Clock signals provide reference timing to every integrated circuit and electrical system. While consumer applications typically use simple quartz crystals for reference-clock generation, other applications have much more sophisticated timing requirements and often require a combination of clocks to provide synchronization, generation and distribution.
In applications like wireless infrastructure and medical imaging, which require high fidelity in their analog-to-digital signal conversion, next-generation designs require higher resolution and faster data-transmission rates. Next-generation, high-performance networking and communications applications require faster transmission rates and higher speed data processing.
In these applications, clock signals play a vital role in the overall architecture. If not designed properly, the system-level performance of these applications can be limited by the performance of the underlying timing solution. Special care must be taken during the device selection and hardware design process to ensure that the clocking design maximizes system performance.
The quality of a clock signal depends heavily on its phase noise and jitter. An ideal clock source would generate a pure sine wave. All signal power (energy) would be generated at one frequency. However, in actuality, all clock signals have some degree of phase-modulated noise. This noise spreads the power of the clock signal to adjacent frequencies, resulting in noise sidebands.
The phase noise is typically expressed in dBc/Hz and represents the amount of signal power at a given sideband or offset frequency from the ideal clock frequency. Radio frequency (RF) and analog-to-digital conversion (ADC) applications require clocks with very low phase noise. In RF applications, increased phase noise can create channel-to-channel interference, degrading RF signal quality. In ADC applications, increased phase noise can limit the signal-to-noise ratio (SNR) and equivalent number of bits (ENOB) of the data converter.
Phase noise is the frequency domain representation of clock noise. Phase jitter, on the other hand, is the time domain instability of the clock signal and is typically expressed in picoseconds (ps) for high-speed clocks. Jitter can be described as the random variation in the actual clock signal’s edges versus its ideal waveform. Phase jitter is the figure of merit in high-speed digital applications including data communications, networking and high-definition video transmission. These applications require multi-gigabit data transmission rates as high as 40 Gbps. Physical-layer transceivers used in networking and HD video rely on low-jitter reference clocks that are internally multiplied within the transceiver to clock the high-speed data transmitted from the device. Excessive jitter can lead to higher bit-error rates that may exceed system-level requirements.
Managing phase noise and jitter in these high-performance applications is a necessity. Often, a jitter-attenuating clock IC or discrete phase-locked loop (PLL) is used to produce low-jitter clocks. A traditional PLL architecture comprises a phase frequency detector (PFD), loop filter (LF) and voltage-controlled oscillator (VCO) as shown in Figure 1 . Often, the PLL loop filter is implemented using discrete components. One of the more challenging elements in designing with high-performance PLLs is how to choose the “right” loop bandwidth for a given application. As with many engineering challenges, this is a tradeoff decision that has to be made at the application level.
Figure 1: Sources of PLL output jitter
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A jitter-attenuating PLL can be used to filter noise from the input clock and produce a low-jitter output clock. Reducing the loop-filter bandwidth increases the amount of jitter attenuation on the reference clock, transferring less jitter from the input to the output. If the reference clock has a significant amount of jitter, using a low PLL bandwidth to filter this noise is typically recommended.
However, it is not always advantageous to use a very-low PLL bandwidth. The chief reason for this is that the relative contribution of VCO noise to a PLL’s output jitter increases as the loop bandwidth decreases . Unless the PLL has a very-low-noise VCO, using a low PLL bandwidth can have the detrimental effect of actually increasing the output-clock jitter. Therein is the tradeoff decision. The PLL bandwidth needs to be set to minimize both VCO and reference jitter. Since the reference clock jitter can vary from application to application, this is a decision that needs to be made independently on each design, as shown in Figure 2 .
Figure 2: Balancing jitter transfer and jitter generation to optimize PLL jitter performance
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Another option is to use a clock IC with an internal VCO, but these devices typically require external loop-filter components that are sensitive to external noise sources. The interface between a PLL’s loop filter and its VCO is one of the most noise-sensitive nodes in a PLL design. Noise that enters a PLL through its external loop filter components will be present on the VCO’s input and will be multiplied by the VCO’s gain factor, increasing the VCO noise and subsequently, the PLL noise in the design.
Solutions using discrete loop filters also increase PLL design and layout complexity. PLL stability needs to be calculated for each unique frequency-plan and loop-bandwidth combination to ensure there is sufficient phase margin in the design. Some high-performance PLL designs use special PCB layout techniques, such as employing guard rings around the loop filter components to provide isolation and minimize leakage current. Since most traditional high-performance clock ICs require multiple, isolated power planes, the loop-filter layout considerations add further complexity to the PCB design.
Figure 3 illustrates a better approach to jitter attenuation based on Silicon Labs’ Si5317 jitter-cleaning clock IC. The Si5317 is a cost-effective, high-performance jitter-attenuating clock based on Silicon Labs’ proven third-generation DSPLL technology. The Si5317 device can accept a noisy reference clock at any frequency from 1 to 710 MHz, and it provides two ultra-low-jitter (0.3 ps rms, 12 kHz to 20 MHz) output clocks at the same frequency. The device’s operating frequency is set using control pins, such that no CPU intervention is required.
Figure 3: The Si5317 jitter-cleaning clock provides a cost-effective jitter-attenuation solution.
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