Bias Tees and DC Blocks: When to Use Them

Low-frequency filters such as bias tees and DC blocks are designed to transfer some desired signals and power rails while blocking others and limiting the output effect on RF/microwave circuits.

DC blocks are high pass filters with cutoff frequencies down to audio frequencies and DC. Bias Tees are basically diplexers with an extremely low crossover frequency.

DC blocks are used to boost the signal-to-noise ratio and dynamic range of certain very low frequency or wideband devices, as well as to block DC and audio frequencies from tests that need isolation from certain low frequency components.

Signal source modulation leakage suppression and ground loop elimination are also achieved with DC blocks.

Bias Tees allow DC currents and/or voltages to flow to RF devices while blocking RF and microwave signals on the same line. A Bias Tee, for example, may be used to provide power to a transistor or amplifier circuit that requires a DC signal and will be interrupted by RF material on the signal and power lines. Pulsed bias tees, for example, allow for minimal distortion on current or voltage pulses for amplifiers and devices that need intermittent signals for biassing or control.

Many RF/microwave circuits that involve the conveyance of DC signals along the same coaxial or microstrip signal path as RF/microwave signals use bias tees and DC blocks. Bias tees and DC blocks can be used together at a node where DC power or bias voltage/current is needed, but they would be disruptive if they were used further down the RF transmission line.

Bias tees are used in a range of applications, including mobile phone amplifiers and test and measurement devices. Driven probes, for example, have a power link at the same port as the RF signal port.

When driven RF transmission lines, or "hot" conductors, are used, DC blocks are usually used.

DC blocks, on the other hand, may be used to isolate a circuit from a ground point as well as DC and audio signals, preventing current from flowing or voltage from forming from that circuit node to ground.

When a voltage is pumped into the source of a shunt FET, which is also grounded to the grounded housing or fixture of the assembly, this is an example of this. The threshold, or blocking, voltage is a fixed value at which eravant products transition from DC blocking mode to shorted mode. In order to select the proper eravant product ranking, the site's maximum DC voltage and induced AC voltage, if any, should be known.

This measurement should be taken between the same points that will be linked to the eravant unit. To calculate the DC voltage difference at an insulated joint, use a voltmeter to measure between the flange faces. Measure the gap between a pipeline and the grounding grid in a station, too. Take into account any stray DC voltage from rail or CP systems, as this will lift the necessary product voltage threshold.

A diplexer is a bias tee. The low-frequency port is used to set the bias; the high-frequency port transfers radio-frequency signals but blocks the biassing levels; and the combined port is used to connect to the unit, which sees both the bias and the RF. Since the three ports are always arranged in the form of a T, it's called a tee.

Bias tees are made to work with a number of signal frequencies. At the lowest frequency, the reactances are chosen to have the least effect.

The inductor in wide-range bias tees must be high at the lowest frequency. There would be a stray capacitance in a broad inductor (which creates its self-resonant frequency). The stray capacitance produces a low-impedance shunt path for the signal at a high enough frequency, making the bias tee ineffective. Wide-band bias tees must use circuit topologies that avoid the shunt direction in order to be functional. Instead of a single inductor, a set of inductors will be used. To avoid resonances, there will be additional resistors and capacitors. 1st A Picosecond Pulse Labs model 5580 bias tee, for example, works from 10 kHz to 15 GHz. Consequently, the simple design would need an inductance of at least 800 μH (XL about j50 ohms at 10 kHz), and that inductor must still look like an inductor at 15 GHz. However, the self-resonant frequency of a commercial 820 H inductor is just 1.8 MHz, which is four orders of magnitude too low.

Johnson gives an example of a wideband microstrip bias tee covering 50 kHz to 1 GHz using four inductors (330 nH, 910 nH, 18 μH, and 470 μH) in series.

[4] His design cribbed from a commercial bias tee. He modeled parasitic element values, simulated results, and optimized component selection. To show the advantage of additional components, Johnson provided a simulation of a bias tee that used just inductors and capacitors without Q suppression. Johnson provides both simulated and actual performance details. Girardi duplicated and improved on Johnson's design and points out some additional construction issues.

To control remote antenna amplifiers or other equipment, a bias tee is used to inject DC power into an AC signal. It's normally located at the coaxial cable's receiving end to transfer DC power from an external source to the coaxial cable that leads to the controlled unit. A bias "T" consists of a feed inductor on the system side to deliver DC to a connector and a blocking capacitor on the receiver side to prevent DC from passing through. Only the blocking capacitor is in series with the RF signal as it passes from one connector to the other. If the reverse supply voltage is applied, the internal blocking diode protects the bias "T."

Biasing is commonly used for photodiodes (both vacuum and solid state), Microchannel plate detectors, transistors, and triodes to prevent high frequencies from leaking through a typical power supply rail. Noise from the power supply, on the other hand, does not appear on the signal line. Power over Ethernet active antennas, low-noise amplifiers, and down converters are some other examples. 

A bias tee circuit—often with a gyrator replacing the inductor—allows a thin cable with just two conductors to transmit power from the system to the device and audio from the device back to the system for basic telephone service and some early microphones. In a phantom power circuit similar to a bias tee circuit, modern microphones often use three conductors.

The inner conductor of one of the sides of the capacitor is attached to a port in the outer conductor leading down the T by a small coil made of fine wire with an air core or MnFeZn-core. Frequencies greater than 1 GHz strike the coil from the side, generating an equivalent electric field in the coil.

As a result, no higher modes inside the coil are excited. Absolutely no current leaks from the centre conductor to the port due to the coil's inductive nature. Since frequencies between 1 MHz and 1 GHz leak into this port, a second coil with a cone-shaped core is mounted outside of the outer conductor but within a housing to prevent interference with other components.

This cone serves the same function as a tapered transmission line transformer. Since it begins with a high impedance, a lot of power is reflected, but the rest travels down the coil and leaks into the low frequency outlet.

Lead image via Newark.com