System Clock Generators: PLL Synthesizer vs. Crystal Oscillator Clock--A comparison

System clock
Generation and distribution of a typical system-timing clock signal comprises functions such as an oscillator source driving a gain amplifier, a translation section to a standard logic level and a clock distribution network. These functions may be accomplished by component chipsets or a highly integrated single package (See Figure 1).

The source of a system-timing clock will require a reliable, precision- timing reference, usually a crystal. This article compares two crystal sources--the XO module and the PLL synthesizer--for a system-timing clock. Key characteristics to focus on are cost, board real estate, frequency accuracy and edge jitter or phase noise.

Today's very complex system designs could distribute numerous clock copy signals at several logic standards and several frequencies. Some boards may also demand tight skew and synchronization design requirements between several devices requiring zero delay buffers and skew tuning buffers. Multiple copies of a clock may require fanout buffering for distribution. Frequency multiples of a clock require a PLL synthesizer. All these requirements may be combined in challenging clock tree designs.

These two primary System Clock sources will each be discussed in regard to their key characteristics, key advantages, and disadvantageous limitations. The primary feature characteristics of frequency, accuracy, and stability will be discussed and analyzed. Applicable jitter basics are reviewed and comparison summary between system design options is presented.

Crystal oscillator clock (XO)
A classic system XO source typically uses a quartz crystal resonator although the discrete two-component solution (separate crystal and IC) remain a design alternative. For oscillator operation, the quartz crystal must be in a dynamic signal loop with a gain amplifier inverter to compensate for crystal losses, adjust for phase shift, and match impedances. The amplifier levels translated to into standard logic output levels for use by a system clock distribution network. For a generalized schematic of the Crystal Oscillator Clock (See Figure 2).

An XO clock is usually available as a hermetically sealed or "canned" module with an internal crystal and integrated circuitry for the translator and output buffer. These canned oscillators are complex to manufacture and may be relatively expensive with long lead times. Unique custom customer requirements such as higher frequencies often drive cost and lead times even higher.

The Crystal Oscillator (XO) Clock is generally limited to a single frequency and has only one single-ended output, or one complementary differential pair. Operation may be in a fundamental or at a harmonic multiple overtone mode.

Frequency is exactly defined as the number of oscillations per second, but is often approximated as instantaneous frequency (reciprocal of the wavelength period) measurement with significant error. Frequency precision refers to the number of significant digits in a frequency measurement. Frequency Accuracy is marginal error (deviation boundary) to a spec nominal or mean target, usually expressed in Parts Per Million (PPM). Crystal operational accuracy is typically measured at 25 degrees C, where effects due to changes in operating temperature, input voltage, aging shock and vibration are most stable.

Frequency Stability is deviation from a reference frequency over such parameters such as temperature, voltage, and time (drift and aging) is typically expressed in Parts Per Million (PPM). Common crystal spec stability values over voltage and temperature ranges are 25, 50 and 100 PPM.

The edge jitter or phase noise of an XO is an independent parameter of accuracy and precision. An XO clock module's total clock jitter should be given in picoseconds, while phase noise is only valid when specified over a sideband frequency range.

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