star operation.