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Broadband Chokes for Bias Tee Applicationsdoc 1193 Broadband Chokes for Bias Tee Applications How to successfully apply a DC bias onto an RF line Insertion loss measured in shunt (ref: 50 Ohms) Introduction 0 The demand for increased bandwidth in data communi- 1 cation is continually increasing, and the integrity of RF 2 signals has become a major design concern. In broadband 3 bias applications, most inductors do not cover enough 4 5 impedance bandwidth. By putting three or four inductors 6 in series, bandwidth can be increased, but DC losses and 7 (dB) S21 filter complexity increase. Broadband chokes provide wide 8 bandwidth in a single inductor package. This document 9 discusses the use of broadband chokes in bias tees. 10 11 12 Broadband Chokes for Bias Tees 1 10 100 1000 10000 The purpose of the inductor in a bias tee, as shown in Frequency (MHz) Figure 1, is to inject a DC bias while isolating the AC signal Figure 2. 4310 LC Characteristics from the DC source. Ideally, any stray AC signal applied to the DC bias line will be isolated by the inductor, preventing Figure 3 shows that the insertion loss from 50 MHz to distortion of the AC signal. 35 GHz is less than 1.0 dB when the BCR inductor is measured in shunt from a transmission line to ground. AC Insertion loss (ref: 50 Ohms) 0 AC only AC + DC -652 0.5 DC -531 1.0 -122 1.5 -802 DC S21 (dB) Figure 1. Equivalent Circuit of a Bias Tee 2.0 For example, a television antenna may need up to 500 mA 2.5 DC injected onto the RF line, while blocking frequencies 0.05 10 20 30 40 from 20 MHz to 2 GHz. Consider the isolation that can be Frequency (GHz) achieved with a Coilcraft 4310LC wideband bias choke. Figure 3. BCR Characteristics Figure 2 shows insertion loss measurement for the coil connected in shunt from a transmission line to ground. Summary This curve illustrates how frequencies below 10 MHz are passed through the inductor to the ground plane, and how The critical determination when choosing RF chokes for bias tees is the frequency range that needs to be blocked. frequencies between 10 MHz and 6 GHz are rejected by Other key parameters are DC resistance, current require- the coil and passed along the transmission line. ments, size and cost. Another application requiring RF isolation is the biasing A broadband bias choke is the key to successfully applying of a pin diode for use in fiber networks. Here, a 200 mAdc a DC bias onto an RF line. Start with identifying the fre- injection is needed on an RF line that carries frequencies quency range of the application and then determine what from 100 MHz to 30 GHz. To meet this requirement, a the other critical parameters need to be. For assistance Coilcraft BCR Broadband Conical Inductor can be used. with your particular wideband choke selection, contact Coilcraft Technical Support. Document 1193 Revised 05/2814.
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