Coaxial SPDs can be mainly classified into gas discharge tube, semiconductor voltage limiting, DC isolation, and quarter-wavelength technologies based on their technical principles. Among these, gas discharge tube and semiconductor voltage limiting types form the core of basic protection, while DC isolation and quarter-wavelength technologies are key designs for achieving specific functions.

1. Gas discharge tube type

Working principle: Based on the principle of gas gap breakdown discharge. When the voltage between the terminals exceeds the insulation strength of the internal inert gas, the gas is ionized to form a plasma, providing a very low impedance path that quickly bypasses the surge current to ground. It belongs to the energy dissipation type of device.

Technical features:

Advantages: The nominal discharge current (In) and maximum discharge current (Imax) are extremely high (typically exceeding 10kA), and the energy handling capability is very strong; the inter-electrode capacitance is extremely low (typically <1pF), resulting in negligible impact on insertion loss and standing wave ratio for high-frequency signals.

Disadvantages: Relatively long response time (in the hundreds of nanoseconds); large dispersion in ignition voltage, resulting in higher residual voltage; in DC or low-frequency AC systems, there may be a "sustaining current" phenomenon after discharge due to the system voltage maintaining the arc, which needs to be addressed in the circuit design.

Application Role: Primarily used as a first-level (coarse protection) device or for primary protection of broadband radio frequency signals, responsible for discharging most of the lightning strike or switching surge energy.

2. Semiconductor voltage-limiting type

Working principle: It utilizes the nonlinear voltage-current characteristics of a semiconductor PN junction. When the surge voltage exceeds its clamping voltage, the device impedance drops sharply, absorbing or bypassing the overvoltage energy and limiting the voltage between the terminals to a predetermined value (clamping voltage). It belongs to the voltage clamping type of device. Main components include:

Transient Voltage Suppressor (TVS) diode: Based on avalanche or Zener breakdown principles, it has an extremely fast response time (picoseconds) and precise clamping voltage.

Semiconductor discharge tube (TSS): Based on the thyristor principle, it is a "switching" device that maintains a very low on-state voltage after conduction.

Technical features:

Advantages: Extremely fast response speed; low and consistent clamping voltage (Up), providing a high level of protection; low on-state voltage in TSS type devices, facilitating high current discharge.

Disadvantages: The current handling capacity and single-pulse/multi-pulse energy tolerance are far lower than those of gas discharge tubes; the junction capacitance of TVS diodes is relatively high, which may have a certain impact on ultra-high-frequency signal transmission.

Application roles: Often used as a second-level (fine protection) or single-level protection, for protecting voltage-sensitive chip interfaces, especially suitable for low-power, high-frequency digital and radio frequency signal lines.

3. DC isolation technology

Working principle: A high-voltage radio frequency decoupling capacitor is connected in series in the signal path of the protection circuit. This capacitor presents a low impedance to AC signals in the operating frequency band, ensuring smooth signal transmission; it presents a high impedance to DC and low-frequency surges, thus blocking the DC component.

Main objective:

Solving GDT current continuation issues: When used in conjunction with a GDT, it can effectively block the system's DC operating voltage, ensuring that the GDT reliably extinguishes the arc after a surge. This is an essential auxiliary design for reliable GDT applications.

System DC potential isolation: Used in RF systems requiring DC potential isolation, such as base station antenna ports with remote power feeding.

Technical essence: This is a critical auxiliary circuit design that ensures the normal operation of core protection devices (especially GDTs) and meets the system's electrical interface requirements, rather than being an independent type of protection.

4. Quarter-wavelength short-circuit protector technology

Working principle: Based on transmission line theory. A transmission line with a characteristic impedance of Z0 and a length of λ/4 (where λ is the wavelength at the operating frequency), when short-circuited at its end, ideally presents an infinite impedance at the input. At the operating frequency, the signal passes through without loss; when a low-frequency surge (whose frequency corresponds to a wavelength much greater than this physical length) arrives, this structure acts as a low-impedance path, directing the energy to the ground.

Technical features:

Advantages: Purely passive physical structure, no lifespan limitations, and extremely high reliability; excellent insertion loss and voltage standing wave ratio (VSWR) performance near the operating frequency; high power handling capacity, capable of withstanding high continuous wave power.

Disadvantages: Narrow bandwidth protection, only transparent to narrowband signals near the designed center frequency; physical size is related to the operating frequency, resulting in larger size for low-frequency applications; high manufacturing cost.

Application scenarios: This product is specifically designed for surge protection in high-power, narrow-band radio frequency transmission systems, such as the output terminals of communication base station transmitters in specific frequency bands and radar transmission channels. It is a high-performance, dedicated solution for these types of applications.