What Is a Clock Buffer?
A clock buffer is an IC used to minimize the delay difference between clock pulses reaching multiple logic circuits in a printed circuit board system and to match (synchronize) the timing.
When operating multiple logic circuits, it is necessary to synchronize the circuits. A signal is needed to control the entire system, and the periodic signal of a certain frequency used for this purpose is a clock (like a metronome in music).
The group of circuits that operate in conjunction with the clock is called the synchronous circuit. The clock branches off in the system and propagates to several logic circuits. However, if this clock is not synchronized, the entire system may not operate as desired due to malfunctioning of logic circuits, and thus, a clock buffer is required.
Uses of Clock Buffers
Clock buffers are used in a wide range of applications, including personal computers, communication systems, and industrial equipment.
Specific applications are as follows:
- Digital logic circuits (CPUs, FPGAs, etc.)
- Data converters (analog-to-digital conversion)
- High-speed interfaces (USB, Serial-ATA, PCI-Express, etc.)
- Frequency conversion for wireless systems
As an example, the CPU (central processing unit), which is the core of PC operation, uses clock signals to synchronize each circuit (control unit, arithmetic unit, registers, interface with storage devices, and interface with input/output devices) to control its operation. The clock signal is used to synchronize the circuits.
For any of these applications, it is essential to synchronize the exchange of information and data among multiple circuits more accurately for stable operation.
Principle of Clock Buffers
Clock buffers are classified into “Non-PLL Buffer” and “PLL Buffer” according to the circuit type.
The main difference is the use of an internal phase-locked loop (PLL). The features of each are as follows:
- Non-PLL Buffer
The input signal is distributed without a PLL, resulting in less jitter degradation (periodic blurring). - PLL Buffer
Extremely small delay between input and output signals.
Since clock buffers can output multiple clock signals, it is possible to generate clocks that can be used by multiple circuits operating at the same frequency by providing a clock signal source and clock buffers for each.
This has the advantage of reducing component costs and facilitating printed circuit board layout. Other features of some products include level conversion and ease of configuration.
How to select Clock Buffers
Select the optimal device for the system to be designed after checking the device data sheet for the required input signal specifications, output clock frequency, number of signals, voltage, jitter, skew, input-output delay, clock buffers supply voltage, package dimensions, pin count, power consumption, cost, etc.
Ideally, the clocks distributed to each group of circuits in the system should switch at the same time 100% of the time, but this is rarely possible in reality. However, it is important to select a device with excellent characteristics to prevent circuit malfunctions.
Other Information on Clock Buffers
Usage and Role of Clock Buffers
To generate a stable clock that can be used in a system, a clock signal source and a clock product that supplements the clock signal source and distributes it appropriately to multiple circuits are required.
1. Clock Signal Source
Resonators and oscillators are used as clock signal sources. A resonator is a pendulum with a specific frequency.
It does not operate alone and can be divided into the ceramic, crystal, and SAW categories. An oscillator is a product that consists of a circuit that starts a pendulum to oscillate and keeps it oscillating (oscillation circuit) and a resonator in a package. Crystal, SAW, silicon, and MEMS oscillators are available.
2. Clock Products
Clock buffers are clock product that supplements the clock signal source. The output signal of the oscillator is input to the clock buffers. The role of the clock buffers is to output multiple clock signals of the same frequency as the input signal.
Other clock products that complement oscillators include clock generators (which can generate multiple frequencies), jitter cleaners (which improve jitter performance), and RF synthesizers (which output high frequencies), and designers need to choose the one that best suits their system requirements, including specifications and cost.