by IDT
There are five timing budget parameters which influence the total system timing error of a quartz crystal and oscillator combination: frequency tolerance, frequency stability, aging, load capacitance; and oscillator accuracy. In this post, we will discuss the basic guidelines in selecting the proper quartz crystal to meet a design’s timing budget.
Frequency Tolerance
Also known as calibration accuracy, frequency tolerance is the amount of frequency deviation from a specified center frequency at ambient temperature (referenced at 25°C). In addition, this deviation is associated with a set of operating conditions including load capacitance and drive level. Similar to the following four parameters, it is specified in units of ppm (Parts per Million).
Frequency Stability
Frequency stability is the amount of frequency deviation from the ambient temperature frequency over the operating temperature range. This deviation is associated with a set of operating conditions including:
– Operating Temperature Range, Load Capacitance, and Drive Level.
This parameter is specified with a maximum and minimum frequency deviation, expressed in percent (%) or parts per million (ppm). The primary factors that determine frequency stability are:
– Type of quartz cut and angle of the quartz cut.
Some of the secondary factors include mode of operation, drive level, load capacitance, and mechanical design. Crystal manufacturers typically specify this on their datasheets.
Aging
Aging is the systematic change in frequency with time due to internal changes in the crystal which is related to the crystal contamination and drive level. Over time, particles drop off or fall onto the quartz surface, hence slightly changing the resonant frequency. Aging is often expressed as a maximum value in parts per million per year [ppm/year]. The rate of aging is typically greatest during the first 30 to 60 days after which time the aging rate decreases. The following factors effect crystal aging:
– absorption and desorption of contamination on the surfaces of the quartz
– stress relief of the mounting and bonding structures
– material outgassing
– seal integrity
This specification can vary among manufacturers. Crystal manufacturers typically specify this on their datasheets.
Load Capacitance (CL)
The fourth parameter to consider is load capacitance (CL). A crystal can be characterized for either series or parallel load resonant mode of operation. Both modes are physically the same; but tuning allows them to operate in a different area of the crystal reactance curve. For most applications, IDT recommends using parallel resonant crystals which require using external load capacitance. It is important to consider some of the board parasitic and internal capacitance of the oscillator before adding the load. The correct method is to calculate all the board parasitics; then add the required capacitance to equal the specified load capacitances. You can minimize the variation for load capacitance by using smaller package capacitors with small tolerances.
Oscillator Accuracy
The fifth parameter to consider is oscillator accuracy. Many times, designers ignore this parameter but process shifts in the silicon, temperature, and voltage can have an effect on the center frequency. The process shift parameters dominate this variation. The amount of internal load capacitance in the oscillator and the trim sensitivity of the quartz crystal can minimize this variation. The datasheet does not typical specify a crystals trim sensitivity but you can request that information from the manufacturer. Trim sensitivity shows the effect on frequency due to load capacitance. Most of the time, properly designing and manufacturing an oscillator on an established process significantly reduces this variation.
Example
Below is an example of a crystal timing budget calculation. First, we choose the appropriate crystal. For the example below, we are targeting 50 ppm accuracy for the system. Figure 1 shows an example of a crystal electrical specification. Most manufacturers have similar values and variables.
(Figure 1. Example of a Crystal’s Electrical Specifications)
Frequency Tolerance = ±15 ppm
Frequency Stability = ±15 ppm
Aging = ±10 ppm total for 10 years
The accuracy of the oscillator across temperature, voltage and process is ±3.5 ppm. This is assuming a trim sensitivity of 7 ppm/pF, a 10% process shift and 5 pF of internal load capacitance (CL).
The load capacitance accuracy, which will include board and pin parasitics, is equal to ±0.5ppm. This is assuming a trim sensitivity of 7ppm/pF, minimal PCB process shift, 1% tolerance load capacitors and external load capacitance of 7 pF. The 5 pF internal and 7pF external load capacitance will fulfill the required 12 pF load capacitance to properly tune the crystal.
The sum of all the parameters is the total system timing error.
Maximum overall timing error = 15 + 15 + 10 + 3.5 + 0.5 = 44 ppm
For any concerns or questions regarding these crystal specifications, please contact the manufacturer. If your application requires a tighter accuracy system timing error, contact the manufacturer for a new custom part number. IDT does not have an exclusive preferred quartz crystal source. IDT devices function properly with many of the mainstream and established quartz crystal manufacturers.
About IDT
IDT develops a broad range of low-power, high-performance mixed-signal semiconductor solutions. You can find more information about IDT’s timing products at www.idt.com/go/timing. You can also find answers to commonly asked questions about IDT’s products, applications, and technology in the IDT Knowledge Base.
