Frequency Control

Ultra-low jitter clock synthesiser

23rd March 2016
Joe Bush

A low phase noise integer-N synthesiser with an integrated VCO and an ultra-low jitter clock distribution section has been introduced by Linear Technology that is well suited for clocking data converters. The LTC6951 features four current mode logic (CML) outputs, each equipped with an independent low noise clock divider and digital delay block to cover a wide frequency range between 1.95MHz and 2,700MHz.

With a total of 115fsRMS absolute jitter (SNR method), the LTC6951 delivers the low jitter clocks necessary to achieve SNR when clocking data converters with high input frequencies and fast output data rates.

The LTC6951 introduces three intelligent schemes to simplify output clock expansion and the creation of large clock trees employed in systems with multiple daughter cards or with a large number of data converters. Linear Technology’s proprietary EZSync output synchronisation method guarantees repeatable and deterministic phase relationships between all clock divider outputs on the LTC6951 and accompanying EZSync supporting devices.

The ParallelSync multi-chip parallel synchronisation feature allows the outputs of multiple LTC6951 ICs to be re-timed to the common reference clock. This permits reference aligned synchronisation in the reference clock domain with easy-to-meet nanosecond range set-up and hold time requirements. The EZ204Sync JESD204B subclass 1 compliant synchronisation method builds on the previous two approaches and enables the generation of the SYSREF and DEVCLK signals essential to this JEDEC standard across multiple parallel connected LTC6951 ICs along with any other EZSync compatible clock devices.

Designing with the LTC6951 can be done using the LTC6951Wizard simulation and design tool, available for free download at The LTC6951Wizard software provides appropriate PLL settings and loop filter component values with a click of a button, and predicts the individual output’s phase noise and jitter. Besides performance simulation, the LTC6951Wizard GUI features a scope plot that simulates time domain results of the LTC6951 outputs based on the clock divider, delay and synchronisation settings, simplifying the design process and assisting in the circuit debugging phase.

The LTC6951 is specified over the full operating junction temperature range from -40 to 105°C. It is available in a 5x7mm, 40 lead plastic QFN package.

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