Bias-T (DC Biaser) is a three-port network device, and the three ports are RF port RF, DC bias port DC and RF DC port RF&DC. Because these 3 ports are often arranged in the shape of T, it is called Bias-T. In the broadband amplifier feed circuit, the high frequency is affected by the parasitic capacitance parameters, and the performance drops sharply, so the feed needs to use Bias-T.
The DC port of Bias-T is composed of a feed inductor, which is used to add DC bias to prevent the AC signal of the RF port from leaking to the power supply system. Under ideal conditions, the DC port will not have any influence on the signal of the RF port; the RF port is composed of A DC blocking capacitor is used to input the RF signal and block the DC voltage of the bias port at the same time; the RF&DC port is connected to the device, and the device can see the DC bias voltage and RF signal at the same time. If the internal components of Bias-T choose ultra-broadband, high-frequency inductors and capacitors that are close to idealization and have no resonance point, then when Bias-T is used to set the DC bias point of some electronic components, it will not interfere with other components.
Bias-T key parameters
For Bias-T, the more important indicators are DC bias voltage and current; RF and DC isolation; RF bandwidth, group delay, insertion loss and return loss, etc., and then explain each The meaning of the indicator.
DC side bias current
The bias current at the DC terminal is input from the DC terminal, and the current passing through the inductor is mainly limited by the maximum current that the DC port inductor can withstand. The larger the value of the bias current at the DC terminal, the better the index. When the actual current exceeds the upper limit of the bias current, the inductance of the DC port is in a state of current saturation. The performance of Bias-T will be affected in this state, and the device may be damaged due to excessive current, resulting in Bias-T not working properly.
RF terminal and DC terminal isolation
Isolation refers to the ability of the inductor to prevent the radio frequency signal from the RF port from flowing to the DC port, and the unit is usually expressed in dB. In theory, the greater the isolation, the better. If the isolation index of a certain Bias-T is poor, RF signals will leak into the power supply system, affecting system performance. Figure 1-2 is a schematic diagram of the isolation between the RF end and the DC end. In the figure, the input signal power of the RF port is 40dBm. When the isolation is 10dB, the signal power received at the DC end is 30dBm, which is equivalent to 1W. , may cause irreparable damage to the power supply system; when the isolation is 40dBm, the signal power received at the DC end is 0dBm, which is equivalent to 1mW, and will not cause interference or damage to the power supply system.
RF bandwidth
The RF bandwidth is the frequency range between the upper 3dB cut-off point and the lower 3dB cut-off point, which is the passband of Bias-T. The 3dB cut-off point is the point at which the average power in the passband is attenuated by 3dB. The wider the RF bandwidth, the better. If the signal frequency range exceeds the passband of Bias-T, the signal outside the passband of Bias-T will be greatly attenuated.
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