High-specification resistors for telecom applications are found in networking equipment, voice communications, VOIP, routers, bridges, multiplexes, fiber/wire converters, filtering, impedance matching, signal conditioning, and power management.
The adoption of Internet of Things (IoT) devices, the growth in technologies like 5G and the move to VOIP has created a need for smaller (surface mount) and more efficient resistor components.
In this post, we discuss the key performance characteristics to consider when selecting a resistor for a telecom application. We’ll cover the trade-offs involved in choosing the appropriate resistor technology. Whether it’s Thin Film, Metal Film, Thick Film, or Wirewound resistors.
Resistor Performance Characteristics
For telecom applications, the main resistor performance characteristics to consider are:
- Resistor value and tolerance.
- Power rating.
- Temperature coefficient of resistance (TCR).
- Frequency response.
- Industry standards.
When choosing any electronic component, there is always a trade-off between performance and cost. It is important to consider the specifications (including industry standards) and choose wisely. Ensure the resistor meets the requirements in the given environment, but don’t over-specify.
The first objective is to determine the required resistance value for the resistor. Then consider the tolerance value needed to meet the system requirements. In some telecom applications resistor tolerance is not an issue, in others, it is critical.
Power levels in telecom systems vary depending on the application. It’s crucial to select a resistor that can safely handle the maximum power without overheating or degrading performance. The operating environment can impact the resistors ability to dissipate heat. In some cases, it may be necessary to use cooling, heatsinks, or to derate the resistor.
Related to the above, is the resistors Temperature Coefficient of Resistance (TCR). The TCR indicates how a resistance value changes with temperature. If ambient temperature variations are an issue, choose a resistor with a low TCR.
Finally, consider the operating frequency range of the telecom circuit. For high-frequency applications, it is important to select resistors with low parasitic capacitance and inductance to minimise signal distortion and attenuation. In precision applications, resistor noise can be an issue.
Selecting A Resistor Technology
Resistor size, power handling capability and frequency response limit the use of some resistor technologies.
Thin film resistors exhibit low TCR and low noise characteristics. They are the first choice in applications where minimal signal distortion is critical.
The relatively high inductance of wire wound resistors means their high-frequency performance is poor compared to thick film and metal film. They are used in applications where high-frequency performance is not a primary concern.
The high-frequency performance of metal film and thick film resistors is adequate for many applications. Microwave and millimetre-wave frequencies equipment use thin film and specialised high-frequency resistors.
As discussed, high frequency performance rules wirewound resistors out of many telecom applications. Thin film resistors are not suitable for higher power applications or where a risk of pulse or surge events exists. Hence thick film resistors and metal film are the most common, general-purpose, types for telecom applications.
Both metal film and thick film are available in surface mount variants. Thick film resistors (in general) have superior power-handling capabilities. They can also use heatsinks to dissipate heat in higher power applications.
Metal film resistors are less robust than thick film under extreme environmental conditions. They also have a limited resistance range compared to thick film resistors.
In conclusion, thick film resistors are the lowest cost and thick film and metal film are the most general purpose. Wirewound and thin film have specific performance issues that restrict their use in many applications.
The small size and high-frequency performance of thick film resistors make them ideal for telecom applications. They are robust, can deal with power surges and survive in high-temperature applications.