Zero-bias bypass switching circuit using mismatched 90 degrees hybrid

CROSS REFERENCE TO RELATED APPLICATIONS

P-0001[0001] This application is a Continuation-In-Part application of U.S. application Ser. No. 10/120,750, filed Apr. 11, 2002, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] This invention is generally directed to the function of providing an RF bypass function for tower-mounted low noise amplifiers (LNAs).

BACKGROUND OF THE INVENTION

[0003] The present invention is directed to the operation of amplifiers, such as low noise amplifiers in a tower-mounted active antenna system, such as an antenna system used in a base station for wireless communications. Generally speaking, such base stations employ a number of amplifiers, including low noise amplifiers (LNAs). In the case of active antenna systems, the LNAs are mounted on the tower adjacent to the radiating elements or antennas. On the signal receive side of such an installation, a low noise amplifier is typically employed at the tower top for improving the signal-to-noise ratio of the received signal 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. Failure of tower-mounted equipment presents certain maintenance issues. 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, including the LNAs.

[0004] One such fail-safe mode of operation for a tower-mounted LNA is an automatic bypass capability for bypassing the low noise amplifier (LNA) when a failure mode of the amplifier is detected. Such a failure mode in the LNA results in a significant degradation of the signal output by the LNA. A failure may occur, for example, due to the loss of DC bias current supplied to the tower mounted equipment, which might occur for a number of reasons. In cases of a defined failure mode of the LNA, the LNA may be automatically or otherwise bypassed, feeding the received signals directly from the antenna to the base station equipment, typically at the enclosure at the base of the tower. While such a bypass may degrade the signal-to-noise ratio somewhat in the absence of the amplifier, it is usually considered preferable to the signal degradation, which takes place when the amplifier is in a failure mode. Therefore, it is desirable to provide a bypass arrangement, to bypass an LNA, which is in a predefined failure mode from the absence of a DC control bias.

[0005] Currently, relatively expensive and cumbersome RF or microwave-compliant relay circuits have been used as one way to accomplish this bypass function. The switching devices or relays must meet stringent requirements to assure signal quality, including a relatively low insertion loss and relatively high isolation. In addition, a relay system is subject to the usual shortcomings of mechanical switching systems, including relatively low speed, high expense, relatively large size and susceptibility to various mechanical failure modes.

[0006] Therefore, it is desirable to bypass a failed LNA of a system in order for the system to continue functioning properly. However, it is also desirable that the bypass function be handled inexpensively and with low loss. A robust, solid-state construction is also desirable. Additionally, the design should function in the bypass mode with all DC input power removed to the mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

P-0007[0007] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given below, serve to explain the principles of the invention.

P-0008[0008]FIG. 1 is a schematic of a zero-bias bypass switching circuit in accordance with one embodiment of the invention.

P-0009[0009]FIG. 2 is a schematic of a zero-bias bypass switching circuit in accordance with another embodiment of the invention.

P-0010[0010]FIG. 3 is schematic of a zero-bias bypass switching circuit in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

P-0011[0011] To address the objectives noted above and other objectives in the art, the invention provides a bypass capability that is low-loss, and relatively inexpensive. The solid-state design is also compact and robust.

P-0012[0012] 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 mechanical components. In another embodiment of the invention, shown in FIG. 2, a further improvement of insertion loss and isolation is realized.

P-0013[0013] Referring to FIG. 1, one embodiment of a zero-bias bypass switching circuit or switch in accordance with the invention is illustrated in circuit schematic form. In one embodiment, the switch or switching device includes a bypass circuit portion <b>10</b>, which in the illustrated embodiment comprises a pair of series-coupled quarter wavelength lines <b>12</b>, <b>14</b> that are coupled between an input <b>16</b> and an output <b>18</b> of the circuit. In the illustrated embodiment, the two-quarter wave lines <b>12</b> and <b>14</b> are coupled in series by a DC blocking capacitor <b>20</b>.

P-0014[0014] The input <b>16</b> is coupled in series with an input of the amplifier or LNA <b>22</b>, which is to be bypassed by the switching circuit or device of the invention. Similarly, the output <b>18</b> is coupled in series circuit with the output of this amplifier <b>22</b>. A pair of switching diodes, <b>24</b>, <b>26</b>, are coupled between the ends of the respective quarter wavelength lines <b>12</b> and <b>14</b>, which are coupled to the capacitor <b>20</b> and a radio frequency (RF) ground. Additional switching diodes <b>30</b> and <b>32</b> are coupled in series respectively with the input <b>16</b> and output <b>18</b> of the amplifier <b>22</b>. Respective DC blocking capacitors <b>34</b>, <b>36</b> are coupled respectively intermediate the diodes <b>30</b>, <b>32</b> and the input and output of the amplifier <b>22</b>.

P-0015[0015] A source of control bias <b>40</b> for the switching circuit is coupled by way of suitable current limiting resistors <b>42</b> and <b>44</b> and additional quarter wavelength lines <b>46</b> and <b>48</b> to respective junctions of the diodes <b>30</b> and <b>32</b> and the capacitors <b>34</b> and <b>36</b>. The polarities of the connections of the diodes <b>24</b>, <b>26</b>, <b>30</b>, and <b>32</b> are such that with the control bias <b>40</b> present or operative, all of the diodes <b>24</b>, <b>26</b>, <b>30</b>, and <b>32</b> are biased into a conductive condition that is forward biased. The effect of this will be to deliver the input signal at the input <b>16</b> through the amplifier <b>22</b> to the output terminal <b>18</b>. Conversely, when the control bias <b>40</b> is absent, the diodes <b>24</b>, <b>26</b>, <b>30</b>, and <b>32</b> will be in a nonconductive condition, such that the signal path will flow from the input <b>16</b> through the quarter wavelength lines <b>12</b> and <b>14</b> to the output <b>18</b>.

P-0016[0016] In one practical embodiment, the quarter wavelength lines <b>12</b>, <b>14</b>, and <b>46</b>, <b>48</b> may be formed as microstrips on a suitable dielectric substrate.

P-0017[0017] While the circuit of FIG. 1 presents an operative device, better performance in terms of insertion loss and isolation can be obtained by the circuit in FIG. 2, which adds a number of resonant circuits, as well as using pairs of diodes <b>124</b>, <b>126</b>, <b>130</b>, and <b>132</b> in parallel in place of the single diodes <b>24</b>, <b>26</b>, <b>30</b>, and <b>32</b> of FIG. 1. In addition, in the circuit of FIG. 2, the diodes are M-Pulse Microwave P/N diodes of the type MP5X7084 from M-Pulse Microwave, San Jose, Calif., or equivalent devices. With these diode components selected in the circuit of FIG. 2, insertion loss as low as 0.15 dB and isolation as high as 60 dB is obtainable.

P-0018[0018] While the circuit of FIG. 2 is similar functionally to the circuit of FIG. 1, a number of additional components are utilized to form resonant circuits. In this regard, in the circuit of FIG. 2, the control bias voltage <b>40</b> is applied via a quarter wavelength line <b>148</b> to diodes <b>132</b> at the output side of the amplifier <b>22</b> which will be seen to be connected in reverse polarity to that shown in FIG. 1. Input diodes <b>130</b> to the amplifier <b>22</b> are coupled via a quarter wavelength line <b>146</b> to a DC ground. Both of these diode pairs <b>130</b>, <b>132</b> are provided with a parallel resonant circuit comprising inductors <b>62</b>, <b>60</b> and DC blocking capacitors <b>64</b>, <b>66</b>. The inductors <b>62</b> and <b>60</b> will resonate with the respective associated diodes <b>130</b>,<b>132</b> when in the off or nonconductive state; that is, they are resonant with the residual off state capacitance of the respective associated diodes <b>130</b>, <b>132</b>. The remaining inductors shown in FIG. 2 are chokes.

P-0019[0019] Also in FIG. 2, an additional or third quarter wavelength line <b>70</b> is provided in series between the quarter wavelength lines <b>112</b>, <b>114</b>, which implement a similar function as the quarter-wavelength lines <b>12</b>, <b>14</b> in FIG. 1. DC blocking capacitors <b>134</b>, <b>136</b> in the embodiment of FIG. 2 are coupled directly to the input <b>116</b> and output <b>118</b>.

P-0020[0020] In FIG. 2, the diodes <b>24</b>, <b>26</b> from FIG. 1 are replaced by respective pairs of diodes <b>124</b>, <b>124</b><i>a </i>and <b>126</b>, <b>126</b><i>a, </i>which are coupled with similar DC bias circuits <b>80</b>, <b>82</b>. The capacitors <b>90</b>, <b>92</b> and <b>94</b>, <b>96</b> in these resonant circuits are selected to series resonate with the diodes <b>124</b>, <b>124</b><i>a </i>and <b>126</b>, <b>126</b><i>a </i>in the on, or conductive state; that is, with the on state residual inductance of these diodes. The other capacitors shown in the respective circuits <b>80</b> and <b>82</b> are bypasses.

P-0021[0021] Sufficient isolation around the feedback path or leakage path provided by the bypass is important for proper operation. That is, with the LNA <b>22</b> in the circuit, feedback is to be minimized. The LNA gain is about 15 dB so the isolation must be considerably greater than 15 dB. Even at 25 to 30 dB isolation there can be amplitude and group delay “ripples” in the passband. Therefore, the goal is at least 50 dB isolation in the “normal” mode, i.e. with the LNA <b>22</b> in the path.

P-0022[0022] Diodes <b>24</b>, <b>26</b> in FIG. 1 approximate a short when the LNA <b>22</b> is in the path. This cuts off the feedback through the loop. However, the quarter wave lines make this RF “short” look like an RF open circuit at the input and output ends <b>16</b>, <b>18</b>.

P-0023[0023] Referring to FIG. 2, the resonating inductances and capacitances are intended to minimize the diodes' ON-state impedances. While the resistive component cannot be changed, the reactance component can be cancelled out by use of the resonating elements. This also enhances the performance of the quarter wave lines and their ability to simulate an open circuit at one end when a diode is in conduction at the other end. It is desired for all of the diodes to have the lowest possible on resistance. As a practical matter, this is on the order of 1 Ohm.

P-0024[0024] Also in FIG. 2, the biasing scheme is changed from FIG. 1, such that the control bias is applied only at the output side. The polarities of the diodes are now arranged so that the bypass provides a DC bias path to the anode side of the diodes <b>130</b> as well, whose cathodes remain at a ground bias at the quarter wave line <b>146</b>. The appropriate DC bias is also provided at the anodes/cathodes of the respective diodes <b>124</b>, <b>124</b><i>a, </i><b>126</b>, <b>126</b><i>a </i>by the circuit shown in FIG. 2. The quarter wave lines <b>146</b> and <b>148</b> are such that the DC bias can be applied without affecting the RF performance; that is, these look like open circuits to the RF or microwave signal.

P-0025[0025] The inductances characterized as chokes are essentially AC blocks and DC shorts. Conversely, the capacitors characterized as bypasses form AC shorts and DC blocks.

P-0026[0026] The circuit of FIG. 1 will operate acceptably at relatively low frequencies, for example, on the order of 100 MHz, although the quarter wave line lengths may be inconvenient. However, in order to get the noted performance of 0.15 dB insertion loss and 60 dB isolation, when transmitting or receiving at gigahertz frequencies, the somewhat more complex circuit of FIG. 2 is utilized. It will be noted that the use of solid state components and circuitry in place of relays provides additional advantages in that the DC power requirements of the solid state devices are quite low as compared to relatively high power requirements of relays, and also there are no mechanical switching transients with the use of solid state devices, as in the embodiments of the invention shown herein. Moreover, the present cost of the solid-state circuit of FIG. 2 is on the order of ten to fifteen percent of circuits using microwave relays as the bypass switching devices.

P-0027[0027]FIG. 3 illustrates another alternative embodiment of a bypass circuit, or switch, which may be utilized with tower-mounted LNAs to bypass the LNAs in the absence of a DC control bias, as noted above. The embodiment of FIG. 3 reduces the number of diodes, such as PIN diodes that are required for the bypass function. This is a significant improvement, as such PIN diodes are one of the more expensive components in the inventive circuit disclosed herein. For example, the embodiment illustrated in FIG. 3 utilizes only two PIN diodes in the disclosed embodiment wherein that embodiment illustrated in FIG. 2 utilizes eight PIN diodes. In addition, there are no series PIN diodes prior to the LNA device as with the embodiments seen in FIG. 1 and FIG. 3. As such, input loss with appropriate choice of PIN diodes can approach 0.05 dB.

P-0028[0028] Referring to FIG. 3, the bypass apparatus or bypass switching circuit <b>150</b> is configured for utilizing an LNA configured as a pair of individual amplifier circuits <b>152</b>, <b>154</b>. The inputs and outputs of the amplifier circuits <b>152</b>, <b>154</b> are coupled to 90° hybrid couplers <b>156</b>, <b>158</b>. The operation of such hybrid couplers is known in the art, and based on the signals and impedance conditions at the various ports of the hybrid couplers. The hybrid coupler circuits <b>156</b>, <b>158</b> are utilized herein as signal dividers and signal combiners, respectively. The output ports <b>160</b>, <b>162</b> of the hybrid coupler circuit <b>156</b> are coupled as inputs to the amplifier circuits <b>152</b>, <b>154</b>, respectively. Port <b>164</b> is utilized as an RF signal input, while port <b>166</b> is utilized as an isolated bypass port, as discussed further herein below.

P-0029[0029] In the output coupler circuit <b>158</b>, the ports <b>168</b>, <b>170</b> are coupled to the outputs of the amplifier circuits. Coupler circuit <b>158</b> acts as a signal combiner for the outputs of the amplifier circuits <b>152</b>, <b>154</b> such that the combined signal is available at the output port <b>172</b>. A suitable termination circuit <b>176</b>, such as a 50 Ohm load to ground, terminates the other output port <b>174</b>. The coupler circuit <b>156</b> and its effective output is controlled by a switching circuit or circuits coupled to the output ports. Referring to FIG. 3, coupled to each output port <b>160</b>, <b>162</b> of the coupler circuit <b>156</b>, are switching circuits including respective quarter wavelength lines <b>178</b>, <b>180</b> coupled to PIN diodes <b>182</b>, <b>184</b>. Therefore, the ends of the quarter wavelength lines opposite the output ports <b>160</b>, <b>162</b> are terminated through a respective PIN diode to ground. The PIN diodes <b>182</b>, <b>184</b> are coupled to a suitable control bias (not shown) for biasing them ON or to an ON state during normal operation of the amplifier circuits <b>152</b>, <b>154</b>, such as LNA amplifier circuits. When the diodes <b>182</b>, <b>184</b> are biased ON and coupled to ground, they represent a low impedance to ground at one end of the lines <b>178</b>, <b>180</b>. Through the respective quarter wavelength lines <b>178</b>, <b>180</b>, a high impedance is presented at the output ports <b>160</b>, <b>162</b> of the coupler circuit <b>156</b>, hence, they have minimal impact to the transmission of the signals. Therefore, in normal operation with a control bias, an input signal at port <b>164</b> is divided and applied to the amplifier circuits <b>152</b> and <b>154</b>. The outputs of the amplifier circuits are then combined through hybrid coupler circuit <b>158</b> and delivered as an output signal at port <b>172</b>.

P-0030[0030] In a bypass operation, wherein the control bias is no longer applied to the diodes <b>182</b>,<b>184</b>, the diodes are turned OFF or to an OFF state (i.e., zero biased). This generally presents a high impedance condition at the ends of the quarter wavelength lines <b>178</b>, <b>180</b> proximate the diodes. Then, at the output ports <b>160</b>, <b>162</b>, a low impedance mismatch or “short circuit” condition is presented. If the output ports <b>160</b>, <b>162</b> of the hybrid coupler circuit <b>156</b> are terminated with such equal “short circuits,” the signals at those ports will be all or mostly reflected back into the hybrid coupler circuit, <b>156</b>. Signals reflected back into ports <b>160</b>, <b>162</b> will couple coherently to the bypass port or isolated port <b>166</b> of the coupler circuit <b>156</b>. In order to provide a proper match at port <b>166</b>, regardless of the mode of the output switching circuit or switch <b>192</b>, an isolator circuit <b>190</b>, such as a Ferrite isolator, is coupled inline between port <b>166</b> and output switch <b>192</b>. An additional function of the Ferrite isolator is to increase the attenuation from port <b>172</b> to port <b>166</b> in normal operational mode. This may be necessary to maintain the stability and integrity of the signal. Although the hybrid output ports <b>160</b>, <b>162</b> can be unbalanced with any significant mismatch large enough to produce a reasonably low loss from the input port <b>164</b> to the isolated port <b>166</b>, an open “short circuit” condition on the output ports <b>160</b>, <b>162</b> reduces the possibility of intermodulation created by the amplifier circuits <b>152</b>, <b>154</b>. Then, the primary source of any intermodulation would become the performance of the PIN diodes in their OFF state. Additionally, for minimum loss through the coupler <b>156</b> in bypass mode, the complex impedance at ports <b>160</b>, <b>162</b> should be as identical as possible.

P-0031[0031] Alternatively, the lines <b>160</b>, <b>162</b> might be half wavelength lines, and the switching might be utilized to pass the signals out of ports <b>160</b>, <b>162</b> of coupler circuit <b>156</b> when the diodes <b>182</b>, <b>184</b> are OFF and out of the bypass port <b>166</b> when the diode is ON. This approach would, however, make “zero-bias” bypass mode impossible.

P-0032[0032] To further streamline circuit <b>150</b> of the invention, and to minimize costs, the output of the amplifier circuits is not switched through an “open mismatched hybrid” methodology, as with the inputs to the amplifier circuit. In accordance with one aspect of the present invention, switching circuit <b>192</b> may be an inexpensive relay, which is utilized at the output <b>172</b> in a standard configuration. Since the gain of the amplifier circuits <b>152</b>, <b>154</b> precedes such a switching circuit <b>192</b>, the performance requirements of any relay utilized for the switching circuit <b>192</b> are not as stringent as they may be at a switching circuit utilized at the input to the amplifier circuits. Accordingly, through operation of switching circuit <b>192</b>, the overall output <b>194</b> of circuit <b>150</b> is selected either from the output port <b>172</b> or from the bypass or isolator port <b>166</b> through the isolator circuit <b>190</b>. When a bypass is desired, a signal is directed through the bypass port <b>166</b> to the output port <b>194</b> through switching circuit <b>192</b>. Utilizing a relay for the switching circuit <b>192</b> provides broadband isolation. Furthermore, with such a design, a problematic feedback path is avoided.

P-0033[0033] Accordingly, the embodiment of FIG. 3 provides a desirable bypass switching function, which is robust, straightforward and not particularly complex, is easily operated, and is relatively inexpensive due to a small number of solid-state parts, such as PIN diodes, utilized in the design.

P-0034[0034] While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.