When selecting a crystal or designing an oscillator circuit, pF and ppm are two commonly encountered specifications. Although both affect frequency performance, they represent different parameters.
pF (picofarad) is a unit of capacitance, commonly used to specify the load capacitance of a quartz crystal. ppm (parts per million) expresses frequency deviation relative to a nominal frequency.
Understanding these parameters is essential for proper crystal selection and accurate clock operation.
pF stands for picofarad, a unit of capacitance. In quartz crystal specifications, it commonly refers to Load Capacitance (CL).
A passive quartz crystal requires an oscillator circuit, such as a Pierce oscillator, to generate a clock signal. The load capacitance is the effective capacitance presented to the crystal by the external circuit.
Crystal datasheets typically specify a nominal load capacitance, such as 8 pF, 12 pF, or 20 pF. Matching the circuit's effective load capacitance to this value helps the crystal operate at its specified nominal frequency under the reference conditions.
In a typical Pierce oscillator circuit, two capacitors, C1 and C2, are connected from the crystal terminals to ground.
The approximate effective load capacitance is:
CL = C1 × C2 C1 + C2 + Cstray
Where:
CL: Effective load capacitance
C1, C2: External load capacitors
Cstray: Stray capacitance from PCB traces, IC pins, pads, and other parasitic effects
For a simplified calculation, Cstray is often estimated at approximately 2–5 pF, although the actual value depends on the circuit and PCB layout.
When C1 = C2, the equation becomes:
C1 = C2 = 2 × (CL − Cstray)
For example, if a crystal specifies CL = 12 pF and the estimated stray capacitance is 3 pF:
C1 = C2 = 2 × (12 − 3) = 18 pF
Therefore, two external capacitors of approximately 18 pF may be selected as an initial design value.
Load capacitance affects the oscillation frequency of a quartz crystal.
Higher effective CL: The oscillation frequency generally decreases relative to the specified load condition.
Lower effective CL: The oscillation frequency generally increases relative to the specified load condition.
Excessive frequency deviation may affect communication timing, timekeeping accuracy, and wireless frequency performance.
Therefore, selecting the appropriate load capacitance is an important part of passive crystal circuit design.
ppm stands for parts per million. It expresses the relative difference between the actual frequency and the nominal frequency.
1 ppm represents a frequency deviation of one part per million.
For example, a 10 MHz crystal with a frequency tolerance of ±10 ppm has a maximum initial frequency deviation of:
10 MHz × 10 1,000,000 = 100 Hz
Its frequency may therefore range from 9.9999 MHz to 10.0001 MHz under the specified measurement conditions.
In crystal datasheets, ppm is commonly used for several parameters:
Frequency Tolerance
The initial frequency deviation from the nominal frequency, typically measured at a reference temperature of 25°C.
Frequency Stability over Temperature
The frequency variation across the specified operating temperature range, according to the reference conditions defined in the datasheet.
Aging
The gradual change in frequency over time, often specified in ppm per year.
In general, tighter ppm specifications indicate more stringent frequency performance requirements and may increase component cost.
Different electronic systems have different frequency accuracy requirements.
General MCU clocking: Requirements may fall around ±30 to ±50 ppm, depending on the application.
USB and Ethernet: Some designs require frequency accuracy within ±50 ppm or tighter, depending on the interface standard and system architecture.
Wireless communication: Applications such as GSM, LoRa, and Bluetooth may require tighter frequency accuracy, sometimes around ±10 ppm or better, depending on the system.
Precision measurement and GNSS: Certain applications require stability of ±2.5 ppm, ±1 ppm, or better. Temperature-compensated crystal oscillators (TCXOs) may be suitable for such requirements.
These values are illustrative. Actual requirements should always be confirmed against the relevant IC datasheet, communication standard, and system specifications.
Although pF and ppm describe different parameters, they are closely related in passive crystal applications.
A mismatch in load capacitance can shift the crystal's operating frequency, introducing additional frequency error expressed in ppm.
For example, even if a crystal has an initial frequency tolerance of ±10 ppm, an incorrect load capacitance may introduce additional frequency deviation.
The magnitude of this deviation depends on the crystal's electrical characteristics, including its motional capacitance and shunt capacitance, as well as the actual circuit conditions.
Therefore, the additional frequency error cannot be determined from the load capacitance difference alone.
Proper load capacitance matching is essential for achieving the intended frequency accuracy.
Start by checking the IC datasheet to confirm the required frequency and load capacitance.
Select a crystal with a compatible CL specification, then calculate the external capacitors using:
C1 = C2 ≈ 2 × (CL − Cstray)
The actual stray capacitance should be considered during PCB design and verification.
YXC crystal series such as YSX321SL, YSX221SL, and YSX211SL offer different load capacitance options for various circuit requirements.
For active oscillators, three key specifications should be checked:
Supply voltage
Output type (such as CMOS or LVDS)
Frequency stability (ppm)
YXC oscillator series such as YSO690PR and YSO212PU provide options for applications requiring reliable clock signals. Specific electrical characteristics and operating conditions should be verified against the corresponding datasheets.
For applications requiring tighter frequency stability, temperature-compensated crystal oscillators (TCXOs) or oven-controlled crystal oscillators (OCXOs) may be considered.
For example, the YXC YSO510TP series offers frequency stability options down to ±2.5 ppm and compact package options, making it a potential choice for wireless modules and GNSS-related applications, subject to system requirements.
pF and ppm describe different but related aspects of crystal frequency performance.
pF is associated with load capacitance and influences the operating frequency of passive quartz crystals, while ppm indicates frequency deviation or stability under specified conditions.
Understanding both parameters helps engineers select suitable timing components, design proper oscillator circuits, and achieve the required clock accuracy.
Explore YXC crystal and oscillator solutions at www.yxcxtal.com.
