Why Do Network Devices Use 25MHz and Its Multiples as Frequencies?

Why Do Network Devices Use 25MHz and Its Multiples as Frequencies?

In network devices such as network cards, switches, routers, and Ethernet equipment, the selection of crystal oscillator frequency is determined by the device's core functions, chip requirements, and communication protocol standards. The use of crystal oscillators with a frequency of 25MHz and its multiples is mainly due to this frequency system's close alignment with Ethernet standards, communication protocols, and chip architectures, which enables simplified design, reduced costs, and improved performance. A detailed explanation is provided below:

I. Why Choose 25MHz as the Base Frequency?

25MHz is not an arbitrarily selected value. It is widely used because it has an integer multiple relationship with several core frequencies in network communication, facilitating frequency multiplication or division.

1.1 Meets Ethernet Standard Frequency Requirements

Fast Ethernet (100BASE-TX): Uses a 25MHz clock as the data transmission frequency.

Gigabit Ethernet (1000BASE-T): Uses a 125MHz clock, which is 25MHz × 5.

10GbE (10 Gigabit Ethernet) and above: Although the main clock frequency is higher (e.g., 156.25MHz), it can usually be derived by multiplying the 25MHz crystal oscillator via a PLL.

This indicates that 25MHz, as the base clock frequency, can directly or indirectly support the operating frequencies of PHY/MAC for mainstream Ethernet speeds.

II. Technical Implementation Convenience

2.1 Easy Frequency Multiplication via PLL

Network chips (such as PHY, MAC, and switch chips) often use PLL (Phase Locked Loop) circuits internally to generate the required operating clock frequencies. 25MHz is a moderate input frequency that can efficiently generate common high-speed frequencies, such as:

25MHz × 2 = 50MHz

25MHz × 4 = 100MHz

25MHz × 5 = 125MHz

25MHz × 6.25 = 156.25MHz (for 10GbE)

Since 25MHz is a base frequency with easy-to-calculate low integer multiples, it simplifies PLL design, enables faster locking, and ensures more stable output.

2.2 Compatible with Mainstream Chip Requirements

Most network device chip manufacturers (such as Broadcom, Realtek, Marvell, Intel, etc.) explicitly support 25MHz crystal oscillator input in the input clock requirements of their PHY or MAC chips, or provide operating modes for 25MHz input.

III. Advantages in EMC, Circuit Design, and Cost

3.1 Better EMI Control and High Signal Integrity

Compared to crystal oscillators with higher frequencies (e.g., 50MHz, 100MHz, 125MHz, 150MHz, and 200MHz), 25MHz offers significant advantages in terms of Electromagnetic Interference (EMI):

A lower fundamental frequency results in fewer harmonic interferences;

PCB routing is easier to comply with EMI design specifications;

Lower requirements for enclosures and power integrity, facilitating system integration.

3.2 Mature, Stable, and Cost-Effective Crystal Oscillators

25MHz crystal oscillators are among the most widely used frequencies in the electronics industry, with the following advantages:

Abundant off-the-shelf products and a wide variety of types;

Low manufacturing costs and affordable procurement prices;

A mature supply chain with short lead times and stable quality.

This is highly attractive to network device manufacturers engaged in large-scale production.

IV. Application Example in Practical Design

Taking a Gigabit Ethernet switch as an example:

The main control chip requires an input operating frequency of 125MHz;

In the design, a 25MHz crystal oscillator can be used, and the frequency can be multiplied to 125MHz via the chip's built-in PLL;

Meanwhile, the 25MHz clock can be distributed to other modules (such as PHY, MAC, or clock synchronization modules) through a clock tree;

This achieves a unified clock source, simplifies the clock architecture, and reduces system complexity and costs.

Summary

The reasons why network devices use 25MHz and its multiples as crystal oscillator frequencies can be summarized as follows:

Hangjing Crystal Oscillators are widely used in network devices (network cards, switches, routers, Ethernet). In addition to industrial-grade products, we also offer wide-temperature high-stability crystal oscillators (-55~+125℃) and can provide crystal oscillator application and matching solutions. If you have relevant needs, please feel free to contact Hangjing's sales or technical personnel.