Why Review a Clock Device

Clocks are often chosen last and with little thought, yet their jitter and their configuration decide the performance of the interfaces and the converters they drive. This review examines Renesas's programmable clock generators, the VersaClock family and the high-performance generators, from the perspective of an FAE who supports customers through design-in, with attention to jitter, configuration and layout.

Jitter and Phase Noise

Jitter is the timing error on the clock edges, and phase noise is the same error described in the frequency domain. Both affect high-speed interfaces, because the timing error appears directly in the link, and both affect data converters, because the sampling clock's error appears in the measurement. A clock with low jitter preserves the performance of the load, and a clock with high jitter can limit a system that is otherwise capable, which is why the clock belongs in the signal-chain design.

Where the Requirement Comes From

The acceptable jitter comes from the load, not from the clock: an interface standard or a converter datasheet sets the requirement, and the clock must meet it. Choosing the clock for the jitter the system tolerates, rather than for the lowest jitter available, keeps the design and the cost sensible.

Configuration and Flexibility

The strength of a programmable generator is its flexibility. A device such as the 5P49V6975 produces multiple configurable output frequencies from one reference, so it replaces several fixed oscillators with one part, and its configuration can be stored on-chip so it starts up correctly without external programming. In our bench work, that flexibility simplified the clock tree considerably, and it removed the risk of a wrong oscillator being fitted in production.

One Part for Several Products

Because the outputs are configurable, one device can serve several products or several versions of a product, which simplifies inventory and reduces the number of parts to qualify. That flexibility also lets a design change its clock frequencies without a board change.

Reference and Layout

The reference and the layout decide the jitter you achieve as much as the device does. Keep the reference close and clean, use a solid ground, and route the output traces as controlled-impedance lines away from the switching supplies. A clock routed near a switching regulator will carry that noise into the interface it drives, which is a common cause of an interface that works on the bench but fails in the product. Measuring the jitter with the real load and the real layout is the only way to know the result.

Measuring Jitter

Jitter should be measured with the real load, using the correct instrument and bandwidth, because the load and the routing affect the result. A figure measured into a test load can differ from what the interface sees, so the measurement must reflect the real conditions. BeiLuo's FAE team can measure the clock in our lab.

Verdict

Renesas's programmable clock generators are a sound choice for designs that need several clocks and low jitter. They replace fixed oscillators with one flexible device, they start up correctly from their stored configuration, and they deliver the low jitter that high-speed interfaces and converters require, provided the reference and the layout are handled with care. Confirm the jitter against your load, keep the reference clean, and measure the result with the real conditions. BeiLuo stocks mainstream Renesas timing parts and supports clock design in our lab.