Understanding power issues in SDI products (Part 1)

Broadcast video products continue to push the boundaries of performance, many beyond the published industry standards. There are a broad range of components required to design a product that meets the numerous industry requirements like jitter, eye timing and signal rise/fall, to mention a few. Each component in the video signal path can have an impact on the product's performance. The analog and mixed-signal serial digital interface (SDI) components–such as cable equalizers, cable drivers, reclockers, serializers and deserializers (Ser/Des)–form a critical portion of the video signal path.

They have power requirements, specifically for V_CC voltage ripple, that are often understated or misunderstood with regards to their impact on SDI video quality. Focus on this design parameter can make the difference between a product whose performance is acceptable and exceptional. This article will discuss the issues and related tradeoffs that V_CC ripple has on the performance of these video components.

The V_CC ripple specification for SDI components like those shown in figure 1 is typically not found in the device's product datasheet because the ripple limitations depend on a particular product's specific performance needs.

For example, what video data rates will be supported? What cable length (cable reach) is required at the product input? Meeting the product design specification requires managing the imposed jitter at each stage of the SDI signal path.

The V_CC ripple on an SDI component can have a direct impact on the jitter and therefore, the resulting product performance. Designers of SDI components make every attempt to make their devices tolerant to power supply ripple and noise. However, understanding the V_CC ripple requirements for these devices and the associated performance tradeoffs is still an important consideration. This is especially true for 3G data rates and newer components which enable long cable reaches.

Cable equalizers
The cable equalizer (EQ) is the receive component in the SDI signal path and provides both amplification for and restoration of the incoming video signal. A 3G video signal can be attenuated by as much as 50dB on a 180m length

of cable (Belden 1694A). That means a signal with 800mV of amplitude, at the cable driver, would be reduced to about 3mV after 180m of cable at the input of the EQ. The signal to noise ratio (SNR) plays a key role in powering a 3G EQ like the LMH0394 and LMH0395 (dual output), which can restore a signal after 180-200m of cable loss.