How to find the right Clock Generators AI guide
Written by an AI model from its trained knowledge.
What the AI looks for in Clock Generators
- Output signal standard
- Match the receiver: LVCMOS for general logic and MCUs, LVDS or LVPECL for FPGAs and SerDes, HCSL for PCIe reference clocks (100 MHz) A wrong output format will not meet the receiver's swing or termination needs, and an adapter network adds jitter and parts.
- Jitter
- Compare RMS phase jitter integrated over the same band, usually 12 kHz to 20 MHz; high-speed SerDes and data converters need femtosecond-class parts Jitter is what separates a clock generator for a transceiver or ADC from one for a microcontroller, and vendors quote different bands to look better.
- Output count and frequency plan
- Check that every required output frequency can be produced at the same time from one reference, not just individually Many parts share a PLL or VCO between outputs, so some frequency combinations are impossible or need fractional synthesis with more jitter.
- Configuration method
- Decide between pin-strapped, factory-programmed or OTP, and I2C/SPI-programmed at startup Register-programmed parts need firmware or an EEPROM before they output the right clock, which matters when the clock is needed at boot.
- Reference input
- Confirm whether the part needs an external crystal (and its load capacitance), an oscillator, or accepts a clock input The reference sets the accuracy floor and an unsuitable crystal can fail to start or pull off frequency.
- Supply and I/O voltage
- Core and output-bank voltages must match the board rails and the receiver logic levels Mismatched output-bank voltage gives wrong logic levels even when the frequency is right.
Brands that matter
- Skyworks (Silicon Labs timing): Took over the Silicon Labs timing line, including widely used hobby and low-jitter programmable clock generators.
- Texas Instruments: Broad range from simple fanout clocks to low-jitter LMK and CDCE devices with good design tools.
- Renesas (IDT): Large catalog of PCIe and network clock generators inherited from IDT.
- Analog Devices: High-performance clock generation for data converters and RF systems.
- Microchip: Programmable and PCIe clock generators, including parts from its Micrel acquisition.
- SiTime: MEMS-based clock generators and oscillators with field-programmable frequencies.
What to avoid
- Unbranded breakout boards that do not identify the clock chip or the reference crystal frequency.
- Comparing jitter figures quoted over different integration bands.
- Fine-pitch QFN or BGA parts if you plan to hand-solder without a breakout board.
- Register-programmed parts when you have no microcontroller or EEPROM to configure them at power-up.
Questions
Is a clock generator the same as an oscillator?
No. An oscillator produces one fixed frequency, while a clock generator takes a reference and synthesizes one or more outputs, usually with an internal PLL.
Can I use any 100 MHz clock for PCIe?
No. PCIe reference clocks are differential HCSL-type signals with specific jitter limits, so use a part specified for PCIe.