AN RFMD® APPLICATION NOTE Isolation at High Input Power: SPDT Switch Comparison Introduction With the increasing popularity of the LTE standard now prevalent across many of today's wireless service providers and developers, the demand for high-performance RF components is growing rapidly. LTE developers today seek to implement high-linearity, high-isolation, low-insertion-loss, and low-control voltage switches into their flagship designs. For example, in a cellular base station applications with LTE signals running at +33dBm average output power, and peaks reaching up to +40dBm, even with duty cycles as low as .01%, a high-performance switch is crucial for reliable operation. Choosing the right high-power SPDT switch can be challenging, since performance at high input power levels is rarely specified. So why is high isolation important for an LTE switch? High isolation essentially ensures the integrity of the active path by reducing influence of signals from the other channel. This becomes especially critical if you are dealing with multiple high-peak power signals in a tight frequency bandwidth allocation typical for LTE. RFMD has a few high-performance, high-power 10W SPDT GaAs switches in its lineup such as FMS2031-001, RFSW8000, and RF1201. The comparison presented here is for the FMS2031-001 and RFSW8000. RFSW8000 The RFSW8000 is a 10W, single-pole, double-throw (SPDT) GaAs pHEMT transmit/receive switch designed for high-performance wireless applications including LTE and WiFi. This wideband switch has been designed for use from DC to 6.5GHz, where extremely high linearity, high isolation, low insertion loss, and small package size are required. FMS2031-001 The FMS2031-001 is a 10W, single-pole, double-throw, (SPDT) GaAs pHEMT transmit/receive switch. The switch offers excellent power handling capability and harmonic performance. The FMS2031-001 is designed for use in LTE, WiMax, L-, S-, and Cband wireless applications and WiFi access points where high-linearity switching is required. Parameter Frequency Range Peak Power Switching Speed Specification FMS2031-001 RFSW8000 Unit DC to 6000 DC to 6500 MHz 41 40 dBm 350 300 ns Insertion Loss .5 .4 dB OIP3 60 59 dBm Isolation 32 29 dB 2nd Harmonic -85 -80 dBc 3rd Harmonic Dimensions -85 -80 dBc 3x3 2x2 mm Condition RF MICRO DEVICES®, RFMD®, Optimum Technology Matching®, Enabling Wireless Connectivity™, PowerStar®, POLARIS™ TOTAL RADIO™ and UltimateBlue™ are trademarks of RFMD, LLC. BLUETOOTH is a trademark owned by Bluetooth SIG, Inc., U.S.A. and licensed for use by RFMD. All other trade names, trademarks and registered trademarks are the property of their respective owners. ©2012, RF Micro Devices, Inc. AN120731 7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical support, contact RFMD at (+1) 336-678-5570 or [email protected]. 1 of 4 Isolation at High Input Power: SPDT Switch Comparison Isolation at High Input Power The RFSW8000 and FMS2031-001 are close in performance, however depending on which requirement is crucial to meet, there are some trade-offs. The FMS will have better harmonics, and higher power handling capability, but this comes at the expense of increased size and slightly higher insertion loss than the RFSW. Isolation at high input power is another important specification for LTE applications, as addressed above. The isolation performance, RF1-ANT and RF1-RF2, has been characterized for both switches under equal conditions. The isolation at high input power characterization was performed on two sets of three FMS and RFSW switches. The test was performed with control voltage down to 3V. Standard RFMD evaluation boards were used, and temperature was ambient at 25°C. Two frequencies of operation were tested: 880MHz and 1960MHz (capturing most LTE bands). The power sweep was performed from +27dBm up to +40dBm (10W) and isolation was measured. The DC-blocking capacitor value used on RF IN/ OUT was 1nF, however the use of smaller capacitors such as 100pF to 300pF is permissible. The DC-blocking capacitors are needed in the circuit, and their value depends on frequency of operation. Setup The basic setup involves a high-power amplifier capable of providing POUT of at least 10W. The directional couplers are used to monitor the power level going in and coming out of the switch. The load is rated for high power and serves as a 50 termination. The spectrum analyzer measures the power level of the inactive or isolated path during switch operation. SA 20dB ATT Sig Gen High Power PA 50W Directional Coupler 1 SPDT Switch Power Meter 1 Directional Coupler 2 50 Termination 100W Power Meter 2 Figure 1. Setup Block Diagram AN120731 7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical support, contact RFMD at (+1) 336-678-5570 or [email protected]. 2 of 4 Isolation at High Input Power: SPDT Switch Comparison Results 31 30.8 30.6 30.4 30.2 30 29.8 29.6 29.4 29.2 29 28.8 28.6 28.4 28.2 28 RFSW8000(3V,1nFcaps) IsolationversusPIN (RF1ANTandRF1RF2at1960MHz) Isolation(dB) Isolation(dB) RFSW8000(3V,1nFcaps) IsolationversusPIN (RF1ANTandRF1RF2at880MHz) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 31 30.8 30.6 30.4 30.2 30 29.8 29.6 29.4 29.2 29 28.8 28.6 28.4 28.2 28 27 28 29 30 31 32 InputPower(dBm) 28 Isolation(dB) SW1_1960_RF1_ANT SW2_1960_RF1_ANT SW3_1960_RF1_ANT SW3_880_RF1_RF2 SW1_1960_RF1_RF2 SW2_1960_RF1_RF2 SW3_1960_RF1_RF2 29 30 31 32 33 34 35 36 37 38 39 27 40 28 29 30 31 32 33 34 35 36 37 38 SW2_880_RF1_ANT SW3_880_RF1_ANT SW1_1960_RF1_ANT SW2_1960_RF1_ANT SW3_1960_RF1_ANT SW2_880_RF1_RF2 SW3_880_RF1_RF2 SW1_1960_RF1_RF2 SW2_1960_RF1_RF2 SW3_1960_RF1_RF2 31 32 33 39 40 39 40 InputPower(dBm) SW1_880_RF1_RF2 30 40 34 33.9 33.8 33.7 33.6 33.5 33.4 33.3 33.2 33.1 33 32.9 32.8 32.7 32.6 32.5 32.4 32.3 32.2 32.1 32 SW1_880_RF1_ANT 29 39 FMS2031001(3V,1nFcaps) IsolationversusPIN (RF1ANTandRF1RF2at1960MHz) FMS2031001(3V,1nFcaps) InsertionLossversusPIN (880MHzand1960MHz) InsertionLoss(dB) InsertionLoss(dB) 38 SW3_880_RF1_ANT 1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 34 35 36 37 38 39 40 1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 27 28 29 30 31 InputPower(dBm) AN120731 37 SW2_880_RF1_RF2 RFSW8000(3V,1nFcaps) InsertionLossversusPIN (880MHzand1960MHz) 28 36 SW2_880_RF1_ANT InputPower(dBm) 27 35 SW1_880_RF1_RF2 Isolation(dB) 27 34 SW1_880_RF1_ANT FMS2031001(3V,1nFcaps) IsolationversusPIN (RF1ANTandRF1RF2at880MHz) 36.6 36.5 36.4 36.3 36.2 36.1 36 35.9 35.8 35.7 35.6 35.5 35.4 35.3 35.2 35.1 35 33 InputPower(dBm) 32 33 34 35 36 37 38 InputPower(dBm) SW1_880_IL SW2_880_IL SW3_880_IL SW1_880_IL SW2_880_IL SW3_880_IL SW1_1960_IL SW2_1960_IL SW3_1960_IL SW1_1960_IL SW2_1960_IL SW3_1960_IL 7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical support, contact RFMD at (+1) 336-678-5570 or [email protected]. 3 of 4 Isolation at High Input Power: SPDT Switch Comparison Conclusions As can be observed from the plots, the FMS2031-001 has better isolation performance than the RFSW8000 at +40dBm peak power levels. At high power levels, FMS2031-001 exhibits isolation of +36dB at 880MHz, while the RFSW8000 reaches up to +30dB. At 1960MHz, the FMS2031-001 isolation was +33.4dB, while RFSW8000 registered 29.4dB, on average. The reason for the small difference of approximately 1dB in RF1-ANT and RF1-RF2 isolation is due to the superior ability of both switches to maintain active path insertion loss even at higher input power levels. It was expected that insertion loss would degrade, making RF1-RF2 isolation significantly worse as the power level was swept up to +40dBm, however both switches maintained their insertion loss specifications throughout the power sweep. From the data, it can be concluded that FMS2031-001 is better suited for LTE applications, where path isolation is critical at peak power levels of +40dBm for performance, while the RFSW8000 is a better fit for WiFi, where isolation at high power is not as critical. AN120731 7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical support, contact RFMD at (+1) 336-678-5570 or [email protected]. 4 of 4