Currently, data transport of high definition serial digital interface (HD-SDI) signals at 3 Gb/s is state of the art and standardized by the Society of Motion Picture & Television Engineers (SMPTE). For return loss and voltage standing wave ratio (VSWR) measurements, suitable equipment is available. With upcoming ultra high definition (UHD) signals and related high frequencies, the discrepancies between available 50 Ω measuring equipment and defined 75 Ω video standards increase. This paper explains the influence and importance of return loss in a video system and shows a suitable test method which Neutrik believes is the best for high frequencies. Further return loss measurements compare Neutrik`s new rearTWIST UHD BNC connector, which is specifically designed for high frequencies, with UHD optimized connectors from common manufacturers up to 18 GHz (ST 2082-1). Signals with high bandwidth (e.g. UHD) and resulting high frequencies, it can be a critical factor. In addition also the VSWR is a function of the reflection coefficient, which describes the power reflected from the end of the system and is another representation of the return loss.

With the transition to UHD signals, including 4K and 8K signals with their higher data rates up to 24 Gb/s (ST 2083-1 pending) and related increased clock frequencies, the impedance of BNC connectors becomes more important than ever. Every impedance deviation has a negative influence on the return loss and VSWR values. In order to maintain the square waveform of digital signals harmonics (table 1) up to the 5th order should be considered. Taking the 3rd harmonic as a limit represents a reasonable compromise between signal waveform and reliable test results.