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
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support, contact RFMD at (+1) 336-678-5570 or customerservice@rfmd.com.
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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
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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
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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 customerservice@rfmd.com.
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