Bias tee

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A tee bias is a three-port network used to adjust the DC bias point of some electronic components without interfering with other components. Tee bias is a diplexer. Low frequency ports are used to adjust the bias; high frequency port pass radio frequency signal but block biasing level; combined port connected to the device, which sees bias and RF. This is called tee because 3 ports are often set in the form of T .

Video Bias tee

Design

Conceptually, the bias tee can be seen as an ideal capacitor that allows AC through but blocking DC bias and ideal inductors that block AC but allow DC. Although some tee bias can be made with simple inductors and capacitors, the wideband band of bias is much more complicated because the practical component has a parasitic element.

Tee bias is designed for transmission line environments. Typically, the characteristic impedance Z ₀ will be 50 ohm or 75 ohms. The capacitor impedance ( X _C ) is chosen to be much smaller than Z ₀ , and inductors impedance (< i> X _L ) is selected to be much greater than Z ₀ :

${\ displaystyle {\ begin {aligned} X_ {C} & amp; = {\ frac {1} {\ omega C}} = {\ frac {1} {2 \ pi fC}} \ ll Z_ {0}, \\ X_ {L} & amp; = \ omega L = 2 \ pi fL \ gg Z_ {0}, \\\ end {aligned}}} Â Â$

where is ? is the angular frequency (in radians per second) and f is the frequency (in hertz).

Tee bias is designed to operate at a frequency range of signals. Reactance is chosen to have minimal impact on the lowest frequency.

For various biases of bias, the inductor must be large at the lowest frequency. The large inductor will have a stray capacitance (which creates its own resonant frequency). At a sufficiently high frequency, the stray capacitance presents a low impedance shunt path for the signal, and the bias tee becomes ineffective. T-shirts of broadband bias should practically use a circuit topology that avoids the shunt path. Instead of one inductor, there will be a series of inductor strings. In addition, there will be additional resistors and capacitors to prevent resonance. For example, the Picosecond Pulse Labs 5580 bias tee model works from 10 kHz to 15 GHz. As a result, a simple design would require inductance at least 800Ã? H ( X _L about j 50 ohms at 10 kHz), and that inductor should still look like an inductor at 15 GHz. However, the commercial inductor 820 Â ° H has a self resonance frequency of only 1.8 MHz - four orders too low.

Johnson gives an example of a wideband microstrip bias graph that includes 50 kHz to 1 GHz using four inductors (330 HH, 910 HH, 18 μμH, and 470 μμH) in series. The design is conjured up from a commercial bias tee. He modeled the values ​​of parasitic elements, simulated results, and the selection of optimized components. To demonstrate the advantages of additional components, Johnson provides a simulation of a tee bias that uses only inductors and capacitors without pressing Q . Johnson provides details of actual performance and simulations. Girardi is duplicated and improved on Johnson's design and shows some additional construction issues.

Maps Bias tee

Apps

A bias pole is used to insert DC power into an AC signal to power a remote antenna amplifier or other device. It is usually positioned on the receiving end of the coaxial cable to provide DC power from an external source to the coaxial cable running to the powered device. The "T" bias consists of a feed inductor to send a DC to the connector on the side of the device and the blocking capacitor to keep the DC from passing through to the receiver. The RF signal is connected directly from one connector to the other by simply blocking the capacitor in series. The internal blocking diode prevents damage to the "T" bias if an inverted supply voltage is applied.

Tee bias is used in various applications, but is generally used to provide RF signals and (DC) power to remote devices where running two separate cables will not be profitable. Biasing is often used with photodioda (vacuum and solid state), Microchannel disk detectors, transistors, and triodes, so that high-frequency signals do not leak onto common power rails. Conversely, noise from the power supply does not appear on the signal channel. Other examples include: Power over Ethernet, active antenna, low sound amplifier, and downward converter.

Phone lines for old phone services and some early microphones use biased tee circuitry - often with gyrators replacing inductors - this allows thin cables with only 2 conductors to send power from system to device, and send audio from the device back to the system. Modern microphones often use 3 conductors in a ghost circuit that are very similar to a biased tee circuit.

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Construction

There are several tee bias designs.

Certain construction

The horizontal rod construction of T is based on coaxial cable which is rigid with air as dielectric. The radius is chosen to be as large as possible without allowing a higher mode. The design of the "T" bias is based on the power that goes to the remote device, but is not visible to the base station or receiver. This is done by using capacitors at the RF output terminals, effectively creating open circuits for DC currents. The incoming RF signal, or derived from the antenna, is output for DC power. The front-end of this "T" bias usually consists of a bandpass filter, a low noise amplifier, and a mixer coupled to a local oscillator.

Capacitor

At one point a small piece is cut from the center of the conductor, therefore the capacitor is formed and the low frequency is blocked. This type of capacitor has an advantage that is almost invisible to higher frequencies. To continue the frequency up to 1 MHz, the capacitance must be increased. A dielectric such as a NPO multiplies capacitance by a factor of 65. The thickness of the capacitor must be minimal without causing electrical damage in the dielectric, this means to avoid any peaks in the electric field and this means fine electrodes with rounded and dielectric edges protruding between electrodes (door handles design). Stack capacitors can be used, but every capacitor requires access to the surface of the inner conductor, because if hidden behind another capacitor the high frequency will not see it, because the electric field takes a lot of time to travel through the dielectric with high dielectric constant

Coil

A small coil made of fine wire with an air core or MnFeZn-core connects the inner conductor to one side of the capacitor with a port on the outer conductor leading down T. The frequency above 1 GHz strikes the coil from the side and applies the same electric field to the entire coil. Therefore, there is no higher, vibrant mode inside the coil. Because the inductance of the coil there is almost no leakage current from the center of the conductor to the port. The frequency between 1 MHz and 1 GHz does leak into this port, so there is a second coil with a conical core outside the outer conductor, but inside the housing to avoid interference with other components. This cone acts like a tapered transmission channel transformer. It starts with high impedance, so a lot of power will be reflected, but the rest will travel to the coil and there is some leakage to the low frequency port.

Oscillation

Each oscillation in a capacitor or coil or a lapped LC circuit is muted by the dielectric and core. Also a small coil must have about 10 ohms of resistance for further moist oscillation and avoid the ripple on the transmitted spectrum.

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See also

• Diplomatic

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References

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Further reading

• Minnis, Brian J. (1996), Designing Microwave Circuits with Exciting Synthesis , Art House, ISBN 0-89006-741-4 Ã,

  Also his paper on the microstrip tops and stripline bias

  Minnis, BJ (June 1987), "Decade of Bandwidth Bias T for MIC Applications up to 50 GHz", Theory and Microwave Technique, IEEE Transactions on , IEEE, 35 (6): 597-600, doi: 10.1109/TMTT.1987.1133711 Ã,

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External links

• Baylis, Charles; Dunleavy, Lawrence; Clausen, William (October 2006), "Tees Bias Design for Pulsed-RF Pulsed Testing Systems, Using Accurate Component Models" (PDF) , Microwave Journals < span title = "ctx_ver = Z39.88-2004 & amp; rft_val_fmt = info% 3Aofi% 2Ffmt% 3Akev% 3Amtx% 3Ajournal & amp; rft.genre = article & amp; rft.jtitle = Microwave Journal & amp; rft.atitle = Teas Bias Design for a Pulsed-bias% 2C Pulsed-RF Test System Using Accurate Components Model & amp; rft.date = 2006-10 & amp; rft.aulast = Baylis & amp; rft.aufirst = Charles & amp; rft.au = Dunleavy% 2C Lawrence & amp; rft.au = Clausen% 2C William & amp; rft_id = http% 3A% 2F% 2Fwww.modelithics.com% 2Fpaper% 2F394.pdf & amp; rfr_id = info% 3Asid% 2Fen. wikipedia.org% 3ABias tee ">
• Hicks, Brian; Erickson, Bill (May 21, 2008), Design Considerations of Bias-T for LWA (PDF) Ã, (15Ã, MHz to 115Ã, MHz single 4.7à , ÂμH inductor design)

Source of the article : Wikipedia