12G-SDI Physical Layer Analysis and signal Quality Measurement
The quality of the SDI signal is important to both equipment manufacturers and equipment installers as this determines the maximum cable length over which the signal can be sent and successfully decoded. The quality of the SDI signal can be defined by the waveform shape and the level of jitter present.
Physical Layer Analysis of the actual analogue signal is essential for manufacturers, installers and infrastructure engineers to ensure that the SDI signal quality, whether it be HD, 3G, 6G or 12G, is a as good as possible and that jitter is within specified limits. Tools include auto-measuring Eye Pattern display, jitter meters, jitter histogram and jitter frequency spectrum instruments.
The shape of the Eye Pattern will show return loss issues and the transition or slew rate which is important because it determines the total cable length that the signal can be sent to a receiving device for successful decoding. The best way to measure the quality of the SDI signal is to measure the rise and fall times from the Eye Diagram.
The waveform shape will be different for SD, HD, 3G, 6G and 12G SDI signals. For SD the waveform will be more like a square wave and for 12G the waveform will be more like a sine wave. This is due to the attenuation of the harmonic frequencies as the fundamental clock frequency increases.
Regardless of the SDI clock rate, the most important aspects of the waveform are the amplitude, rise time and fall time. Overshoot and undershoot on the waveform are indicative of phase differences in the harmonic frequencies of the signal but if they are within specified limits should not affect the overall cable length.
Even if the waveform shape is perfect, the level of jitter will affect the total cable length. This is because the effect of jitter is accentuated as the signal level reduces because the rise / fall time of the signal becomes longer. So it is essential to remove jitter from the eye waveform and monitor it separately. Omnitek’s Ultra 4K Tool Box employs this method for this reason.