Question: What is the Total PPM clock accuracy of a WirelessUSB based system?
Answer: Total PPM clock accuracy is the sum of base PPM, temperature PPM, aging PPM, and trim sensitivity PPM.
Base PPM: The base PPM is also known as the frequency tolerance of the crystal being used. Frequency tolerance is the allowable deviation from nominal frequency. Tolerance is usually specified in ± PPM, at +25° C and a specific load capacitance.Typical tolerances are from ± 10 to 50 PPM.
Temperature PPM:Temperature PPM is also known as frequency stability. Frequency stability is the allowable deviation, in parts per million (PPM), over a specified temperature range. Deviation is referenced to the measured frequency at +25° C. Typical frequency stability numbers range from ±10 to 30 PPM
Aging PPM: Aging is the change in the frequency of a quartz crystal unit with the passage of time. A typical aging PPM is 2 PPM per year.
Trim Sensitivity: Trim sensitivity is a measure of the incremental fractional frequency change for an incremental change in the value of load capacitance. Trim sensitivity is expressed in terms of PPM/pF. Typical trim sensitivities range from 5 to 30 PPM/pF.
Total PPM for a WirelessUSB LP system should be less than ±30 PPM.
