Clock Generator Basics and Applications

This page covers clock generator basics, including an application note for programmable clock signal generators and a list of clock signal generator manufacturers or vendors. Many digital circuits require a clock signal, which is a periodic square wave at a controlled frequency. The circuit that generates this clock signal is called a clock signal generator.

There are two main types of clock generators:

• Fixed: Generates a clock signal at a fixed frequency.
• Programmable (or Variable): Generates clock signals with varied input and output frequencies.

Clock signal generators have many applications, including:

• Wireless base stations for LTE, LTE-Advanced, etc.
• Picocells, femtocells, and small cells
• Sampling clocks for ADCs and DACs
• 1 Gigabit and 10 Gigabit Ethernet
• FPGA and processor clocks
• Line cards used in telephone exchanges
• Fiber Channel
• PCI Express cards

Programmable Clock Signal Generator Application Note

Figure 1: Clock generator from Silicon Labs (Si5380) used as a programmable clock signal generator in base station design.

As shown in Figure 1, the clock generator (e.g., Silicon Labs Si5380) provides a sampling clock as input to an ADC and DAC. It also provides a reference frequency signal as input to a synthesizer, which is used to provide the conversion frequency signal for RF up-conversion and down-conversion.

This clock generator boasts very low phase noise and excellent jitter performance.

The following table outlines the key features of the Silicon Labs clock generator (part number: Si5380):

Specification	Feature supported
Input frequency range	Differential: 10 MHz to 750 MHz LVCMOS: 10 MHz to 250 MHz
Output frequency range	Differential: 480 kHz to 1.47456 GHz LVCMOS: 480 kHz to 250 MHz
Phase noise	-159 dBc/Hz
Spurious	-103 dBc (Max.)
Serial interface	I2C, SPI

Clock Generator Manufacturers or Vendors

The following are popular clock generator manufacturers/vendors:

1. Integrated Device Technology, Inc.
2. ON Semiconductor
3. Texas Instruments
4. Silicon Labs (www.silabs.com)
5. Analog Devices