Bias tees allow the insertion of direct current (DC) voltages into a signal path without disrupting the existing signal in that path. They are used to supply an active device such as an antenna amplifier, laser diode, photodiode, or optical modulator with a bias current or bias voltage while allowing high speed, radio frequency (RF) signals to pass through with minimum signal degradation. Typically, bias tees are used with active devices that are also remote, such as a satellite dish or TV antenna on a rooftop. By allowing the active device to receive signal and power over one set of cables instead of two, bias tees reduce the number of cables that are required.
How Bias Tees Work
Bias tees are three-port, coaxial components that accept DC voltage from a power supply. They are often box-like and, as their name suggests, shaped like the letter T. Because DC and radio signals are relatively easy to separate, bias tees allow power to be sent over the same coaxial cable used to transmit RF signals. The low-frequency port is used to set the DC bias or offset. The high-frequency port passes the RF signals but blocks the biasing levels, which would otherwise cause signal distortion. The combined port connects to the active device.
When selecting bias tees, industrial buyers should select products with designs that support the application’s current level. For both low-current and high-current applications, bias tees feature a DC block in the center conductor of the coaxial line. As their name suggests, DC blocks are designed to prevent or block the flow of direct current through RF circuits. In this way, these capacitors (C) serve as high-pass filters that prevent 0-Hz DC voltages from interfering with RF components.
For low-current applications, a resistor (R) provides the connection between the DC input and the coax cable’s center conductor. To avoid loading the coaxial line, the resistance value for this electronic component is greater than the impedance. For higher-current applications, bias tees use an inductor (L) instead of a resistor because the power drop across R or the power dissipated in R would be too great.
Connectors, Frequency, and Impedance
When selecting bias tees, buyers need to specify the correct RF connector type for their installation. Connectors carry numbered or lettered designations, and differ in terms of attributes such as size and coupling mechanism. Most importantly, however, RF connectors are rated for a specific impedance and frequency range. For example, Type C connectors provide constant 50-ohm impedance but may be used with 75-ohm cable below 300 MHz. Typically, cable and satellite TV applications require 75 ohms.
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RF Band Pass Filters
RF band pass filters are active or passive circuits that pass signals from a specific frequency band and reject signals from out-of-band frequencies. Surface acoustic wave (SAW) filters, crystal filters, and cavity filters are types of band pass filters.
RF Band Reject Filters
RF band reject filters are tuned circuits that prevent the passage of signals within a specified band of frequencies. These devices are also known as bandstop or notch filters.
RF High Pass Filters
RF high pass filters pass signals from high frequencies and reject signals from low frequencies.
RF Low Pass Filters
RF low pass filters pass signals from low frequencies and reject signals from high frequencies.