Zero-bias bypass switch

#This invention is generally directed to the function of providing a solid-state RF bypass for tower-mounted low noise amplifiers (LNA) when DC power to the LNA fails.

#While the invention may find other applications, the invention is described herein with specific reference to the problem of bypassing amplifiers such as low noise amplifiers in a tower-mounted active antenna system, such as a base station for wireless communications. Generally speaking, such base stations employ a number of low noise amplifiers, which in the case of so-called active antenna systems are mounted in the tower adjacent the radiating elements or antennas. On the receive side of such an installation, a low noise amplifier is typically employed at the tower top for improving signal to noise ratio and providing an amplified signal to other base station equipment which may also be tower mounted, but which is more typically mounted in a base station enclosure at the base of the tower. Since access to the tower mounted equipment requires a worker to actually climb the tower, it is considered desirable to arrange for various fail-safe devices or modes of operation of the tower mounted equipment.

#One such mode of operation is an automatic bypass capability for bypassing the low noise amplifier (LNA) when a failure mode of the amplifier is detected. The invention provides a bypass upon loss of the DC bias current supplied to the tower mounted equipment, which might occur for a number of reasons. Generally, electromechanical relays have been required to perform the bypass function when DC power is lost.

#In the drawings:

#FIG. 1 is a schematic of a zero-bypass switch in accordance with one embodiment of the invention; and

#FIG. 2 is a schematic of a zero-bypass switch in accordance with another embodiment of the invention.

#Using PIN diodes with low “zero voltage” capacitance, a bypass switch design is possible which achieves the desired goals. Insertion loss of the switch is no more than approximately 0.5 dB total (0.25 dB for each section) in the “through” path, and the switch has at least around 25 dB isolation between alternate paths. This design has loss in the normal “through” path comparable to far more expensive relays. It is a solid state design with no electromechanical components. In another embodiment of the invention, shown in FIG. 2, a further improvement of insertion loss and isolation is realized.

#Referring to FIG. 1, one embodiment of a zero-bypass switch in accordance with the invention is illustrated in circuit schematic form. The switch or switching device includes a bypass circuit portion 10 which in the illustrated embodiment comprises a pair of series-coupled quarter wavelength lines 12, 14 which are coupled between an input 16 and an output 18 of the circuit. In the illustrated embodiment, the two quarter wave lines 12 and 14 are coupled in series by a DC blocking capacitor 20.

#The input 16 is coupled in series with an input of the amplifier or LNA 22 which is to be bypassed by the switch or switching circuit or device of the invention. Similarly, the output 18 is coupled in series circuit with the output of this amplifier 22. A pair of switching diodes 24, 26 are coupled between the ends of the respective quarter wavelength lines 12 and 14 which are coupled to the capacitor 20, and a radio frequency (RF) ground. Additional switching diodes 30 and 32 are coupled in series respectively with the input 16 and output 18 of the amplifier 22. Respective DC blocking capacitors 34, 36 are coupled respectively intermediate the diodes 30, 32 and the input and output of the amplifier 22.