A Wideband General Purpose PIN Diode Attenuator Application Note Introduction PIN diode based AGC attenuators are commonly used in many broadband system applications such as: cable or fiberoptic TV, wireless CDMA, etc. A popular attenuator design utilized over the instantaneous frequency range from 10 MHz to beyond 2 GHz is the PI network. The benefit of this design is its broadband constant impedance, wide dynamic range and good compatibility with AGC signals. The PIN diode is used as a current controlled resistance component in the PI network. PIN diodes are low cost, low distortion elements available in commonly used small size plastic packages. This application note describes the design of a high performance, PIN based 4 diode PI attenuator utilizing the low cost SMP1307-011 diode in a plastic SOD-323 package. Performance is characterized from 10 MHz 3 GHz. PI Attenuator Fundamentals For matched broadband applications, especially those covering low RF frequencies (to 5 MHz) through frequencies greater than 1 GHz, PIN diode designs are commonly employed. The circuit configurations most popular are the TEE, bridged TEE and the PI. All these designs use PIN diodes as current controlled RF resistors whose resistance values are set by a DC control, established by an AGC loop. Figure 1 shows the basic PI attenuator that uses 3 PIN diodes. It also shows the expressions that determine the resistance values for each PIN diode as a function of attenuation. Figure 2 displays the value of PIN diode resistance for a 50 Ω PI attenuator. Note that the minimum value for the shunt diodes, R 1 and R 2, is 50 Ω. This application note describes the design and performance of a PI attenuator that uses 4 PIN diodes as shown in Figure 3. The benefit of the 4 diode circuit is its symmetry that allows for a simpler bias network and a reduction of distortion due to cancellation of harmonic signals in the back to back configuration of the series diodes. RF Input D 1 (R S1 ) Diode Resistance (Ω) RF Input 10000 1000 100 10 D 3 (R S3 ) Figure 1. PI Attenuator R S1 = R S2 R S3 D 2 (R S2 ) RF Output 1 0.1 1 10 100 Attenuation (db) Figure 2. Attenuation of PI Attenuators D 1 (R S1 ) D 3 (R S3 ) D 2 (R S2 ) Figure 3. 4 Diode PI Attenuator RF Output Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com 1
Attenuator Circuit Model In the Libra IV model shown in Figure 4, the PIN diode pairs, X 3 /X 4 and X 1 /X 2, are symmetrically biased from two DC sources. A 5 V reference DC voltage source (V REF ) provides adequate biasing to keep the RF resistance of the shunt diodes X 2 and X 3 near 50 Ω at high attenuation while the series diodes, X 4 and X 1, are at high resistance values.the values of biasing resistors SRL 3, SRL 2, SRL 1, SRL 5 and SRL 4 were selected to provide low SWR for the full attenuation range. Attenuation is controlled by the control voltage source (V CTL ), ranging from 1 6 V. This source supplies forward bias current to the series diodes, X 4 and X 1, through a wideband, high impedance ferrite inductor, X 7 (Taiyoyuden model FBMH4525) and resistors SRL 5, SRL 4 and SRL 6. Capacitors C 6 and C 7 simulate the effect of the coaxial connectors (on our test boards we used SMA connectors). Shunt connected capacitors, SRLC 7 and SRLC 11, were inserted to compensate the parasitic inductances of the decoupling capacitors, SRLC 4 and SRLC 6. These parasitic inductances strongly affect attenuator performance at frequencies beyond 2 GHz. The PI type C-L-C circuit between series diodes SRLC 8, L 1 and SRLC 9, was used to increase the maximum isolation at higher frequencies while improving insertion loss at low attenuation. Figure 5 illustrates the effect of connecting or not connecting this C-L-C circuit. A clear 5 8 db improvement in isolation is demonstrated. Capacitors SRLC 12, SRLC 10 and SRLC 5 provide RF ground for the shunt diodes. The separation of the biasing path onto two branches SRL 2 and SRL 1 was to reduce RF coupling between input and output, which will affect maximum attenuation especially at high frequencies, due to the parasitic series inductances. 2 Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com
Figure 4. Attenuator Model for Libra IV Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com 3
-30-30 Without Correction -35 Attenuation (db) -40-50 -60 With Correction Attenuation (db) -40-45 -50-55 -60 2 pf 10 nf -65-70 0 1 2 3 Frequency (GHz) -70 0 1 2 3 Frequency (GHz) Figure 5. The Effect of Compensation Circuit Figure 6. The Effect of Capacitor SRLC 5 The values of the bias resistors were optimized for optimum SWR performance over the entire attenuation range. The intent was to keep the values of SRL 5 and SRL 4 as low as possible to ensure maximum forward current in the series diodes, X 4 and X 1, but high enough not to affect insertion loss. The input and output circuits are not symmetrical as may be seen from the values of capacitors SRLC 12 and SRLC 10 (10 nf each), compared to SRLC 5 (2 pf). The SRLC 5 value was selected to improve high frequency isolation by compensating the parasitic series inductance of shunt diode, X 2, and its own parasitic inductance. This compensation was found helpful in improving isolation by several db at frequencies higher than 1 GHz; however, as a result, the SWR of the output port SWR is increased at lower frequencies. Most applications are not sensitive to high output SWR, but if necessary, symmetricity of the attenuator may be established by increasing SRLC 5 to 10 nf. Figure 6 shows the effect of changing SRLC 5 from 2 pf to 10 nf. If implemented, there will be no significant effect on the input SWR, because of the high isolation between input and output, and no effect on attenuation or SWR at the minimum attenuation. Figure 7. Attenuator Model Test Bench for Libra IV The linear test bench used for the analysis of the above attenuator is shown in Figure 7. 4 Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com
SMP1307 SPICE Model The SMP1307-011 is a silicon PIN diode with a thick I-region (175 µm) and a long carrier lifetime (TL = 1.5 µs). This results in a variable resistance device with a wide variation of resistance vs. current capable of operating with low distortion as an attenuator element. The diode is packaged in a SOD-323. The SPICE model for the SMP1307-011 varactor diode defined for the Libra IV environment, is shown in Figure 8 with a description of the parameters employed. In this model, two diodes were used to fit both DC and RF properties of PIN diode. The PIN diode built-in model of Libra IV was used to model behavior of RF resistance vs. DC current, while PN-junction diode model was used to model DC voltage-current response. Both diodes were connected in series to insure the same current flow, while PN-junction diode was effectively RF short-circuited with the capacitor C 2 = 10 11 pf. The portion of the RF resistance, which reflects residual series resistance, was modeled with R 2 = 2.2 Ω. This is shunted with the ideal inductor L 1 = 10 19 nh to avoid affecting DC performance. Capacitances C G, C P and inductor L 2 reflect junction and package properties of SMP1307-011 diode. The described model is a linear model that emulates the DC and RF properties of the PIN diode when the signal frequency is higher then: 1300 1300 = = 0.0425 MHz W[µm] 2 175 2 For more details on the properties of the PIN diode refer to Reference 1. Tables 1 and 2 describe the model parameters. They show default values appropriate for silicon varactor diodes that may be used by the Libra IV simulator. Some of the values of PIN diode built-in model of Libra IV were not used. Those are marked Not used in the tables. Figure 8. SMP1307-011 Model for Libra IV Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com 5
Parameter Description Unit Default IS Saturation current (Not used) A 1.9E-9 V I I-region Forward Bias Voltage Drop V 0.08 UN Electron Mobility cm**2/(v*s) (Not used) cm**2/(v*s) 900 WI I-region Width (Not used) M 1.2e-4 R R I-region reverse bias resistance Ω 4E5 CMIN PIN punchthrough capacitance F 0 TAU Ambipolar lifetime within I region (Not used) S 1E-12 R S Ohmic resistance Ω 0 C JO Zero-bias junction capacitance F 1.8E-15 VJ Junction potential V 1 M Grading coefficient - 1.01 KF Flicker-noise coefficient (Not used) - 0 AF Flicker-noise exponent (Not used) - 1 FC Coefficient for forward-bias depletion capacitance (Not used) - 0.5 FFE Flicker noise frequency exponent (Not used) - 1 Table 1. Silicon PIN Diode Values in Libra IV Assumed for SMP1307 Model Parameter Description Unit Default IS Saturation current A 1.1E-8 R S Series resistance Ω 1.48 N Emission coefficient (Not used) - 2.2 TT Transit time (Not used) S 0 C JO Zero-bias junction capacitance (Not used) F 0 VJ Junction potential (Not used) V 1 M Grading coefficient (Not used) - 0.5 E G Energy gap (with XTI, helps define the dependence of IS on temperature) EV 1.11 XTI Saturation current temperature exponent (with EG, helps define the - 3 dependence of IS on temperature) KF Flicker noise coefficient (Not used) - 0 AF Flicker noise exponent (Not used) - 1 F C Forward-bias depletion capacitance coefficient (Not used) - 0.5 B V Reverse breakdown voltage (Not used) V Infinity I BV Current at reverse breakdown voltage (Not used) A 1e-3 ISR Recombination current parameter (Not used) A 0 NR Emission coefficient for ISR (Not used) - 0 IKF High-injection knee current (Not used) A Infinity NBV Reverse breakdown ideality factor (Not used) - 1 IBVL Low-level reverse breakdown knee current (Not used) A 0 NBVL Low-level reverse breakdown ideality factor (Not used) - 1 T NOM Nominal ambient temperature at which these model parameters were derived C 27 FFE Flicker noise frequency exponent (Not used) 1 Table 2. Silicon PIN Diode Values in Libra IV Assumed for SMP1307 Model 6 Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com
The model DC current voltage response calculated by Libra IV is shown in Figure 9A together with the measured data. It shows very good compliance of our model DC properties with measured results. Figure 9B shows internal RF resistance after the parasitic capacitances C G, C P and inductor L 2 were deembeded. Here again, the measured and simulated results are in agreement. Current (ma) 100 10 1 0.1 Measured Simulated Resistance (Ω) 10000 1000 100 10 Measurement Simulation 0.01 0.4 0.6 0.8 1.0 Voltage (V) Figure 9A. DC Voltage Current Response of SMP1307-011 1 0.01 0.1 1 10 100 1000 Current (ma) Figure 9B. RF Resistance vs. Current for SMP1307-011 Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com 7
Attenuator Design, Materials, Layout and Performance The circuit diagram for the 4 diode PI attenuator is shown in Figure 10. The PCB layout is shown in Figure 11. The board was made of standard, 30 mil thick, FR4 material. The bill of materials used is shown in Table 3. Designator Value Part Number Footprint Manufacturer C1 10,000 p 0603AU103JAT9 0603 AVX C2 0.5 p 0603AU0R5JAT9 0603 AVX C3 10,000 p 0603AU103JAT9 0603 AVX C4 10,000 p 0603AU103JAT9 0603 AVX C5 10,000 p 0603AU103JAT9 0603 AVX C6 10,000 p 0603AU103JAT9 0603 AVX C7 2 p 0603AU2R0JAT9 0603 AVX C8 0.5 p 0603AU0R5JAT9 0603 AVX C9 0.5 p 0603AU0R5JAT9 0603 AVX C10 0.5 p 0603AU0R5JAT9 0603 AVX C11 10,000 p 0603AU103JAT9 0603 AVX C12 10,000 p 0603AU103JAT9 0603 AVX D1 SMP1307-011 SMP1307-011 SOD-323 Skyworks Solutions D2 SMP1307-011 SMP1307-011 SOD-323 Skyworks Solutions D3 SMP1307-011 SMP1307-011 SOD-323 Skyworks Solutions D4 SMP1307-011 SMP1307-011 SOD-323 Skyworks Solutions L1 1x8 mm MSL 1x8 mm (Printed on PCB) L2 1x8 mm MSL 1x8 mm (Printed on PCB) L3 2.2 nh LL1608-F2N2S 0603 TOKO L4 FBMH4525 FBMH4525_HM162NT 1810 TAIYO-YUDEN R1 560 CR10-561J-T 0603 AVX R2 100 CR10-101J-T 0603 AVX R3 1 k CR10-102J-T 0603 AVX R4 1 k CR10-102J-T 0603 AVX R5 1 k CR10-102J-T 0603 AVX R6 560 CR10-561J-T 0603 AVX Table 3. The Attenuator Bill of Materials 8 Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com
L 1 (1x8 mm) D1-D4 SMP1307-011 C 1 D 1 R 2 100 L 3 2.2 n L 4 FBMH4525 D 3 C 11 V TUNE 0 6 V C 12 L 2 (1x8 mm) RF Input C 2 0.5 pf R 1 560 D 2 C 8 0.5 pf R 3 1 k C 9 0.5pF R 4 1 k D 4 C 10 0.5 pf R 6 560 RF Output C 3 C 4 R 5 1 k C 5 C 6 C 7 2 pf V REF 5 V Figure 10. Attenuator Circuit Diagram Figure 11. Attenuator PCB Layout Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com 9
The measured attenuation of this circuit and the simulated results obtained with the model in Figure 8 are shown in Figure 12A and 12B respectively. The model fits measurement results very well in the attenuation extremes, but has a small deviation from measurements in the middle of the attenuation range. This may be attributed to the imperfection of diode RF resistance model shown in Figure 9B. Figure 13 shows measured input SWR at different control voltages. SWR is well below 2 across the entire range of frequencies and attenuation levels as predicted by the model. A plot of attenuation vs. control voltage at temperatures of 23 C and 85 C temperatures are shown in Figure 15. The graph shows that the temperature performance is very stable, with less than 0.5 db variation over the 62 C excursion at the highest attenuation. S21 (db) 0-10 -20-30 -40-50 8 V 6 V 5 V 9 V 12 V 4 V 3 V 2 V 1.5 V -60 0.00 1.00 0 V Frequency (GHz) Figure 12A. Measured S 21 V CONTROL 7 V 2.00 3.00 VSWR Attenuations (db) 2.0 1.9 1.8 V CONTROL 1.7 1.5 V 1.6 6 V 4 V 1.5 5 V 7 V 1.4 8 V 1.3 12 V 1.2 2 V 1.1 3 V 9 V 1.0 0.00 1.00 2.00 3.00 0-10 -20-30 -40-50 Frequency (GHz) 0 V Figure 13. Measured SWR F = 3 GHz F = 1.98 GHz F = 910 MHz -60 F = 490 MHz F = 70 MHz -70 0.00 2.00 4.00 6.00 Control Voltage (V) 8.00 10.00 12.00 Figure 14. Attenuation vs. Control Voltage 0 S21 (db) 0-10 8 V 6 V 5 V 4 V -20 2 V -30-40 1.5 V -50-60 0 V 0.00 1.00 2.00 3.00 Frequency (GHz) S21 (db) -10-20 -30-40 -50-60 0.00 1.00 5 V 2 V 0 V Frequency (GHz) +23 C +85 C 2.00 3.00 Figure 15. Attenuation vs. Temperature Figure 12B. Simulated S 21 10 Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com
Figure 16 shows output third order intercept point (IP3) vs. control voltage. The measurement was performed at 900 MHz using a single tone 1 W input power. IP3 was derived from the third harmonic using the method described in Reference 2. Output IP3 (dbm) 80 70 60 50 40 30 20 10 0 5 10 15 20 25 30 35 40 45 50 Attenuation (db) Figure 16. Third Order Intercept Point vs. Attenuation at 900 MHz References 1. Gerald Hiller, Design with PIN Diodes, Application Note, Skyworks Solutions, Inc. 2. Gerald Hiller, Predict Intercept Points in PIN-Diode Switches, Microwaves & RF, Dec. 1985. 3. Robert Caverly and Gerald Hiller, Distortion in PIN Diode Control Circuits, IEEE Trans. Microwave Theory Tech., May 1987. List of Available Documents 1. The 4-diode PIN attenuator simulation project files for Libra IV. 2. The 4-diode PIN attenuator circuit schematic and PCB layout for Protel EDA Client 1998 version. 3. The 4-diode PIN attenuator PCB Gerber photo-plot files. Skyworks Solutions, Inc., 1999. All rights reserved. Skyworks Solutions, Inc. [781] 376-3000 Fax [781] 376-3100 Email sales@skyworksinc.com www.skyworksinc.com 11
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