What are the "Four Heavenly Kings" for decrypting high-speed PCBs: Dk, Df, impedance, and insertion loss?

2026,03,14

As PCBs move towards the era of high frequency and high speed, printed circuit boards are no longer just the "base" that supports components. In addition to their conductive function, they also have the function of transmitting high frequency and high speed signals. When we talk about the electrical performance of high-speed PCBs, dielectric constant (Dk), loss factor (Df), characteristic impedance (Z0), and insertion loss are four unavoidable keywords. They are interrelated and together determine the transmission quality of signals on the circuit board.

1、 The definition and units of the four major indicators

1. Dielectric constant (Dk/ε r): The dielectric constant of the "deceleration zone" of a signal is a physical quantity that measures the ability of a material to store electrical energy under the action of an electric field. Simply put, it reflects the degree of "obstruction" that a signal experiences when propagating through a medium. Its definition is usually the ratio of the capacity of a capacitor made of this material as a medium to the capacity of a capacitor of the same size made of vacuum as a medium, so it is a dimensionless relative value (usually expressed as ε r).

·Numerical significance: The Dk of common FR-4 sheets is between 4.2 and 4.8, while the Dk of high-frequency sheets such as PTFE (polytetrafluoroethylene, commonly known as Teflon, Teflon) is usually between 2.2 and 3.0. The lower and more stable the Dk value, the faster the signal propagation speed, and the more favorable it is for high-frequency transmission.

2. Loss factor (Df/tan δ): The "thief" loss factor of energy, also known as the dielectric loss tangent or dissipation factor, is a parameter used to characterize the energy loss of dielectric materials in alternating electric fields due to the hysteresis effect or leakage caused by dielectric polarization. It represents the ratio of the portion of signal energy that "leaks" into the insulation board to the energy stored in the board, and is also a dimensionless physical quantity.

·Numerical meaning: The smaller the Df value, the better. The Df of ordinary FR-4 is usually around 0.02, while the Df of high-frequency high-speed materials (such as Rogers RO4350B) can be as low as 0.0037 or even lower. The smaller the Df, the smaller the signal heating and attenuation caused by the material itself.

3. Characteristic impedance (Z0): The characteristic impedance of the "ID card" of the transmission line is the ratio of the instantaneous voltage to the instantaneous current encountered when the signal propagates on the transmission line, measured in ohms (Ω). It is not a simple DC resistor, but a comprehensive characteristic determined by the distributed resistance (R), inductance (L), conductance (G), and capacitance (C) of the transmission line. In high-frequency environments, the characteristic impedance can be approximately simplified as Z0=√ (L/C).

·Numerical significance: In PCB design, the common impedance control for single ended signal lines is 50 Ω or 75 Ω, while differential signals are usually 90 Ω or 100 Ω. Maintaining impedance continuity (i.e. impedance matching) is the key to preventing signal reflection.

4. Insertion Loss (IL): The "road toll" of a signal. Insertion loss refers to the degree of attenuation of output power relative to input power after a signal passes through a transmission line, typically expressed in decibels (dB). It is a macroscopic final performance indicator that directly reflects the "cost" the signal incurs on its transmission path. Its mathematical definition is S21 = -10 * log(Po/Pi), where Pi is the input power and Po is the output power.

· Numerical significance: The smaller the absolute value of insertion loss, the better (i.e., the closer the dB value is to 0). For example, an insertion loss of -3dB means that the signal power is lost by half. In practical testing, the unit of insertion loss is usually db/inch. Why is it like this? In engineering applications, for standardized measurement, insertion loss is usually accompanied by a length unit (such as dB/inch or dB/cm), but in theoretical definitions and system link budgets, it is a pure dB value.

· Why is it usually written as dB/inch (or dB/cm): "Specific conditions" in the definition. The essence of insertion loss is the attenuation ratio of output power relative to input power. Since the longer the signal travels on the transmission line, the greater the attenuation, it is meaningless to simply say "the insertion loss is 3dB" - it must be specified on what length of transmission line it was measured. Therefore, in order to standardize the performance of materials in data sheets, manufacturers usually normalize the insertion loss to unit length, with common units including: · dB/inch: inches, commonly used by American sheet manufacturers (such as Rogers and Isola). · dB/cm: centimeters, commonly used by European and Asian manufacturers. · dB/m: meters, mainly used for describing very low-loss RF cables. · Why there is confusion: Context of the two expressions · Material property context (unit length dB): When we select materials, saying "the insertion loss of material A is 0.7dB/inch @ 10GHz" refers to the attenuation of 0.7dB per inch of transmission line at 10GHz frequency. This reflects the loss characteristics of the material itself. · System link context (total dB): When designers calculate the total attenuation of an actual transmission line (such as a 10-inch-long trace), they calculate it as 0.7dB/inch × 10 inch = 7dB (plus other losses such as connectors). At this point, the "total insertion loss of this link is 7dB" does not include the length unit, because it is the total attenuation value of the specific path. · Conversion and supplementary explanation · These two units are convertible: · 1 dB/inch ≈ 0.394 dB/cm · 1 dB/cm ≈ 2.54 dB/inch For simulation software or network analyzer testing, although the final displayed curve axis unit is dB, when setting the length of the device under test, the instrument has already considered the length factor through techniques such as "de-embedding", and the calculated result is actually the total loss dB value under the specific path.

When referring to "insertion loss of a material", it is usually accompanied by a length unit (such as dB/inch) to facilitate comparison of the merits of different materials. When referring to "insertion loss of a specific channel", it is usually simply written as dB, indicating the total attenuation of that path

Author:

Mr. lingchao

E-mail:

mr.xu@lingchaopcb.com

Phone/WhatsApp:

+86 13780181891