Why Are 25MHz and Its Multiples Chosen as Frequencies for Network Equipment?

In network devices such as network interface cards (NICs), switches, routers, and Ethernet equipment, the selection of crystal oscillator frequencies is determined based on the core functionality of the device, chip requirements, and communication protocol standards.

The use of 25MHz and its multiples is primarily because this frequency system is closely aligned with Ethernet standards, communication protocols, and chip architectures, leading to simplified design, reduced costs, and improved performance. Below is a detailed explanation of this topic:

I. Why Is 25MHz Chosen as the Base Frequency?

25MHz is not an arbitrarily selected value. It is widely adopted because it has an integer multiple relationship with some core frequencies in network communications, making it convenient for frequency multiplication or division.

Compliance with 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 beyond: Although the main clock frequency is higher (e.g., 156.25MHz), it can typically be derived from a 25MHz crystal oscillator via a Phase-Locked Loop (PLL).

This demonstrates that 25MHz, as a fundamental clock frequency, can directly or indirectly support the operating frequencies of PHY/MAC layers for mainstream Ethernet rates.

II. Convenience in Technical Implementation

1.Ease of Frequency Multiplication Using PLL

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

·25MHz × 2 = 50MHz

·25MHz × 4 = 100MHz

·25MHz × 5 = 125MHz

·25MHz × 6.25 = 156.25MHz (used for 10GbE)

Because 25MHz is a base frequency with easily calculable low integer multiples, it simplifies PLL design, enables faster locking, and ensures more stable output.

2.Compatibility with Mainstream Chip Requirements

Most network device chip manufacturers (e.g., Broadcom, Realtek, Marvell, Intel) explicitly support 25MHz crystal oscillator input in their PHY or MAC clock input requirements or provide operating modes compatible with 25MHz input.

III. Advantages in EMC, Circuit Design, and Cost

1. Better EMI Control and Signal Integrity

Compared to higher-frequency crystal oscillators (e.g., 50MHz, 100MHz, 125MHz, 150MHz, and 200MHz), 25MHz offers significant advantages in electromagnetic interference (EMI) mitigation:

·A lower fundamental frequency results in reduced harmonic interference.

·PCB routing more easily complies with EMI design specifications.

·Lower requirements for casing and power integrity facilitate system integration.

2.Maturity, Stability, and Cost-Effectiveness of Crystal Oscillators

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

·Abundant off-the-shelf products with a wide variety.

·Low manufacturing costs and affordable procurement prices.

·Mature supply chain, short lead times, and stable quality.

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

IV. Application Examples in Practical Designs

Take a Gigabit Ethernet switch as an example:

·The main control chip requires a 125MHz operating frequency input.

·A 25MHz crystal oscillator can be used in the design, with the chip’s internal PLL multiplying the frequency to 125MHz.

·Simultaneously, the 25MHz clock can be distributed to other modules, such as PHY, MAC, or clock synchronization modules, via a clock tree.

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

Summary

The use of 25MHz and its multiples in network equipment can be attributed to the following reasons:

Hangjing Crystal Oscillators are widely used in network devices (NICs, switches, routers, and Ethernet equipment). In addition to industrial-grade products, we also provide wide-temperature-range high-stability crystal oscillators (-55°C to +125°C) and offer crystal oscillator application and matching solutions. If you have any requirements in this regard, please feel free to contact Hangjing sales or technical support.