Keeping the Beat With Selectable Frequency Oscillators

Oscillators within digital systems provide beats that organize communication throughout the design. Information moves with beats maintaining elegant communication. Syncopated data flows along its pathways according to controls given on specified beats. Beats give life to the system with frequency and frequency is initiated with parts often coined the heartbeat.

The heartbeat uses crystals or other technology such as MEMS to produce frequency. Common features driving selection of oscillators are needs for off-chip or integrated driver circuitry and ability to reject noise. Purity of clock signal generation prevents jitter and thereby avoids injecting noise into the system. Quiet clocks ensure a healthy heartbeat and allow maximum bit rates for system communication.

Material used to transfer frequency through driver circuits must be compatible. Using buffers and internal level shifters allow interface with competing material technologies and drives options for configuration. Single-ended clocks may be translated into differential outputs. Incorporated drivers are able to divide down frequencies and to buffer inputs for multiple outputs. Features allowing options for use with multiple technologies enable flexibility for designing within mixed signal systems.

Consider Material and Selectables for Oscillator Driver Circuitry

Mixed signal systems often use multiple clocks within its overall design. Clock trees are configured for independent clocking blocks that may be synchronized to overall system communication needs. Using off-chip clock sources such as an oscillator with integrated drive circuitry allows customized options for linking design blocks.

Linking to design blocks requires optimization of jitter and power supply noise rejection. Clocks exist within noisy environments and choosing the right part for application requires knowledge of tolerances. Jitter specifications measure the purity of the clock and less jitter means less noise injected by the oscillator. Chipsets served by oscillators specify tolerance and reference designs are recommended for fine-tuning final selection.

Oscillators organize thoughts with frequency

Compatibility with system material technologies provides requirements for selecting parts. Choices for inputting one technology while outputting another are available. Technologies supported for either input or output nodes takes specific form with material fabrication. Typical materials found in oscillators are TTL, CML, CMOS, ECL, HCSL, LVDS, or LVPECL. Below we take a look at several varieties of inputs and outputs, in addition to selectable features for dividing frequency to suit system needs.

• CMOS Complementary Metal-Oxide Semiconductor. Material and topology with high noise immunity and low power consumption.

• HCSL High-speed Current Steering Logic. Open-emitter output used to drive PCIe.

• LVDS Low-Voltage Differential Signaling. A technical standard, TIA/EIA-644, defining protocol for twisted pair communication signals.

• LVPECL Low-Voltage Positive Emitter-Coupled Logic. Power-optimized differential-signaling system used in high-speed and clock distribution circuits.

• MEMS Microelectromechanical systems. Technology found in ICs that merges at nanoscale to accommodate more functionality per volume and may include moving nanomechanical parts.

• OE Output Enable. A control signal driving output from the part.

• OSC Oscillator. A device for generating oscillating electric currents or voltages by nonmechanical means.

• PCIe Peripheral Component Interconnect Express. A high-speed serial computer expansion bus standard.

• PSNR Power Supply Noise Rejection. The capability of an electronic circuit to suppress any power supply variations to its output signal.

• SDH Synchronous Digital Hierarchy. A standardized protocol that transfers multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs).

• SONET Synchronous Optical Network. A standardized digital communication protocol that is used to transmit a large volume of data over relatively long distances using a fiber optic medium.

Oscillators clock organized information through time

The oscillators highlighted below cover several options for inclusion within mixed signal systems. There are options to configure frequency at the factory or to configure frequency through selectable pins on the part. Some may be used within CMOS systems or others may have options for use with a variety of materials such as CMOS or ECL. Additional options include single-ended to differential outputs and buffering for multiple signal outputs.

DSC2311 Crystal-less^™ Configurable Two-Output Clock Generator

This is a dual output CMOS clock generator. Each output is custom set at the factory to your specifications. The outputs may be driven simultaneously allowing synchronizing to the system design needs. The part boasts high reliability qualified to high shock and vibration making it suitable in automotive and industrial applications. It has an output enable/disable feature for energy savings. It is contained in a VDFN package requiring a single external bypass capacitor.

The DSC2311 is a crystal-less^™ clock generator that is factory-configurable to simultaneously output two separate frequencies from 2.3 MHz to 170 MHz. The clock generator uses proven silicon MEMS technology to provide low jitter and high frequency stability across a wide range of supply voltages and temperatures. By eliminating the external quartz crystal, crystal-less clock generators significantly enhance reliability and accelerate product development, while meeting stringent clock performance criteria for a variety of consumer electronics, communications, and storage applications.

Found on page 1 of Microchip DSC2311 Crystal-less^™ Configurable Two-Output Clock Generator datasheet

Lead-free and RoHS-compliant, this space-saving oscillator finds use in consumer electronics, camera and imaging modules, home automation, industrial and power conversion, mobile communications, internet, and sensor devices. It is also used in solid state, hard drive, and flash drive storage solutions.

SY89876L Differential IN to LVDS OUT Programmable Clock Divider

The SY89876L is a programmable clock divider with dual outputs in selectable frequencies. The divider accepts various high-speed inputs such as CML, LVPECL, LVDS, or HSTL and outputs LVDS in programmed frequencies. It is available in a 16-pin MLF^® package and operates over a -40℃ to 85℃ temperature range.

This is a low-skew, low-jitter device capable of accepting a high-speed clock input signal and dividing down the frequency using a programmable divider ratio to create a lower speed version of the input clock. Available divider ratios are 2, 4, 8, and 16, or straight pass-through. The /RESET input asynchronously resets the divider. In the pass-through function (divide by 1) the /RESET synchronously enables or disables the outputs on the next falling edge of IN (rising edge of /IN).

Functional Block Diagram found on page 1 of Micrel SY89876L Programmable Clock Divider datasheet

Si562 Ultra Series^™ Quad Crystal Oscillator

This is a configurable crystal oscillator able to drive four output frequencies. Output topologies available are LVPECL, LVDS, CML, HCSL, CMOS, and Dual CMOS output options. Output options are configurable due to internal buffers able to generate either complementary or in-phase outputs.

The Si562 Ultra Series^™ oscillator utilizes Silicon Laboratories’ advanced 4th generation DSPLL^® technology to provide an ultra-low jitter, low phase noise clock at four selectable frequencies. The device is factory-programmed to provide any four selectable frequencies from 0.2 to 3000 MHz with <1ppb resolution and maintains exceptionally low jitter for both integer and fractional frequencies across its operating range. The Si562 offers excellent reliability and frequency stability as well as guaranteed aging performance. On-chip power supply filtering provides industry-leading power supply noise rejection, simplifying the task of generating low jitter clocks in noisy systems that use switched-mode power supplies. Offered in industry-standard 3.2x5mm and 5x7mm footprints, the Si562 has a dramatically simplified supply chain that enables Silicon Labs to ship custom frequency samples 1-2 weeks after receipt of order. Unlike a traditional XO, where a different crystal is required for each output frequency, the Si562 uses one simple crystal and a DSPLL IC-based approach to provide the desired output frequencies. This process also guarantees 100% electrical testing of every device. The Si562 is factory-configurable for a wide variety of user specifications, including frequency, output format, and OE pin location/polarity. Specific configurations are factory-programmed at time of shipment eliminating the long lead times associated with custom oscillators.

Block diagram found on page 1 of Silicon Labs Ultra Series^™ Crystal Oscillator Si562 Data Sheet

With configurable quad outputs available in frequencies from 200 kHz to 3000 MHz this Si562 is suitable for use in coherent optics, Ethernet, broadcast video, servers, switches, storage, NICs, search acceleration, test and measurement, clock and data recovery, and FPGA/ASIC clocking.

Oscillators may be found in diverse configurations to suit your application. Oscillators with included drive circuitry may be found for any type of logic be it CMOS or HCSL, LVDS or LVPECL. Above we looked at vendor offerings for a CMOS-specific part and two parts with diverse configurations available at either input or output of the oscillator. Additional features included options for either single-ended or differential outputs. Many configurations are available and finding the right one for your system is at your fingertips.

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