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Low-cost bias circuits serve HF and VHF bands

-September 14, 2000

Bias circuits are passive networks that you use to apply dc excitation to various active circuits. The monitor tee, which is also known as the bias tee, has been commercially available for more than 40 years at microwave frequencies. The original products provide useful frequency ranges of two to one through five to one. More recent cost-effective bias tees cover 0.1 MHz to more than 4 GHz. Other designs are available that extend well into higher microwave-frequency bands (Reference 1). Another bias circuit is the bias-passing attenuator, which is also commercially available at microwave frequencies.

You can realize simple bias circuits at HF and VHF frequencies with minimal engineering and at much lower cost than those that must operate at microwave frequencies. You can obtain usable performance over a frequency range exceeding two decades. You can optimize the cost of these bias circuits by integrating them into subsystems and systems using surface-mount fabrication.

The monitor tee is a three-port network (Figure 1). One inline port handles both dc and RF. The second inline port handles only RF. The shunt port passes only dc. With values of L=1 mH and C=0.1 µF, the measured insertion loss from 0.5 to 100 MHz for inline transmission is less than 0.2 dB in a 50W test setup. The shunt port terminates in 50W.

The bias passing attenuator is a two-port network containing a fixed pad attenuator, input and output dc blocking capacitors, and a bridging RF choke (Figure 2). The values for L, C1, C2, and R1 to R3 in the figure create a nominally 6-dB attenuator. From 0.5 to 100 MHz, measured insertion loss is 6±0.5 dB in a 50W test setup.

Other bias-passing circuits include lowpass, highpass, and bandpass filters. The LC lowpass Wfilter has inherent bias-passing capability via the cascaded series inductors. The LC highpass filter needs additional circuit elements for bias passing. Sometimes, this bias-passing circuit substantially degrades the stopband selectivity of the highpass filter. You can design the LC bandpass filter for bias passing using coupled shunt resonators. The filter input and output couplings must all be inductive. Also, the shunt inductors of the individual resonators must be floating with series RF bypass capacitors for ground returns.

REFERENCE

1. Andrews, JR, "Broadband Coaxial Bias Tees," Application Note AN-1d, Picosecond Pulse Labs, Copyright February 1998.

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