A bidirectional amplifier (BDA) is an active RF device that is widely used in a variety of telecommunication, radar, and other RF sensing applications. Although conceptually a bidirectional amplifier performs a relatively simple operation, the internal electronics can be somewhat complex. Moreover, many applications favour certain names and types of bidirectional amplifiers, which makes identifying a bidirectional amplifier in the wild somewhat difficult.
The essential functions of a BDA are to amplify the signals transmitted and received by the device. This means that signals generated by a telecommunication device and intended to be transmitted will benefit from a higher output signal and the low power signals received by the antenna of the BDA will also experience some gain. The goal of a BDA is to increase the transmit and receive signals without adding significant noise or distortion.
To do this, a BDA is typically placed inline as a ,booster, or ,repeater, in a communication system. This can be done by installing a BDA on a tower together with the transmit/receive antenna or inline with a coaxial or waveguide transmission line to add gain to signals travelling between two distant points.
To apply gain to both the transmit and receive signals a BDA requires both a power amplifier (PA) in the transmitter side signal chain and a low-noise amplifier (LNA) in the receive side signal chain. Moreover, a BDA consists of a means of separating the transit and receive signal chains from each other, so that the high-power transmit signals aren’t fed into the LNA (crosstalk) and the receive signal chain is optimised to reduce loss and minimise added noise to the received signals.
A bidirectional amplifier works by separating the transmit and receive signals into separate signal chains. Once separated, the transmit and receive signals are then increased in signal strength using either a PA for the transmit signal or a LNA for the receive signal. A BDA is installed in such a way that the receive signals to the radio or transmission line are increased in strength and isolated from the transmit signals and vice versa.
There are a variety of methods to separate the transmit and receive signals, with RF switches, duplexers, or circulators. A BDA that uses duplexers or circulators and allows for simultaneous transmission and reception functions is known as a full-duplex bidirectional amplifier. The difference between a duplexer-based and a circulator-based BDA is that a duplexer is a type of bandpass filter that separates signals from the input to the two outputs based on frequency, where as circulator separates the signals at the input based on direction. A BDA that uses RF switches to separate the transmit and receive signal chains can only allow for transmission or reception at a given time and is known as a half-duplex BDA. Typically, a BDA has either coaxial connector interconnect or waveguide interconnect depending on the type of system or application the device is being used in.
For more information contact Andrew Hutton, RF Design,
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