Bias T

Bias T

A bias “T” is used to insert DC power into an AC signal to power remote antenna amplifiers or other devices. It is usually positioned at the receiving end of the coaxial cable to pass DC power from an external source to the coaxial cable running to powered device. A bias “T” consists of a feed inductor to deliver DC to a connector on the device side and a blocking capacitor to keep DC from passing through to the receiver. The RF signal is connected directly from one connector to the other with only the blocking capacitor in series. The internal blocking diode prevents damage to the bias “T” if reverse supply voltage is applied.

Design

The design of a bias "T" is based upon power going out to the remote device, but not being seen by the base station or receiver. It does this by using a capacitor on the RF output terminal, effectively creating an open circuit for the DC current.citation | inventor1-last = Jovanovich | inventor1-first = Alan | inventor2-last = Lam | inventor2-first = For Sander | title = Zero loss bias "T" | issue-date = 2001 | patent-number = 6229408 | country-code = US] The incoming RF signal, or the one from the antenna, is the output for the DC power. This front-end of a bias "T" typically consists of a bandpass filter, a low noise amplifier, and a mixer coupled to a local oscillator.

Applications

Bias "T" has been used in a variety of applications, but is generally used to provide an RF signal and power to a remote device where running two separate cables would not be advantageous.Citation | title = Bias T for Antenna Mount Preamps | date = 2007 | year = 2007 | url = http://www.hamtv.com/pdffiles/biasT.pdf | accessdate = 2008-08-08 ] Examples of this are: Power over EthernetCitation | title = Power Over Ethernet Bias T Power Module | url = http://download.intel.com/support/wireless/wlan/pro2011b/accesspoint/poe_diagram.pdf | accessdate = 2008-08-08 ] , active antennas, low-noise amplifiers, and down convertersCitation | title = WR-BT-650 HF/VHF Power Injector (Bias 'T') | url = http://www.winradio.co.uk/home/bt-650.htm | accessdate = 2008-08-08 ]

External links

S parameters for a bias T [http://qucs.sourceforge.net/tech/node47.html]

References


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