1. Field of the Invention
This invention relates to microwave switches. More particularly, this invention relates to high power microwave switches employing PIN diodes utilized in a single pole double throw (SPDT) configuration.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
For many years, switches have been used in the electrical arts to provide a means for isolating a portion of an electrical circuit. In its simplest form, a single pole/single throw (SPST) switch resides in one of two positions. In a xe2x80x9cclosedxe2x80x9d position, the switch allows a signal to pass from an input port to an output port. In an xe2x80x9copenxe2x80x9d position, the switch prevents a signal from passing from the input port to the output port. A theoretically perfect switch has no series resistance or shunt admittance in the xe2x80x9cclosedxe2x80x9d position and has either infinite series resistance, infinite shunt admittance, or both in the xe2x80x9copenxe2x80x9d position. A single pole/double throw (SPDT) switch selects between two separate output ports.
The earliest switches used in microwave applications were mechanical switches, but they suffered from many limitations. In response to these limitations, solid state switches were developed. These solid state switches use semiconductor devices in a variety of different configurations to provide the xe2x80x9copenxe2x80x9d and xe2x80x9cclosedxe2x80x9d positions, and many employ PIN diodes as their controllable elements. PIN diodes, as is well known, are diodes that are formed from a silicon wafer containing nearly equal P type and N type impurities on opposing sides of the wafers. In the middle of the wafer there exists a barrier layer of silicon with little or no doping known as the intrinsic layer. The intrinsic layer has a relatively long recovery time that causes PIN diodes to be relatively slow in comparison to regular diodes. Higher frequency applied signals, such as those in the radio and microwave frequencies, do not cause the PIN diodes to become rectifying and forward biased during the positive portion of the signal cycle. In their unbiased or reverse biased states, the PIN diodes have a series resistance that is typically in excess of 1000 ohms and a small junction capacitance. In their forward biased state, the PIN diodes have a series resistance of approximately one to two ohms. Because of their switching and electrical characteristics, PIN diodes function well in high frequency solid state switch applications.
In one general configuration PIN diodes with their anodes connected to a transmission line segment that spans from the switch input to the switch output are employed. The cathodes of the PIN diodes are connected to ground. In the xe2x80x9cclosedxe2x80x9d position, the PIN diodes are unbiased or reverse biased, presenting a high impedance between the transmission line segment and ground. This allows the signals to pass from the switch input to the switch output. In the xe2x80x9copenxe2x80x9d position, the PIN diodes are forward biased, presenting a very low impedance to the RF signals and creating both a reflection to the signal and a shunt to ground. In this position the signals do not reach the output of the switch.
In solid state antenna arrays, the low noise amplifier (LNA) on the receiver side must be protected from the high power transmit pulse to the antenna. This protection is typically provided by a limiter and circulator which in combination give about 30 dB of protection. For semi-active arrays (such as those in the Firefinder Block II radar) the transmit pulse can be very high, as much as 250 watts or more, at the subarray level. For such applications, at least about 47 dB of isolation is required. This additional protection can be provided by a SPDT PIN diode switch. This switch must be able to handle this high power level and must also be able to protect the low noise amplifier in the event a bias failure takes away the ability of the system to reverse bias the PIN diodes. The switch isolation is much higher when the PIN diodes are reverse biased than when they are in their OFF state, but unbiased.
The relevant art contains a number of references that disclose SPDT switches that use PIN diodes. U.S. Pat. No. 4,267,538 to Assal, et al. teaches a multiport PIN diode switch, but it is concerned with achieving optimal impedance matching between its various input and output ports and not with high power switching problems. U.S. Pat. No. 5,142,256 to Kane teaches another type of PIN diode switch, this one utilizing multiple PIN diodes spaced along a transmission line segment. The emphasis here is to select a particular one of the multiple PIN diodes to vary the apparent transmission line length. Again, there is no treatment of the problems of high power switching. U.S. Pat. No. 5,440,283 to Nendza discloses another PIN diode switch, this one being concerned with avoiding the use of DC blocking capacitors and quarter wavelength transmission line segments and using lumped circuit elements instead to decrease the attenuation and increase the bandwidth of the switch. Finally, U.S. Pat. No. 5,109,205 to Hart et al. describes yet another type of PIN diode switch that seeks to avoid the apparent disadvantages of using a common DC and RF ground plane with the attendant need to use blocking capacitors by providing a separate DC ground connection.
Thus, although the patents discussed above provide for significant improvements in the art, there remains an ongoing need for further improvements in the design of high power microwave switches, particularly in the provision of a practical and effective bias failure mode for very high power applications involving semi-active radar arrays with transmit power above 50 watts.
The need in the prior art for an improved high power SPDT switch is addressed by the present invention which provides at least two SPST PIN diode switch assemblies connected in series to provide for adequate isolation of the more sensitive receiver side of the radar transceiver and one SPST diode switch assembly on the transmit side of the transceiver, these two sides of the SPDT switch being oppositely biased at any one time. In the event of a bias failure the residual isolation provided by the at least two SPST assemblies is sufficient to protect the receiver section of the radar system from the high power pulses from the transmitter.
The SPST assembly either passes or blocks the transmission of an RF signal from one side of the assembly to the other. The assembly has a central node to which is connected an input transmission line and an output transmission line. Also connected to the central node are an inductive resonator and an open circuit resonating stub. The PIN diode is connected between the other side of the inductive resonator and ground. The PIN diode is also connected to a biasing means that supplies a small forward bias (about +1 volt) or a large negative bias (between about xe2x88x9220 to about xe2x88x92100 volts). This DC bias voltage is isolated from the rest of the radar system by blocking capacitors on the transmission lines leading to and from the SPST assembly. One of the SPST assemblies is used on the transmit side of the radar system and two or more of the SPST assemblies are used on the receive side. A bias controller circuit switches the bias to the single SPST assembly back and forth between forward and reverse on the transmit side of the radar while switching the bias in the opposite sense to the two or more SPST assemblies on the receive side. The operating characteristics of the SPDT switch are described in more detail below.