APPLICATION NOTE APN1007: Switchable Dual-Band 170/420 MHz VCO for Handset Cellular Applications Introduction Modern multiband cellular handsets use multiple voltage control oscillator (VCO) functions to accommodate the many down/up conversions in the intermediate frequency (IF) portion. Using separate VCOs would cause a substantial increase in cost and size of the radio frequency (RF) section. This component overload can be resolved by implementing band switchable VCOs. In many commercial switchable VCOs, the reactive elements in the tank circuit are switched. This function is usually performed with PIN diodes. The disadvantage of this solution is that in the closed circuit state (“diode on” state) there is VCC current flowing through the diode. This VCC current carries electrical noise which directly modulates the VCO frequency. Therefore, the noise spectrum may grow significantly beyond the PLL filter range. This type of switching also limits the switching to within 10–15 percent of the center frequency, due to the strong effect of PIN diode series resistance on the tank circuit losses, increasing phase noise. In this paper, we describe a new switchable VCO solution which employs switching between separate tank circuits. This reduces the effect of PIN diode series resistance on VCO noise performance and results in virtually unlimited switching range and individual optimization of each tank circuit. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200317 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 1 APPLUICATION NOTE • APN1007 170_420_Dual_Resonator Figure 1. VCO Model Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 2 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200317 Rev. A APPLUICATION NOTE • APN1007 The VCO Model In the circuit schematic in Figure 1, a traditional Colpitts structure, the varactors are connected as shunt capacitors. Tank inductors, L4 and L1, providing DC bias to the varactors, are shunted to ground at the common point with capacitor X6. Inductors L4 and L1 are modeled as lossy elements (with Q = 25 at 100 MHz) in parallel with capacitors C5 and C4 of 0.38 and 0.28 pF respectively. This is typical for multilayer inductors of style 0603 (60 x 30 mil footprint), (TOKO Coils and Filters catalogue). The tank inductor values of 12 and 56 nH were optimized to fit the desired 170 MHz and 420 MHz frequency bands. Capacitor X6 is modeled as a series RLC network with the length of transmission line TL3, appropriated to its position on the layout (see later). Series capacitative reactances, X3 and X4 are also modeled as lossy series RLC networks, with their appropriate layout-specific transmission lines, TL4 and TL2. Shunt capacitors, C7 and C6, are due to the effects of multiple components pads. The DC bias resistance, SRL2, was chosen relatively small, 300 Ω, to avoid significant thermal noise generation. The PIN diodes were modeled as parallel RC networks, PRC1 and PRC2, with switching resistances RSW_L and RSW_H, in the low band and high band branches respectively. The appropriate biasing resistors are shown as shunt elements to ground, R2, R1 and R3, respectively. The truth table showing the values of RSW_L and RSW_H for the appropriate low/high switching is shown in Table 1. RSW_L RSW_H State 3Ω 3000 Ω Low band 3000 Ω 3Ω High band The Colpitts feedback capacitances, CDIV1 = 20 pF and CDIV2 = 15 pF, were optimized to provide a reasonable power response over the switching range. The NEC NE68519 transistor was selected for its high gain and low noise performance. The output is supplied from the emitter load resistance, RL1 through the 20 pF coupling capacitor, modeled as a series SLC1 component. Figure 2 shows the Libra Test Bench. In the test bench we define an open loop gain, Ku = VOut/VIn, as a ratio of voltage phasors at input and output ports of an OSCTEST component. Defining the oscillation point balances the input (loop) power to provide zero gain for a zero loop phase shift. Once the oscillation point is defined, the frequency and output power may be measured. We don't recommend using the OSCTEST2 component for the closed loop analysis, since it may not always converge and does not allow clear insight into the understanding of VCO behavior. This is considered an advantage of modeling over a purely experimental study. Figure 3 shows the Default Bench. The variables used for more convenient tuning during performance analysis and optimization are listed in a “variables and equations” component. SMV1142-011 and SMV1408-011 SPICE Models SPICE models for the SMV1142-011 and SMV1408-011 varactor diodes defined for the Libra IV environment, are shown in Figure 4 and Figure 5 with a description of the parameters employed. Table 1. Truth Table Capacitor X1 improves the low band matching of the tank circuit, insignificantly affecting high band performances of the tank, because of the high resistance of PRC1 in the low band state. This component may be removed for narrower frequency switching. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200317 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 3 APPLUICATION NOTE • APN1007 Figure 2. Libra Test Bench Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 4 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200317 Rev. A APPLUICATION NOTE • APN1007 Figure 3. Default Bench Figure 4. Spice Model for SMV1142-011 Figure 5. Spice Model for SMV1408-011 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200317 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 5 APPLUICATION NOTE • APN1007 Parameter Unit Default Saturation current (with N, determine the DC characteristics of the diode) A 1e–14 RS Series resistance Ω 0 N Emission coefficient (with IS, determines the DC characteristics of the diode) - 1 TT Transit time S 0 CJO Zero-bias junction capacitance (with VJ and M, defines nonlinear junction capacitance of the diode) F 0 VJ Junction potential (with VJ and M, (CJO and M) defines nonlinear junction capacitance of the diode) V 1 M Grading coefficient (with VJ and M, (CJO and VJ) defines nonlinear junction capacitance of the diode) - 0.5 IS Description EG 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 dependence of IS on temperature) - 3 KF Flicker noise coefficient - 0 AF Flicker noise exponent - 1 FC Forward-bias depletion capacitance coefficient - 0.5 BV Reverse breakdown voltage V Infinity IBV Current at reverse breakdown voltage A 1e-3 ISR Recombination current parameter A 0 NR Emission coefficient for ISR - 2 IKF High injection knee current A Infinity 1 NBV Reverse breakdown ideality factor - IBVL Low-level reverse breakdown knee current A 0 NBVL Low-level reverse breakdown ideality factor - 1 TNOM Nominal ambient temperature at which these model parameters were derived °C 27 FFE Flicker noise frequency exponent 1 Table 2. Model Parameters Table 2 describes the model parameters. It shows default values appropriate for silicon varactor diodes that may be used by the Libra IV simulator. According to the SPICE model in Figures 4 and 5, the varactor capacitance, CV, is a function of the applied reverse DC voltage, VR, and may be expressed as follows: CJO ( 1 + VVAR VJ ) M CJO (pF) SMV1142-011 SMV1408-011 + CP This equation is a mathematical expression of the capacitance characteristic. The model is accurate for abrupt junction varactors (like the SMV1408). The model is less accurate for hyperabrupt junction varactors because the coefficients are dependent on applied voltage. To make the above equation work better for the hyperabrupt varactors, the coefficients were optimized for the best capacitance vs. voltage fit as shown in Table 3 and Figure 6. Note: In the Libra model shown in Figure 6, CP is given in picofarads, while CGO is given in farads to comply with the default unit system used in Libra. M VJ (V) CP (pF) 13.4 1 2.2 0 0.7 1.8 21 25 68 0.13 0.6 1.8 LS (nH) 14 SMV1142-011 approximation 12 10 8 SMV1142-011 SMV1408-011 approximation 6 4 SMV1408-011 2 0 0 1 2 3 4 Varactor Voltage (V) Figure 6. Capacitance vs. Voltage Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 6 RS Ω) (Ω Table 3. Optimized Coefficients for Capacitance vs. Voltage Capacitance (pF) CV = Part Number July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200317 Rev. A 5 APPLUICATION NOTE • APN1007 VCO Design, Materials, Layout and Performance Figure 7 shows the VCO circuit diagram. J3 1 C4 100 pF R4 VSW_Low 1.5 k J4 D2 1 CCC SMV1139-011 VCC +3 V 100 pF C8 10 pF L2 56 nH P2 R6 SMP1320-011 3k R1 J1 300 L1 12 nH P1 8 pF Q1 NE68519 C9 100 pF C1 1 C6 SMP1320-011 VTUNE C10 100 pF 470 pF D1 C2 SMV1408-011 20 pF C3 R3 R7 1.5 k 6.8 k 20 pF J5 1 R5 C7 RF 100 15 pF C11 C5 100 pF 30 pF R2 1.5 k J2 1 VSW_High Figure 7. VCO Circuit Diagram Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200317 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 7 APPLUICATION NOTE • APN1007 Table 4 shows the bill of materials used. The PCB layout is shown in Figure 8. The board was made of standard, 30 mil thick, FR4 material. Designator Value Part Number Footprint Manufacturer C1 8 pF 0603AU8R0JAT9 0603 AVX C2 470 pF 0603AU471JAT9 0603 AVX C3 100 pF 0603AU101JAT9 0603 AVX C4 100 pF 0603AU101JAT9 0603 AVX C5 100 pF 0603AU101JAT9 0603 AVX C6 20 pF 0603AU200JAT9 0603 AVX C7 15 pF 0603AU150JAT9 0603 AVX C8 10 pF 0603AU100JAT9 0603 AVX C9 100 pF 0603AU101JAT9 0603 AVX C10 20 pF 0603AU200JAT9 0603 AVX AVX C11 30 pF 0603AU300JAT9 0603 CCC 100 pF 0603AU101JAT9 0603 AVX L1 12 nH LL1608-F12NS 0603 TOKO L2 56 nH LL1608-F56NS 0603 TOKO R1 300 CR10-301J-T 0603 AVX R2 1.5 k CR10-152J-T 0603 AVX R3 1.5 k CR10-152J-T 0603 AVX R4 1.5 k CR10-152J-T 0603 AVX R5 100 CR10-101J-T 0603 AVX R6 3k CR10-302J-T 0603 AVX R7 6.8 k CR10-682J-T 0603 AVX D1 SMV1408-011 SMV1408-011 SOD-323 Skyworks D2 SMV1142-011 SMV1142-011 SOD-323 Skyworks P1 SMP1320-011 SMP1320-011 SOD-323 Skyworks P2 SMP1320-011 SMP1320-011 SOD-323 Skyworks Q1 NE68519 NE68519 SOT-419 NEC Figures 9 and 10 show the measured performance of this circuit and the simulated results obtained with the model in Figure 8. In both low and high band states there is good compliance for frequency response. Some of the difference of the measured data and the simulation is probably due to the 5–10 percent variation of circuit capacitances and inductances from their nominal values. The low band simulated power response was in fair agreement with measured performance, assuming measurement uncertainty of ±1 dB. The power response difference of up to 4 dB for the high band may be due to underestimated circuit component losses. For example, a decrease of inductor Q-quality from 30 to 20 decreases output power about 1 dB; an increase in resistance of switching diode from 3 to 6 Ω will decrease power to about 2 dB. Table 4. Bill of Materials Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 8 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200317 Rev. A APPLUICATION NOTE • APN1007 25 mm 30 mm Figure 8. PCB Layout 450 190 Frequency 8 Frequency Measured points Power 2 150 0 140 -2 Frequency (MHz) Frequency (MHz) 4 4 430 2 420 Measured points 410 0 Power -2 400 Output Power (dBm) 170 Output Power (dBm) 6 160 6 440 180 -4 390 -6 380 130 0 0.5 1.0 1.5 2.0 2.5 3.0 Varactor Voltage (V) Figure 9. Low Band Measured and Simulated Results 0 0.5 1.0 1.5 2.0 2.5 3.0 Varactor Voltage (V) Figure 10. High Band Measured and Simulated Results Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200317 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 9 APPLUICATION NOTE • APN1007 Figure 11 shows the phase noise in the high band state vs. frequency offset. It shows better than -95 dBc/Hz at 10 kHz offset. This measurement was made using the PN9000 Phase Noise Test Set, courtesy of Aeroflex Comstron, Plainview, NY (www.aeroflex.com). Phase noise for both bands was measured with the HP8564E spectrum analyzer. At 10 kHz offset the noise was -95 dBc/Hz for both bands. It was expected that phase noise would be better at the low band. The poorer measured phase noise at the low band may be attributed to the spectrum analyzer method where the internal noise may be too close to the measurement level. It may also be related to the wide-bandwidth matching requirement of the VCO feedback circuit, which makes it difficult to satisfy noise optimums in both bands simultaneously. Our design compromise was to balance noise performance between both bands. In any event, -95 dBc/Hz is considered acceptable for digital cellular applications. Figure 11. High Band Phase Noise vs. Frequency List of Available Documents VCO Related Application Notes 1. HF Switchable VCO Simulation Project Files for Libra IV 1. Varactor SPICE Models for RF VCO Applications 2. HF Switchable VCO Circuit Schematic and PCB Layout for Protel EDA Client, 1998 Version 3. HF Switchable VCO PCB Gerber Photo-plot Files 2. A Colpitts VCO for Wideband (0.95–2.15 GHz) Set-Top TV Tuner Applications 3. A Balanced Wideband VCO for Set-top TV Tuner Applications © Skyworks Solutions, Inc., 1999. All rights reserved. (For the availability of the listed materials, please call our applications engineering staff.) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 10 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200317 Rev. A APPLUICATION NOTE • APN1007 Copyright © 2002, 2003, 2004, 2005, Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. 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Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 200317 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 11