Protecting RF signal paths from accidental DC misconnects or ESD without affecting RF performance

Radio frequency (RF) data collection is a basic part of instrument testing. By their very nature, however, RF ports are connected to less than robust structures inside the test gear. high frequency transistors and integrated circuits (ICs) must have small, low breakdown voltage, and low capacitance junctions to produce the required gains at RF frequencies from the high frequency (HF) to the ultra-high frequency (UHF) bands. Output power comes at a premium in high frequency RF gear, so stages frequently function near their safe operating point. Therefore, there is little reserve to absorb unwanted energy bursts. The designer may also be forced to use topologies that have a direct, direct-current (DC) path from the connector to a sensitive component (e.g., the input of a high-speed Analog-to-Digital (A/D) converter). A DC block is not allowable, but the sampling board can be damaged if the input voltage exceeds just a few volts. The classic method of protection against electro-static discharge (ESD), DC misconnections or over-voltage conditions is to either use zener or Schottky diodes or a combination of resistive-capacitive (RC) networks that are not directly applicable to high frequency RF transmission lines. The insertion losses would be prohibitive and the port may still have poor protection. This paper identifies and addresses performance constraints of existing protection methodologies and present several network theory-based options to enhance and extend the operating range of the analog signals. Several methods of incorporating ESD and DC protection circuitry without affecting the RF signal integrity will also be shown.