WiMAX Broadband Switches – Why PIN Diodes?
By Tim Boles, Distinguished Fellow of Technology, M/A-COM Technology Solutions and James Brogle, Principal Engineer, M/A-COM Technology Solutions

The ever increasing linearity and peak-to-average power requirements of the modulation schemes employed by EDGE, 4G, TD-SCDMA, and the most challenging to date, WiMAX systems in base station, CPE, and even femtocell applications, have led designers to employ PIN diode based control configurations as the preferred solution as the antenna T/R switch. There are several reasons why PIN diode switching components have regained favor for this function. The first is based upon the fact that PIN diodes have a high intrinsic reverse breakdown voltage, 100 volts to 250 volts are easily achieved by proper choice of “I” region thickness, resulting in the ability to reliably handle very high power levels. Secondly, very low insertion loss and high port-to-port isolation can be achieved by choosing anode areas and “I” region thicknesses that optimize the trade-offs between junction capacitance, Cj, and series resistance, Rs. The width of “I” region also is a strong determinant in the overall device linearity, P-1dB, IP3, and EVM, and the resultant high frequency distortion products, and is critical in the PIN diode design mix. Lastly, if either series or series/shunt circuit topologies are employed, packaged PIN diode monolithic switches that have instantaneous multi-octave bandwidths covering 50 MHz to over 6 GHz have been demonstrated. It can be seen that this multi-octave capability easily covers the VHF, UHF, GSM, PCS, DCS, G3, EDGE, G4, and all the WiMAX frequencies in one switch.

The one consideration that must be taken into account with these PIN-based components is the fact that PIN diodes are current controlled devices. While the moderate currents that are required for proper PIN switch operation are certainly a detriment for battery operated applications such as handsets for basestation, CPE, and femtocell usage, where the power supply/current draw is only limited by the wire gauge connection to the power plant, this current requirement should not be a concern.

From a reliability viewpoint, the ultimate limit for device wear out and catastrophic failure is set by peak junction temperature. Peak junction temperatures that exceed maximum recommended limits are always caused by I2R losses due to excessive current flow through the device, generally caused by uncontrolled avalanche breakdown mechanisms initiated by peak voltages generated by the RF swings associated with high incident power levels. As an example, for a series element, the peak and peak-to-peak RF voltage swings under a matched condition is given by Equation (1) and (2) below:

It can be seen that for a 10 watt CW incident signal the peak RF voltage developed is 45 volts and the peak-to-peak swing is double this value or 90 volts.

For less than ideal matching conditions, a mismatch factor based upon the circuit VSWR ratio as shown in Equation (3) needs to be factored into Equation (1) to produce Equation (4). Again, as an example, for a 10 watt CW incident signal with a VSWR mismatch of 1.5:1, the peak-to-peak RF induced voltage swing increases to 107 volts. It can be seen that as the circuit match to the series device degrades, this mismatch factor and the peak voltages that the series element must withstand increase dramatically.

By proper choice of “I” region thickness, a single PIN diode can have reverse breakdown characteristics between 100 volts and 250 volts, easily enabling reliable operation at high incident power levels, including VSWR circuit mismatch effects and the associated RF induced peak voltage swings. In the simplest high frequency PIN diode switch, only one series element per arm is required, enabling low insertion loss, superior isolation, wide bandwidth, high power handling and excellent linearity to be achieved.

While it is certainly possible for GaAs MESFETs to be operated reliably under these high power and VSWR conditions, compromises are required in virtually all of the switch high frequency parameters. Since the typical MESFET employed in high frequency switches is intrinsically a low voltage device, having a drain to source reverse breakdown, Vds, in the 8 volt to 20 volt range, several devices are required to be cascaded in series to reach the necessary standoff voltages of even a 10 watt incident power level, much less account for higher incident powers or VSWR issues. Whether it is a PIN diode or a MESFET transistor, every time an element is added to the switch arm, it typically adds 0.2 dB - 0.5 dB per additional element to a series switch insertion loss. Another design approach employing MESFET active elements that is able to handle incident powers greater than one to two watts is to utilize a shunt switch topology. In this switch configuration, there is virtually no RF energy dissipated in the FET active element and the insertion loss and peak junction temperature is determined by the Q of the transmission line connecting the input to the output port. While this approach is able to handle high power levels, the broadband capability is severely restricted. In order to cover the same 50 MHz to 6 GHz frequency range with this shunt configured switch, as many as seven or eight different external circuits must be employed, each having different matching component values, to realize the necessary quarter wave sections that are required to provide isolation between each switch arm.

MA-COM Technology Solutions has been designing and manufacturing both PIN diode and GaAs MESFET integrated switch solutions for at least two decades. For this reason, MA-COM Technology Solutions is uniquely positioned to be able to recommend the best switch solution, whether PIN or FET based, for the full spectrum of applications addressing the needs of the entire RF, microwave, and mmW marketplaces.

In order to meet the high linearity, power handling, and bandwidth challenges imposed by the frequency allocations and the OFDM modulation schemes that are unique to WiMAX, MA-COM Technology Solutions has developed a family of integrated, silicon PIN diode based switches based upon a patented technology, HMIC – an acronym for Heterolithic Monolithic Integrated Circuit, referring to its creation from two dissimilar materials. In this technology, the two different materials, glass and silicon - thus, the term hetero– are joined into a single monolithic structure at a waferscale level. From this marriage of glass and silicon, a synergism is obtained which enables the high frequency and high power handling properties of the two materials to be optimized enabling complex high frequency circuits to be fashioned in quantities of tens of thousands at a time using standard semiconductor processing equipment and techniques. A typical cross section of the HMIC technology as applied to both series and shunt configured PIN diodes that are employed as elements in high frequency switches is shown in Figure 1.

In Table 1, a parametric comparison of a family of PIN diode T/R switches specifically optimized by the MA-COM Technology Solutions design team for the requirements of WiMAX and high linearity applications, the MASW-000822, MASW-000823, and MASW-000825, is presented. As shown in Figure 2, this series of switches employ a “series only” topology with a single series diode in each arm of the SPDT switch. Further, this switch is designed in a common anode configuration enabling operation from a single positive voltage bias supply. Lastly, while this SPDT switch has a single diode in each of the T/R arms, an asymmetric design approach was used in order to simultaneously optimize the transmit insertion loss and the receive isolation. This asymmetry is shown in a typical photomicrograph of the series only switch design in Figure 3, where it is clear that the anode dimensions, and the resulting differences in junction capacitance and series resistance, of each PIN diode are purposefully different. The functional results of this asymmetric layout can be seen by examining Table 1 and are displayed graphically in Figure 4, where it can be seen that the insertion loss of the Tx arm of the SPDT MASW-000822/23/25 switches is under 0.5 dB while the Rx isolation is approximately 26 dB over the 2.0 GHz to 3.8 GHz range of WiMAX and TD‑SCDMA frequencies.

In a similar manner, Table 2 is a parametric comparison of a second iteration of PIN diode T/R switch technology aimed at the requirements of WiMAX, TD-SCDMA and other high linearity, high power applications, the MASW‑000834, which is operated at two different bias conditions. As shown in Figure 5, this switch employs a series only topology on the Tx arm with a series/shunt configuration in the Rx arm of the SPDT switch. This continues the asymmetric design approach to further enhance the receive isolation while leaving the transmit insertion loss unaffected. This series-series/shunt asymmetry is shown in a photomicrograph of the MASW‑000834 switch die in Figure 6, where it is clear that a shunt diode has been added to the receive path of the switch. It can also be seen in Figure 6 that the transmit diode has been redesigned as a rectangular anode structure to reduce the overall device thermal resistance and significantly increase the device incident power handling. Again, this switch is designed to be operated from a single positive voltage bias supply. Again, the functional improvements in terms of Rx isolation of this asymmetric series/shunt approach can be seen by comparing Table 1 and Table 2, and the graphical data presented in Figure 4, where it can be seen that the receive isolation has been increased by approximately 10 dB, when contrasted with the MASW‑000822/23/25 series only designed switches, into the range of 36 dB to 41 dB by the addition of a shunt diode to the Rx arm of the MASW‑000834 T/R switch. This improvement in Rx isolation was achieved with no cost to the Tx insertion loss, which remains less than 0.5 dB over the 2.0 GHz to 3.8 GHz bands.

The ability of the MASW-000822, MASW-000823, MASW-000825, and MASW-000834 PIN diode based switches to meet and exceed the linearity and power handling requirements of WiMAX, TD-SCDMA, 4G, and other linear systems can be clearly seen by an inspection of Table 1 and Table 2 of the specifications for the P1dB and IIP3 parameters, which are key indicators of the overall device linear performance, reveals that the linearity is limited by the maximum measurement capability of the test equipment and not by the switches.

Another indicator of switch linearity for use in WiMAX systems is the use of EVM, Error Vector Magnitude. In WiMAX, an OFDM (Orthogonal Frequency Division Multiplexing) modulation format is employed, which is a spread spectrum waveform that looks similar to a multi-tone signal. In this modulation scheme, a perfectly linear signal has ~ 0% EVM. As non-linearities are introduced, the EVM starts to increase. The change in EVM % represents the percent change in the vector signal. The EVM percentage then can be correlated to the Bit Error Rate in the system application.

The EVM % is plotted versus incident power for the MASW-000822 series only HMIC PIN diode switch and is shown in Figure 7. In this plot, it can be seen that a 1.0% EVM level is achieved at an incident power of +39 dBm. As the incident power into the switch is increased to +41 dBm, the linearity is still an excellent 1.5% EVM level. Bottom line, whether IIP3 or EVM is used as a measure of linearity and distortion, both techniques demonstrate the excellent performance of this monolithic switch structure and the underlying technology.

Because of the very high intrinsic reverse breakdown voltage capability of PIN diodes, all of the MASW‑000822, MASW‑000823, MASW‑000825, and MASW‑000834 T/R switches, as can be seen in Table 1 and Table 2, are capable of handling very high incident power levels. While these switches utilize an asymmetric design topology on the Rx path, all devices employ a single series diode in the Tx arm, yet are still capable of reliably handling at least 40.0 dBm, 10 watts, of incident RF energy at 3.8 GHz WiMAX frequencies as demonstrated on the MASW‑000822. Within this family of PIN diode switches, the maximum transmit power handling capability is realized in the MASW‑000834 component. As was presented above, the series diode in the transmit path of this switch was optimized not only for low insertion loss but also, via the rectangular anode geometry, for reduced thermal resistance. Lower device thermal resistance translates in to lower junction temperatures and higher incident power handling. It can be seen in Table 2 that the MASW‑000834 switch is capable of handling incident Tx power levels of 47.0 dBm, 50 watts, at 2.0 GHz and 45.5 dBm, 35.5 watts, at 3.8 GHz, covering both the WiMAX and TD‑SCDMA frequency bands. A comparative plot of the incident power handling at 2.0 GHz of the MASW‑000822, MASW‑000823, MASW‑000825, and MASW‑000834, at two different bias currents, relative to the maximum safe operating junction temperature of 175 °C is presented in Figure 8.

In summary, it has been demonstrated that fully monolithic PIN diode based T/R switches capable of simultaneously delivering broadband frequency performance, reliably handling very incident power levels in excess of 10 watts, and providing excellent linearity and low signal distortion in complex modulation formats was achieved utilizing a patented HMIC integration technology. This technology has the ability to produce low loss series PIN diodes that have both high electrical isolation from the ground plane and, a low thermal impedance to maintain a reliable peak junction temperature. Both of these features are critical to the excellent switch characteristics exhibited by the MASW‑000822, MASW‑000823, MASW‑000825, and MASW‑000834 family of PIN diode T/R switches.

M/A-COM Technology Solutions
www.macom.com
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