APN1005

APPLICATION NOTE APN1005: A Balanced Wideband VCO for Set-Top TV Tuner Applications Introduction Modern set-top TV DBS tuner systems require more channel coverage, while maintaining competitive prices. This situation creates tough design goals: to improve performance and simplify design. Balanced VCO configuration could be a competitive circuit solution, since it provides the widest tuning range with practical circuitry and layout. However, tuning margins would be further improved by optimizing the varactor manufacturing process. Skyworks has developed such a process to satisfy the most ambitious wideband design goals. In this publication, we will address the design of the balancedtype voltage control oscillator (VCO) based on the newly developed varactor SMV1265-011 with the unique set of capacitance tuning ratios and Q-quality. VCO Model Figure 1 shows the VCO model built for open loop analysis in Libra Series IV including the SMV1265-011 varactor model. The circuit schematic in Figure 2 shows a pair of transistors in a single feedback loop, connected so that collector currents would be 180° shifted (ideally). A pair of back-to-back connected SMV1265-011 varactors is used, rather than a single one. This allows lower capacitance at the high-voltage range, without changing the tuning ratio. The reason is that, apart from package capacitance, certain mounting fringing capacitances, though small, may strongly affect higher frequency margins. The effects of parasitic capacitances were summarized in the model as C4 and C3, valued 0.4 pF each. These values may vary depending on the layout of the board. Varactor DC biasing is provided through resistors R6 and R8, both 1 k, which may affect the phase noise, but eliminate the need for inductive chokes. This minimizes overall costs and the possibility of parasitic resonances — the usual cause for frequency instability and spurs. The phase corrector DC chokes, SRL1 and SRL2, were modeled as lossless inductors at 33 nH since their losses are dominated by the 30 Ω emitter biasing resistors. DC blocking series capacitances (CSER1 and CSER2) are modeled as an SRC network, including associated parasitics. Their values were optimized to 10 pF providing smooth tuning over the design band. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 1 APPLICATION NOTE • APN1005 Figure 1. VCO Model, Including SMV1265-011 Varactor Model Figure 2. Transistor Pair in Single Feedback Loop Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 2 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200314 Rev. A APPLICATION NOTE • APN1005 The pseudo-resonator inductance is formed by microstrip transmission line TL2, which provides necessary circuit response at high frequencies. This has little effect at the lower band due to the resonator’s dominantly capacitive nature. The function of transmission line TL1 is both feedback and phase alignment — providing flat power response over the tuning range. Power output is supplied from the collectors of X1 or X2 through the series connected resistance and DC blocking capacitance SRC2 and SRC3. DC biasing for both of the transistors is supplied through a resistive divider R1/R3/R2 . The NEC NE68119 bipolar transistors were selected to best fit performances. Note: The circuit is very sensitive to the transistor choice (in terms of tuning range and stability) due to wide bandwidth design requirements. For the model of NEC NE68119 we used the Gumel Poon model of Libra IV with the coefficients provided by CEL RF & Microwave Semiconductors Catalogue,1997-98. In the Libra test bench shown in Figure 3 we defined an open loop gain (Ku = VOUT/VIN) as the ratio of voltage phasors at the input and output ports of an OSCTEST component. Defining the oscillation point requires the balancing of input (loop) power to provide zero gain for a zero loop phase shift. Once the oscillation point is defined, the frequency and output power can be measured. Use of the OSCTEST2 component for the close loop analysis is not recommended, since it may fail to converge in some cases, and doesn’t allow clear insight into the understanding of VCO behavior. This property is considered an advantage of modeling over a purely experimental study. In the default bench shown in Figure 4 the variables used for more convenient tuning during performance analysis and optimization are listed in a “variables and equations” component. Figure 3. Libra Test Bench Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 3 APPLICATION NOTE • APN1005 Figure 4. Default Bench SMV1265-011 SPICE Model Figure 5 shows a SPICE model for the SMV1265-011 varactor diode, defined for the Libra IV environment, with a description of the parameters employed. Figure 5. SMV1265-011 Libra IV SPICE Model Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 4 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200314 Rev. A APPLICATION NOTE • APN1005 Parameter IS RS N TT CJO VJ M EG XTI KF AF FC BV IBV ISR NR IKF NBV IBVL NBVL TNOM FFE Series resistance Description Saturation current (with N, determine the DC characteristics of the diode) Emission coefficient (with IS, determines the DC characteristics of the diode) Transit time Zero-bias junction capacitance (with VJ and M, defines nonlinear junction capacitance of the diode) Junction potential (with VJ and M, defines nonlinear junction capacitance of the diode) Grading coefficient (with VJ and M, defines nonlinear junction capacitance of the diode) Energy gap (with XTI, helps define the dependence of IS on temperature) Saturation current temperature exponent (with EG, helps define the dependence of IS on temperature) Flicker noise coefficient Flicker noise exponent Forward-bias depletion capacitance coefficient Reverse breakdown voltage Current at reverse breakdown voltage Recombination current parameter Emission coefficient for ISR High injection knee current Reverse breakdown ideality factor Low-level reverse breakdown knee current Low-level reverse breakdown ideality factor Nominal ambient temperature at which these model parameters were derived Flicker noise frequency exponent Unit A Ω S F V EV V A A A A °C Default 1e-14 0 1 0 0 1 0.5 1.11 3 0 1 0.5 Infinity 1e-3 0 2 Infinity 1 0 1 27 1 Table 1. Silicon Varactor Diode Default Values Table 1 describes the model parameters. It shows default values appropriate for silicon varactor diodes which may be used by the Libra IV simulator. According to the SPICE model in Figure 4, the varactor capacitance (CV) is a function of the applied reverse DC voltage (VR) and may be expressed as follows: CV = CJO M whole range of the usable varactor voltages is segmented into a number of subranges each with a unique set of the VJ, M, CJO, and CP parameters as given in the Table 2. Voltage Range (V) 0–2.5 2.5–6.5 6.5–11 11–up CJO (pF) 22.5 21.0 20.0 20.0 M 2.0 25.0 7.3 1.8 VJ (V) 4.00 68.00 14.00 1.85 CP (pF) 0.00 0.00 0.90 0.56 + CP ( 1 + VR ) VJ This equation is a mathematical expression of the capacitance characteristic. The model is accurate for abrupt junction varactors (SMV1400 series); however, the model is less accurate for hyperabrupt junction varactors because the coefficients are dependent on the applied voltage. To make the equation fit the hyperabrupt performances for the SMV1265-011, a piece-wise approach was employed. Here the coefficients (VJ, M, CJO, and CP) are made piece-wise functions of the varactor DC voltage applied. Thus, the Table 2. Varactor Voltages These subranges are made to overlap each other. Thus, if a reasonable RF swing (one that is appropriate in a practical VCO case) exceeds limits of the subrange, the CV function described by the current subrange will still fit in the original curve. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 5 APPLICATION NOTE • APN1005 100 Approximation Measured 1.0 0.8 2.6 2.4 Series Resistance (Ω) Capacitance (pF) 10 0.6 0.4 1 0.2 0.1 0 5 10 15 20 25 30 0 2.2 2.0 1.8 1.6 1.4 RS Measured 1.2 0 5 10 15 20 25 30 RS_PWL Varactor Voltage (V) Figure 6. SMV1265 Capacitance vs. Voltage Varactor Voltage (V) Figure 7. SMV1265 Resistance vs. Voltage Figure 6 demonstrates the quality of the piece-wise fitting approach. Special consideration was given to the fit at the lowest capacitance range (high-voltage area) since it dramatically affects the upper frequency limit of the VCO. To incorporate this function into Libra, the pwl() built-in function was used in the “variables” component of the schematic bench. M = pwl (VVAR 0 2 2.5 2 2.500009 25 6.5 25 6.50009 7.3 11 7.3 11.0009 1.8 40 1.8) VJ = pwl (VVAR 0 4 2.5 4 2.500009 68 6.5 68 6.50009 14 11 14 11.0009 1.85 40 1.85) CP = pwl (VVAR 0 0 2.5 0 2.500009 0 6.5 0 6.50009 0.9 11 0.9 11.0009 0.56 40 0.56) CJO = pwl (VVAR 0 22.5 2.5 22.5 2.500009 21 6.5 21 6.50009 20 11 20 11.0009 20 40 20)*1012 Note: While CP is given in picofarads, CGO is given in farads to comply with the default nominations in Libra. (For more details regarding pwl() function see Circuit Network Items, Variables and Equations, Series IV Manuals, p. 19–15). Since the epitaxial layer for this kind of hyperabrupt varactor has relatively high resistivity, the series resistance is strongly dependent on the reverse voltage applied to varactor junction. The value of series resistance (RS) measured at 500 MHz is shown in Figure 7, with a piece-wise approximation of RS also given. The piece-wise function may be used as follows: RS = pwl (VVAR 0 2.4 3 2.4 4 2.3 5 2.2 6 2 7 1.85 8 1.76 9 1.7 10 1.65 11 1.61 12 1.5 40 1.5) Note: The pwl() function in Libra IV is defined for the evaluation of harmonic balance parameters rather than variables. Therefore, although series resistance was defined as dependent on reverse voltage, for harmonic balance it remains parametric and linear. The same applies to capacitance, which remains the same as in the original diode model, but its coefficients (VJ, M, CJO, and CP) become parametric functions of the reverse voltage. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 6 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200314 Rev. A APPLICATION NOTE • APN1005 VCO Design and Performance Figure 8 shows the VCO schematic. C6 1000 VCC1 5-8 V R5 820 C5 100 R12 50 A R3 120 R8 1k R10 2.4k R6 820 R4 120 R7 51 C4 100 V2 NE68119 L1 33 nH C1 10 T1 16 x 0.4 mm R1 R2 L2 33 33 33 nH V1 NE68119 C2 10 T2 15 x 0.7 mm D2 T3 SMV1265 3 X 0.7 mm D1 SMV1265 R11 1000 R9 1k C3 100 VVAR1 Figure 8. VCO Schematic Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 7 APPLICATION NOTE • APN1005 Table 3 shows the bill of materials used. Designators C1 C2 C3 C4 C5 C6 C6 D1 D2 L1 L2 R1 R10 R11 R2 R3 R4 R5 R6 R7 R8 R9 V1 V2 Comment 0603AU100JAT9 (AVX) 0603AU100JAT9 (AVX) 0603AU101JAT9 (AVX) 0603AU101JAT9 (AVX) 0603AU101JAT9 (AVX) 0603AU102JAT9 (AVX) 0603AU102JAT9 (AVX) SMV1265-011 (Skyworks) SMV1265-011 (Skyworks) LL1608-F33NJ (TOKO) LL1608-F33NJ (TOKO) CR10-330J-T (AVX) CR10-242J-T (AVX) CR10-102J-T (AVX) CR10-330J-T (AVX) CR10-121J-T (AVX) CR10-121J-T (AVX) CR10-821J-T (AVX) CR10-821J-T (AVX) CR10-510J-T (AVX) CR10-102J-T (AVX) CR10-102J-T (AVX) NE68119 (NEC) NE68119 (NEC) Footprint 0603 0603 0603 0603 0603 0603 0603 SOD-323 SOD-323 0603 0603 0603 0603 0603 0603 0603 0603 0603 0603 0603 0603 0603 SOT-416 SOT-416 Figure 9 shows the PCB layout. The board is made of standard FR4 material 30 mils thick. The results measured with the circuit in Figure 8, as well as the simulated results obtained with the model in Figure 9, are shown in Figures 10 and 11. Note: The simulated tuning curve in Figure 10 agrees with measured data, which proves the effectiveness of the above piece-wise approximation technique. Note: In the middle of the tuning range there is disagreement between our model and the measured results. This could be attributed to the imperfection of the model, which is highly sensitive to the way different parasitic effects are treated. The other problem of modeling this oscillator case was the convergence of the harmonic balance. To facilitate convergence in this case, we kept the number of harmonics to at least five. The sweeping frequency range is recommended to keep as close to the oscillation point as possible — especially when analyzing the middle band area. In Figure 11, the power response modeled at 7 V was very close to the measurement. Higher measured power is attributed to the analyzer calibration (the calibration error of the analyzer is known to be within a couple of decibels). The general trend of the simulated results reflects the real VCO response almost exactly, which clearly demonstrates the model’s effectiveness. Table 3. Bill of Materials Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 8 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200314 Rev. A APPLICATION NOTE • APN1005 Figure 9. PCB Layout 8 2.2 2.0 Measured 6 4 Measured @ 7 V Frequency (GHz) 1.8 1.6 1.4 1.2 1.0 0.8 0 5 10 Power (dBm) Simulations 2 0 -2 -4 -6 -8 0 5 10 Simulated @ 7 V Measured @ 5 V 15 20 25 30 15 20 25 30 Varactor Voltage (V) Figure 10. Frequency Tuning Varactor Voltage (V) Figure 11. Power Response Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 9 APPLICATION NOTE • APN1005 Table 4 shows the measurement data and shows a useful tuning range of 0.84–2.23 GHz for the applied varactor voltage from 1–27 V. List of Available Documents 1. Balanced Wideband VCO Simulation Project Files for Libra IV. 2. Balanced Wideband VCO Circuit Schematic and PCB Layout for Protel EDA Client, 1998 version. 3. Balanced Wideband VCO Gerber Photo-plot Files 4. Detailed measurement and simulation data. For the availability of the listed materials, please call our applications engineering staff. © Skyworks Solutions, Inc., 1999. All rights reserved. VVAR (V) 0 1 2 4 6 8 10 12 14 16 18 20 25 28 30 Frequency (GHz) 0.788 0.842 0.91 1.144 1.492 1.714 1.848 1.946 2.016 2.066 2.106 2.134 2.198 2.225 2.238 POUT @ 7 V (dBm) 3.5 3.7 3.7 4.8 6.5 6.4 6 5.2 4.8 4.4 4.3 4.4 3.7 3.5 3.4 POUT @ 5 V (dBm) -8.3 -7.6 -6.1 -2.8 1 1.8 1.2 -0.1 -0.9 -1.5 -1.8 -2.4 -3.3 -3.7 -4 Table 4. Tabulated Measurement Data Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com 10 July 21, 2005 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200314 Rev. A APPLICATION NOTE • APN1005 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 • sales@skyworksinc.com • www.skyworksinc.com 200314 Rev. A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • July 21, 2005 11