Use Differential Clock Buffers with Ultra-Low Additive Jitter for Precise Timing Architectures

Modern wireless, high-speed digital, industrial, and embedded systems depend on precision system clocks, yet increasingly complex timing architectures make it difficult to distribute clean signals without accumulating jitter, which increases radio-frequency (RF) phase noise, narrows interface timing margins, and reduces measurement and control accuracy.

As clock trees grow to support more complex applications with higher fanout demands and diverse input/output format requirements, designers need more versatile distribution components that can still maintain the necessary timing precision.

This article discusses the challenges facing designers of precision timing distribution architectures. It then introduces differential clock buffers from Skyworks Solutions and shows how they can be used to meet these challenges.

How careful design addresses clock tree challenges

System designs continue to grow more complex across various application areas, presenting designers with correspondingly greater challenges in accurately delivering precision reference clock signals across larger circuit networks without compromising performance or adding cost. For example, in enterprise-level networking and datacenter applications, robust clock distribution trees must support switch fabrics, multi-domain synchronization, and clock translation between subsystems. In computer systems using high-throughput interconnects such as PCI Express, reliable performance depends on adherence to tight timing margins. In industrial and embedded systems, accurate clock signaling is critical for precision data acquisition and control loops.

In each of these applications, clock buffers play a central role in propagating the reference clock across the clock tree. In doing so, they are expected to operate across multiple signaling formats and voltage domains while minimizing additive jitter, which is the incremental jitter contributed by a clock buffer at each stage of the clock tree. Additive jitter is affected by factors such as slew rate, output format, power supply voltage, and the clock buffer’s performance characteristics.

Slew rate: Although an ideal clock buffer switches at a constant voltage threshold, the switching threshold for real-world devices can vary within a window (Figure 1). The slower the input slew rate, the longer it takes the signal to reach the actual threshold before the buffer switches, inducing additive jitter in the output.

Figure 1: The input slew rate influences additive jitter by affecting how long a signal remains within the buffer’s switching-threshold region. (Image source: Skyworks)

Output format: Different logic families exhibit varying levels of additive jitter because they drive signals with different voltage swings, edge rates, and termination behaviors. Compared to logic families using low-voltage differential signaling (LVDS) formats with smaller swings or slower edges, logic families such as low-voltage positive emitter-coupled logic (LVPECL), with larger swings and faster edges, cross the receiver’s threshold region more quickly. This reduces sensitivity to slight variations in noise or supply conditions during the transition. Termination style and driver topology also influence how uniformly a signal transitions under load, which contributes to differences in jitter behavior across logic families.

Supply voltage: Supply voltage affects additive jitter because variations on the power rail can shift the internal switching thresholds of the buffer circuitry and momentarily alter the effective edge timing as the device regenerates the input clock. When supply noise modulates these thresholds, even slightly, the clock edge may cross the threshold earlier or later than intended, producing additional timing uncertainty. Of course, this effect is more pronounced with slower input edges or logic families that generate smaller voltage swings, where the signal only marginally exceeds the switching threshold.

Clock buffer performance characteristics: The characteristics of the clock buffer ultimately determine how effectively it manages the factors that influence additive jitter across a clock tree.

How differential clock buffers facilitate timing accuracy

As system requirements tighten, Skyworks' SKY535xx clock buffers provide the combination of ultra-low additive jitter and support for different logic families required in precision timing architectures. Their performance and flexibility meet requirements across a wide range of use cases, including PCIe Express Gen1 through Gen7 systems, high-speed networking, timing-critical industrial and embedded systems, clock domain format translation, and synchronization in timing-sensitive wireless and instrumentation applications.

The flexible input stage of these devices features a 3:1 multiplexer supporting two universal any-format inputs (CLK0, CLK1) and one crystal input (XA). Their output stage includes two clock output banks (Bank A and Bank B) that support 10 total differential outputs in the SKY53510, eight in the SKY53580, and four in the SKY53540.

In addition, the SKY535xx devices provide separate supply pins for the core logic (VDD), reference output (REFOUT) clock driver (VDDOC), and each output bank (VDDOA, VDDOB) (Figure 2). They also integrate low-dropout (LDO) regulators to help maintain high power supply rejection while simplifying design by reducing the number of external components needed to support low-jitter operation.

Figure 2: SKY535xx devices support complex clock-tree configurations, including a 3:1 input multiplexer and two independent output banks, enabling low-jitter clock distribution across multiple formats and voltages. (Image source: Skyworks)

To support flexibility across multiple logic families and supply rails, the SKY535xx family’s two universal inputs accept a broad range of widely used clock formats and voltage ranges on CLK0 and CLK1. These formats include LVPECL, LVDS, scaled LVDS (S-LVDS), high-speed current-steering logic (HCSL), current-mode logic (CML), stub series-terminated logic (SSTL), high-speed transceiver logic (HSTL), and AC-coupled low-voltage CMOS (LVCMOS) at 1.8 V, 2.5 V, or 3.3 V.

Designed to operate independently from dedicated 1.8 V, 2.5 V, or 3.3 V sources, the SKY535xx devices’ two output banks can be programmed to generate LVPECL, LVDS, S-LVDS, HCSL, or tristate (Hi-Z) output using the SFOUTA and SFOUTB output signal format control pins for output Bank A and Bank B, respectively (Figure 3).

Figure 3: Dedicated output signal format control pins (SFOUTx) enable independent selection of the output signal format for each of the SKY535xx device's two output banks. (Image source: Skyworks)

Built for high-performance clock distribution, the SKY535xx devices support high-frequency operation with each output format, including direct current (DC) to 3.1 gigahertz (GHz) for LVPECL, DC to 3 GHz for LVDS, and DC to 800 MHz for HCSL. At the same time, they exhibit ultra-low additive jitter across all formats. For example, these devices show an additive jitter of only 35 femtoseconds (fs) RMS (typical) and 47 fs RMS (maximum) for a 156.25 megahertz (MHz) clock in LVPECL format, measured with an integration bandwidth of 12 kilohertz (kHz) to 20 MHz (Figure 4). They demonstrate similar performance across other output formats, with only a modest increase in jitter at lower frequencies.

Figure 4: SKY535xx devices exhibit ultra-low additive jitter across their output logic formats, with only a modest increase in jitter at lower frequencies. (Image source: Skyworks)

The combination of performance and flexibility offered by Skyworks’ SKY535xx devices makes them particularly efficient for supporting complex timing architectures that require multiple clock domains, signaling standards, and voltage levels to coexist without compromising jitter performance. Their scalable fanout supports clock-tree expansion without requiring additional devices that could introduce further additive jitter or timing uncertainty and increase design cost and complexity. Furthermore, by supporting multiple output formats and levels, a single SKY535xx device can serve heterogeneous endpoints, simplifying design and reducing the number of buffer devices required.

To help ensure clean clock signals in extended distribution nets, the SKY535xx family’s REFOUT driver incorporates synchronous output-enable reference (OE_REF) sampling, ensuring that REFOUT begins switching only at defined clock boundaries. This feature helps stabilize downstream timing behavior by avoiding malformed pulses that could otherwise lead to false edge detection or spurious transitions, resulting in ambiguous or incomplete transitions in downstream logic.

Implementing ultra-low-jitter clock distribution solutions

To achieve rated additive jitter performance, Skyworks recommends operating these devices with a differential slew rate of 3.0 V per nanosecond (V/ns) and 1.0 V/ns for single-ended formats. As noted earlier, additive jitter can rise as slew rate falls with any clock buffer. With these devices, however, designers can use the integrated XA crystal input to reduce additive jitter in clock distribution designs that operate at slower frequencies or lower amplitudes, which reduces slew rates. A comparison of additive jitter resulting from driving the CLK0 or XA input with a single-ended sine wave shows that the XA crystal input can exhibit reduced jitter (Figure 5).

Figure 5: At lower frequencies and input amplitudes, driving the XA input with a single-ended sine wave can achieve lower additive jitter than driving the CLK0 or CLK1 input with the same signal. (Image source: Skyworks)

As mentioned earlier, slew rate is only one of the multiple factors that affect jitter in a clock tree. Consequently, the successful implementation of complex clock distribution solutions depends on careful evaluation of proposed configurations and performance measurement.

To this end, Skyworks' SKY53510-EVB evaluation board serves as both an evaluation platform and a reference design for characterizing device performance and validating implementation practices. Designed for ease of use, the board requires no software setup and instead provides multiple jumpers and switches for configuring an onboard 10-output SKY53510 (Figure 6).

Figure 6: The SKY53510-EVB evaluation board provides complete access to the SKY53510 clock buffer's pins via a set of jumpers and switches, simplifying the evaluation of different clock buffer operating configurations. (Image source: Skyworks)

Designers power the board by connecting a wall wart power adapter, a USB cable, or an external 5 V_DC source. Separate jumpers independently configure VDD, VDDOA, VDDOB, and VDDOC for 1.8 V, 2.5 V, or 3.3 V operation and allow the use of the four dedicated onboard LDOs, or an external power source bypassing the LDOs. The device's CLK0 and CLK1 are accessible via subminiature version A (SMA) connectors, supporting differential or single-ended clocks.

Alternatively, designers can drive the SKY53510's XA input using the onboard 54 MHz crystal or an external clock. Output Bank A and Bank B can each be configured independently for LVPECL, LVDS, S-LVDS, HCSL, or Hi-Z using a DIP switch, and each bank includes selectable supply voltages to support level translation and mixed-format clock distribution.

The board's configurable input section allows designers to compare the behavior of differential inputs on CLK0 and CLK1 with the crystal-based drive on XA, assess the impact of appropriate AC and DC termination for different logic families, and evaluate how input slew rate influences additive jitter. The board also includes reference termination networks for LVPECL, LVDS, S-LVDS, and HCSL outputs, providing practical examples for preserving edge quality and minimizing jitter in production layouts.

The board features CAL_IN and CAL_OUT calibration traces that are an exact match in length and trace geometry to the input and output paths, enabling accurate measurement of the propagation delay and output-to-output skew parameters that underlie multi-domain clock distribution performance.

Conclusion

The timing architectures needed for high-performance applications increasingly challenge designers to distribute clean reference clocks across multiple domains and signaling formats. The Skyworks SKY535xx differential clock buffers meet these challenges with their ultra-low additive jitter and flexible input and output options.