Clocks are where a one-line BOM note becomes a field failure. State whether you are buying a quartz crystal, a ceramic resonator, or a clock-oscillator module. Then state frequency, stability over the temperature you actually ship, and the supply if it is a module.
Write the stability with its temperature
A +/-20 ppm crystal at 25 C can be +/-50 ppm from -40 C to 85 C. If the radio or the metering error budget needs the tighter number at the temperature extreme, the part is an TCXO or a different cut, and the price moves. Say the temperature range on the RFQ. Say the aging per year if the product stays in calibration.
For a bare crystal, add load capacitance, shunt capacitance, and maximum drive level. Overdrive ages the crystal. For a module, add supply voltage, output logic level, and standby current. Jitter matters for high-speed digital and for some converters. If you do not have a jitter budget, do not invent a femtosecond number. Say logic clock and the supply.
- Frequency and which tolerance applies at which temperature.
- Crystal load capacitance, or module supply and output standard.
- Package and whether the pad map is an industry common size.
- Operating temperature equal to the product, not the lab.
- Aging, if calibration life is part of the product claim.
| Part | Must be on the PO | Easy to miss |
|---|---|---|
| Crystal | Hz, load pF, temp range, drive | ESR too high for the chip |
| Oscillator module | Hz, Vdd, output, temp stability | Enable pin left floating |
| TCXO | Stability over temp, control voltage if any | Buying a plain crystal instead |
Do not approve on frequency alone
Measure the sample in the real circuit if you can, because load capacitance is the board plus the specified capacitor. A frequency counter on a module is simpler: confirm volts and a clean logic swing. Keep the chip maker’s crystal guideline with the approved datasheet. Substituting a lower ESR or a different load because the frequency matches will shift the clock.
What to lock on the order
Put the frequency, the load or the supply, and the temperature range in the BOM description, not only in a linked datasheet. Buyers approve a line that says 16 MHz and receive a part that meets 16 MHz at room temperature only. If the chip maker names a list of crystals, start from that list and deviate only with a note that says what you recalculated. For a module, add the enable-pin state at power-up so the firmware and the hardware agree. Ask for the stability figure in the same terms as your error budget, over the same temperature. Incoming inspection can be a frequency check on a fixture at room temperature plus a paperwork check that the temperature grade matches the drawing. Do not skip the paperwork check because the room-temperature number looked right.
Questions buyers actually ask
Can I swap a crystal for an oscillator?
Only if the chip pin can take a clock input and the supply current is acceptable. A two-pin crystal needs the on-chip amplifier. A module already oscillates.
What does 20 ppm mean?
It is a fraction of the frequency, usually over a temperature range you must read. Room-temperature tolerance and the full-range figure are different lines.
Does the load capacitance matter?
Yes for a crystal. The wrong load pulls the frequency off. Copy the load the chip maker assumes, then the crystal load.