A Low Intermodulation Directional Coupler and Log Amplifier for High Power VSWR Measurement

There is a clear requirement for the measurement of feeder and antenna VSWR in all high power and high integrity communication systems. Most base stations for new cellular (GSM, PCN etc) systems have such measurement systems. The new digital modulation schemes make these measurements difficult due to the pulsed nature and wide dynamic range of the signals. The typical intermodulation distortion requirements of ¿150 dBc in the receive band make any device which follows the transmit output filter a very critical component. A directional coupler has been developed which allows a very simple through line construction to allow a minimum of intermetall-ic connections which may introduce intermodulation products. This coupler has excellent directivity (30 dB) and sufficient bandwidth to eliminate tuning as the isolated port has a second degree transfer function on transmission. A simple successive detection log video amplifier and associated sampling circuit allow a very low mainline coupling level to be used which prevents additional distortion from the detectors. The log amplifier uses silicon monolithic amplifiers to minimise parts count and cost.