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Combining the virtues of its proprietary low-temperature, co-fired ceramic (LTCC) technology with clever circuit techniques, Mini-Circuits has crafted a high-performance bias tee for applications such as amplifiers, laser diodes and active antennas. Labeled TCBT, the new bias tee is designed to provide superior performance in a tiny surface-mount package that measures 0.15 inch × 0.15 inch × 0.15 inch, saving PCB space for the designer. Although the package is a standard format, it is the smallest bias tee for the frequency range it serves, claims the developer. In fact, this miniature device boasts an unprecedented performance over a wideband of 50 MHz to 6.0 GHz.

According to the company, this patent-pending unit comes in two models. While TCBT-6G operates over a range of 50 MHz to 6.0 GHz, TCBT-2R5G is rated from 20 MHz to 2.5 GHz. Both models are guaranteed to operate at up to 100 mA DC current and 25 dBm RF power. Meanwhile, efforts are under way to further extend the frequency performance of this bias tee by further reducing the parasitics.

The circuit diagram for this high-performance bias tee is shown in Figure 1. In this circuit, L1 is a coil winding around a ferrite core and mounted on the LTCC substrate, while L2 comprises a metal line inside the LTCC substrate. To achieve high performance over the specified frequency range, the circuit combines L1 and L2 inductances. For that, the inductance of L1 is kept large for low-frequency performance, while L2 is small to maintain high-frequency performance. In addition, the DC block capacitor C is also mounted on the substrate. The LTCC offers vias and metal pads to connect terminals of C to the bottom leads with minimum inductance.

Consequently, the parasitic capacitance of L1 and L2 is small. And the isolation between RF and DC ports is also high. To guarantee high isolation between RF and DC ports over the wide frequency range, the DC lead is kept far away from the RF port. The design ensures that RF and DC leads provide least capacitance between these ports. As per the data sheet, typical isolation between RF and DC ports for the -2R5G model is 44 dB in the median range of 200 MHz to 1.25 GHz. Likewise, the -6G unit offers a typical isolation of 28 dB in the 500 MHz to 3.0 GHz range.

Typical insertion loss for the TCBT-6G unit is 0.7 dB with low VSWR of 1.1 (typical). Its VSWR performance for three DC current ratings over the specified frequency range is depicted in Figure 2. Likewise, the TCBT-2R5G offers a typical insertion loss of 0.4 dB with VSWR of 1.05 (typical).

In quantities of 10 to 49, the TCBT-6G is priced at $11.95 each. And in similar quantities, the TCBT-2R5G carries a price tag of $8.95 each.
Mini-Circuits
(718) 934-4500
www.minicircuits.com/TCBT-6G.pdf
www.minicircuits.com/TCBT-2R5G.pdf