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Clock Oscillators Keep Time in MIL Systems

May 1, 2019
Various forms of crystal oscillators provide the signal generation and timing precision essential to the synchronization of many subsystems within military/aerospace systems.

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High-frequency crystals and crystal oscillators (XOs) must survive operating environments from the ground to deep space, providing timing signals in different forms and frequencies to maintain the accuracy of myriad electronic systems from communications through electronic warfare (EW) and radar. These XOs can be specified in a range of package styles to fit the mechanical and electrical requirements of a defense-based application. A quick review of the key performance specifications and a sampling of available oscillator types can help in choosing a clock XO when the time is right.

XOs are available with many different output types, including single-ended and differential waveform types. Single-ended clock signals can be routed on a printed circuit board (PCB) by a single-path transmission line (such as microstrip or stripline). Differential clock signals require a pair of transmission-line paths to channel the complementary differential signals. Single-ended clocks feature simpler circuit designs, while differential clock oscillators offer the suppression of power-supply noise and control of electromagnetic interference (EMI) that comes from the use of balanced lines.

Single-ended oscillator types include sources with sine waves and clipped sine waves, square waves, complementary metal-oxide semiconductor (CMOS) signals, high-speed CMOS (HCMOS) signals, low-voltage CMOS (LVCMOS) signals, and transistor-to-transistor-logic (TTL) signals. Among the differential output types are emitter-coupled-logic (ECL) signals, positive-emitter-coupled-logic (PECL) signals, low-voltage PECL (LVPECL) signals, current-mode logic (CML), low-voltage differential signaling (LVDS), and high-speed current-steering logic (HCSL).

As the names may indicate, the clock outputs differ in power consumption and speed as well as noise levels. In addition to standard clock oscillators, which generate a fundamental-frequency or overtone-frequency output from a crystal resonator, hybrid crystal oscillators integrate additional functions as needed, such as amplifiers, noise-suppression filters, and phase-locked loops (PLLs), to provide high-performance clock outputs in compact housings for applications where space is tight.

In the frequency domain, the spectral purity of different crystal oscillators is usually compared by their single-sideband (SSB) phase noise. In the time domain, for timing applications, the SSB phase noise basically equated to the phase jitter.

Jitter is a measure of the timing consistency of an oscillator’s signal waveform edges, essentially whether all rise times of the rising edges are equal and whether all fall times of the falling waveform edges are equal and occur at the same time. Low jitter refers to little or no deviations in an oscillator’s signal waveform edges and is, of course, preferable for most systems. High jitter can degrade system performance, such as causing a rise in the bit error rate (BER) of serial data transmissions.

Oscillator Varieties

Three types of XOs achieve different levels of stability by controlling thermal effects. Standard XOs offer good stability without additional circuitry. Temperature-compensated XOs (TCXOs) achieve somewhat higher frequency stability at a cost of greater complexity, higher power consumption, and slightly larger package size.

Oven-controlled crystal oscillators (OCXOs) are typically the most stable form of XO, but they are larger and consume more power than XOs and TCXOs. When some adjustment in oscillator frequency is needed, voltage-controlled crystal oscillators (VCXOs) are another form of XO that provides a small amount of tuning around the center frequency by means of an applied tuning voltage.

When sorting through any catalog of XOs, TCXOs, or OCXOs in search of the right fit for a clock oscillator application, output frequency, output signal format, frequency stability, jitter, and package style are key factors. Additional parameters to consider include supply voltage, power consumption, frequency pushing, frequency pulling, frequency tuning speed (for VCXOs), and post tuning drift (for VCXOs).

Modern clock and hybrid clock oscillators are housed in compact packages with a variety of output signal formats. (Courtesy of CTS Corp)

For military applications, crystal oscillators are screened according to applicable standards. These include MIL-STD-883 for bond pull, thermal shock, stabilization bake, temperature cycling, and constant acceleration testing; MIL-STD-202 for gross leak and fine leak; and MIL-STD-55310 for aging. The package styles for commercial clock oscillators involve packages with pins and surface-mount configurations (see figure). Clock oscillator packages are as small as 2.5 × 2.0 mm to meet tight circuit requirements.

A Sampling of Available XOs

What is an example of a low-jitter clock oscillator suitable for defense applications? The TX-707 series TCXOs from Vectron International covers a center frequency range of 8 to 52 MHz with fundamental-frequency HCMOS or clipped sine-wave output signals. Housed in a compact 7- × 5-mm surface-mount-technology (SMT) package, TX-707 TCXOs are shipped with initial frequency stability of ±1 ppm and maintain frequency stability of ±1 ppm after one year and ±4 ppm for 15 years. Models are available for +3.3- or +5.0-V dc supplies (10-mA typical current consumption) at operating temperatures from -40 to +85°C.

Typical SSB phase noise from 50 MHz for the TX-707 series is −82 dBc/Hz offset 10 Hz from the carrier, −113 dBc/Hz offset 100 Hz, −135 dBc/Hz offset 1 kHz, and −155 dBc/Hz offset 100 kHz from the carrier. A jitter specification is not provided, although the clock oscillator features a fast rise time of 5 ns.

For somewhat higher-frequency operation, the BOCS2 series clock oscillators developed by Bliley Technologies are OCXOs available with fundamental-frequency outputs from 1.5 to 60 MHz and third-overtone output center frequencies from 50 to 170 MHz. They can supply CMOS/TTL or HCMOS output types.

The BOCS2 OCXOs maintain frequency of stability of ±3 ppm after one year and ±5 ppm for the first five years for excellent long-term frequency stability. These oscillators exhibit low phase noise, with typical performance of −60 dBc/Hz offset 10 Hz from a 100-MHz carrier, −95 dBc/Hz offset 100 Hz from the same carrier, −125 dBc/Hz offset 1 kHz from the carrier, and −144 dBc/Hz offset 100 kHz from a 100-MHz carrier. The phase jitter is typically only 0.2 ps at 12 kHz to 20 MHz from the carrier.

Available with or without PLLs, these compact OCXOs fit a package measuring just 2.5 × 2.0 mm. The clock oscillators are MIL-STD-202 compliant and designed to meet the demands of QPL 55310 requirements. They can be equipped for supply voltages of +1.8, +2.5, or +3.3 V dc with options for temperature ranges of −20 to +70°C, −40 to +85°C, and −55 to +125°C. Versions are available with frequency versus temperature stability of ±25, ±50, and ±100 ppm.

When frequency tuning is required, a VCXO from Greenray Industries’ Y1600 series provides HCMOS outputs from 10 to 50 MHz with a ±10-ppm frequency adjustment range. The VCXO features initial accuracy of ±3 ppm with an aging rate of less than ±1 ppm for the first year. Even with the convenience of tuning, these clock oscillators feature outstanding phase-noise performance, with SSB phase noise of −105 dBc/Hz offset 10 Hz from a 10-MHz carrier, −135 dBc/Hz offset 100 Hz from the same carrier, −155 dBc/Hz offset 1 kHz from the carrier, and −162 dBc/Hz offset 100 kHz from a 10-MHz carrier. The XO is designed for a +5-V dc supply at operating temperatures from −20 to +85°C.

These are just a few of the many clock oscillators available for timing applications in military and aerospace systems. As with commercial and industry clocks, the trend is for smaller packages operating at lower power levels. Many of the XOs endure rigorous burn-in procedures to ensure high initial stability and excellent long-term stability for the first year and five years thereafter.

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