Activity Dip

Activity dips result from mechanical coupling of the principal resonance mode to one or more interfering modes that exist but are not electrically excited by the sustaining circuit. Resonance frequencies of these modes shift as the environmental temperature changes. At some temperatures, the frequency of the interfering mode(s) may come close to the frequency of the desired mode, causing the main mode to loose energy. This, in turn, causes an increase in the resonator equivalent resistance which manifests as a shift in output frequency. This shift is usually a rapid jump in the frequency over temperature characteristic. After the frequency jumps, the ***ooth frequency curve continues on a similar trajectory as before, but it is shifted up or down due to the jump. This rapid frequency change can cause system problems such as PLL unlock or packet loss. Quartz-based resonators are susceptible to activity dips. However, SiTime MEMS-based resonators are free of activity dips.

Aging

Aging is the change in oscillator frequency, measured in ppm over a certain time period, typically reported in months or years. This change in frequency with time is due to internal changes within the oscillator, while external environmental factors are kept constant.

Allan Deviation

Also known as short-term frequency stability, Allan deviation (ADEV) is the measure of oscillator stability in the time-domain. It represents a frequency change over an interval of time called averaging time. Allan deviation is calculated as the root mean square (RMS) change in successive frequency measurements. The averaging time typically ranges from milliseconds to thousands of seconds depending on the target application. The formula for Allan deviation is shown below, where the y values represent the values of fractional frequency deviation between adjacent clock cycles and M is the sample size. Allan deviation is used for clock oscillators because it converges for more types of oscillator noise compared to standard deviation. Allan deviation converges for white phase modulation, flicker phase modulation, white frequency modulation, flicker frequency modulation, and random walk frequency. Allan deviation does NOT converge for flicker walk frequency modulation and random run frequency modulation.

Clipped Sinewave Output

Clipped sinewave is a common single-ended output format often encountered in TCXO (temperature controlled oscillator) or OCXO (oven controlled oscillator) devices. The main feature of clipped sinewave output is very slow gradual rising and falling edges that resemble portions of the sinewave, hence the name. Slow rise/fall times have several benefits including reduced energy of high-frequency output harmonics that are undesirable in RF applications. This helps achieve good signal integrity with fewer restrictions in the layout rules. The drawback is slightly lower jitter performance at high frequencies compared to LVCMOS output. The diagram below shows a typical clipped sine waveform and the significantly slower rise and fall times.

CML

Current mode logic (CML) is a common oscillator differential output format. It is an open drain type output which means the driver only drives low and that external pull-up resistors are required to pull the clock signal high during the high portion of the clock period. Two voltage swings are commonly supported, 450 mV and 850 mV. The diagram below shows a typical 450 mV waveform. CML is commonly used in telecom infrastructure applications such as wireless base stations.

Cycle to Cycle Jitter

Cycle to cycle (C2C) jitter is defined as the variation in cycle time of a signal between adjacent cycles. It is measured over a random sample of adjacent cycle pairs (JEDEC JESD65B). The suggested minimum sample size is 1,000 cycles as specified by JEDEC. See related terms: Integrated Phase Jitter (IPJ), Long-Term Jitter, Period Jitter, Phase Noise

Differential

In contrast to single-ended output, differential output consists of two complementary signals with 180° phase difference between the two signals. This output type is often used in high-frequency oscillators (100 MHz and above). Differential signals usually have lower voltage swing than single-ended signals, faster rise/fall times, better noise immunity, and are used when better performance or higher frequency is required. The most commonly used differential signally types are LVPECL, LVDS, and HCSL. See related term: Single-Ended

DPPM

DPPM (defective parts per million) quantifies how many units may be defective per 1 million units. This unit of measurement is estimated with certain degree of confidence.

Duty Cycle

Duty cycle is a clock signal specification that is defined as the ratio in percentage between the pulse duration in high state to the period of the oscillator signal. The diagram below illustrates duty cycle % = 100* TH/Period, where TH and Period are measured at the 50% point on the waveform. Typical duty cycle specifications range from 45% to 55%.

Frequency

Frequency is the repetition rate (cycle) of the signal output from the oscillator and is measured in Hertz (Hz) per second. Many applications call for a specific oscillator frequency. Following is a list of standard frequencies and their typical applications.

Frequency Stability

Frequency stability is a fundamental performance specification for oscillators. This specification represents the deviation of output frequency due to external conditions – a ***aller stability number means better performance. The definition of external conditions can differ for different oscillator categories, but usually includes temperature variation. It may also include supply voltage variation, output load variation, and frequency aging. Frequency stability is typically expressed in parts per million (ppm) or parts per billion (ppb) which is referenced to the nominal output frequency.

Frequency vs Temperature Slope

Frequency vs temperature slope, also shown as ΔF/ΔT, is the rate of frequency change due to a 1°C change in temperature. It quantifies sensitivity of the oscillator frequency to ***all temperature variations near the operating temperature point. It is one of the major performance metrics of precision TCXOs that determines if the TCXO is stable enough to support the needs of the target application. Smaller frequency vs temperature slope values mean lower frequency variation due to the temperature change in a confined temperature window. For example, an average system temperature window may be ±5°C. In systems that require time and frequency transfer using IEEE 1588, better frequency vs temperature slope helps improve time error. The unit of measure is in ppm/°C or ppb/°C. Below is a plot of the SiT5356 Elite TCXO showing the frequency slope from 12°C to 13°C with a value of 0.86 pb/°C. This plot shows frequency error vs. the nominal frequency instead of absolute frequency, hence the y-axis label FERROR. The frequency vs. temperature slope is reported as the highest absolute value of slopes observed over the total temperature rage.

Gain Transfer or Kvco

Gain transfer or Kvco is a common characteristic of voltage controlled oscillators (VCXOs) that determines how much output frequency changes in response to a 1-V change in control voltage. This is useful in calculating the characteristics of closed loops that utilize a VCXO.

Hadamard Variance

Hadamard variance is the square of the change in three successive frequency measurements. These measurements are the values of fractional frequency deviation between three adjacent clock cycles and M is the sample size. Hadamard variance converges for white phase modulation, flicker phase modulation, white frequency modulation, flicker frequency modulation, random walk frequency, flicker walk frequency modulation and random run frequency modulation. It is unaffected by linear frequency drift and well suited for ***ysis of Rubidium oscillators. Below is the formula for Hadamard variance, where y represent the values of fractional frequency deviation among three contiguous clock cycles and M is the sample size.

HCSL

High speed current steering logic (HCSL) is a commonly used differential output format used for PCI Express, servers, and other applications. As shown below, it has a typical output swing of 700 mV and swings from 0V to 700 mV.

Holdover

Holdover is a mode of operation used by systems that are synchronized to an external precision frequency and/or time reference, and that have temporarily lost this reference signal. The local oscillator should have the capability to maintain, or holdover, stable frequency and/or time within the defined limits in a system after the loss of the external reference.

Integrated Phase Jitter (IPJ)

Phase jitter is the integration of phase noise over a certain spectrum and is expressed in picoseconds or femtoseconds. The diagram below shows an example integration band between f1 and f2 and the area under this curve is time domain picoseconds or femtoseconds of jitter.

Load

Within the scope of oscillators, load usually refers to capacitive load – the total capacitance driven by the oscillator output. Load consists of the input capacitance of the driven IC, trace capacitance, plus any other parasitics or passive components on the printed circuit board.

Long-Term Jitter

Long-term jitter measures the deviation of clock features from the ideal position over several consecutive clock cycles. This effectively measures how the duration of a number of consecutive clock cycles deviates from its mean value. See related terms: Cycle to cycle (C2C) jitter, Integrated Phase Jitter (IPJ), Period Jitter, Phase Noise