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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 1 APN1005: a balanced wideband vco for set-top tv tuner applications application note 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 balanced- type 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 c 4 and c 3 , valued 0.4 pf each. these values may vary depending on the layout of the board. varactor dc biasing is provided through resistors r 6 and r 8 , 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, srl 1 and srl 2 , were modeled as lossless inductors at 33 nh since their losses are dominated by the 30 ? emitter biasing resistors. dc blocking series capaci- tances (c ser1 and c ser2 ) 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 july 21, 2005 skyworks proprietary information products and product information are subject to change without notice. 200314 rev. a application note ? APN1005 2 figure 1. vco model, including smv1265-011 varactor model figure 2. transistor pair in single feedback loop
application note APN1005 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 the pseudo-resonator inductance is formed by microstrip trans- mission line tl 2 , 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 tl 1 is both feedback and phase alignment ? providing flat power response over the tuning range. power output is supplied from the collectors of x 1 or x 2 through the series connected resistance and dc blocking capacitance src 2 and src 3 . dc biasing for both of the transistors is supplied through a resis- tive divider r 1 /r 3 /r 2 . 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 band- width 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. figure 3. libra test bench in the libra test bench shown in figure 3 we defined an open loop gain (ku = v out /v in ) 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 mea- sured. 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 under- standing 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 opti- mization are listed in a ?variables and equations? component.
skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com july 21, 2005 skyworks proprietary information products and product information are subject to change without notice. 200314 rev. a application note APN1005 4 figure 5. smv1265-011 libra iv spice model 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 4. default bench
application note APN1005 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 voltage range c jo v j c p (v) (pf) m (v) (pf) 0?2.5 22.5 2.0 4.00 0.00 2.5?6.5 21.0 25.0 68.00 0.00 6.5?11 20.0 7.3 14.00 0.90 11?up 20.0 1.8 1.85 0.56 whole range of the usable varactor voltages is segmented into a number of subranges each with a unique set of the v j ,m,c jo , and c p parameters as given in the table 2. parameter description unit default is saturation current (with n, determine the dc characteristics of the diode) a 1e-14 r s series resistance ? 0 n emission coefficient (with is, determines the dc characteristics of the diode) - 1 tt transit time s0 c jo zero-bias junction capacitance (with v j and m, defines nonlinear junction capacitance of the diode) f 0 v j junction potential (with v j and m, defines nonlinear junction capacitance of the diode) v 1 m grading coefficient (with v j and m, defines nonlinear junction capacitance of the diode) - 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 e g , 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 b v reverse breakdown voltage v infinity i bv current at reverse breakdown voltage a 1e-3 isr recombination current parameter a0 nr emission coefficient for isr -2 ikf high injection knee current a infinity nbv reverse breakdown ideality factor -1 ibvl low-level reverse breakdown knee current a0 nbvl low-level reverse breakdown ideality factor - 1 t nom nominal ambient temperature at which these model parameters were derived c 27 ffe flicker noise frequency exponent 1 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 capaci- tance (c v ) is a function of the applied reverse dc voltage (v r ) and may be expressed as follows: 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 hyper- abrupt 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 (v j ,m,c jo , and c p ) are made piece-wise functions of the varactor dc voltage applied. thus, the these subranges are made to overlap each other. thus, if a rea- sonable rf swing (one that is appropriate in a practical vco case) exceeds limits of the subrange, the c v function described by the current subrange will still fit in the original curve. table 1. silicon varactor diode default values table 2. varactor voltages c v =+ c p 1 + c jo m v r v j ()
skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com july 21, 2005 skyworks proprietary information products and product information are subject to change without notice. 200314 rev. a application note APN1005 6 figure 6 demonstrates the quality of the piece-wise fitting approach. special consideration was given to the fit at the lowest capaci- tance 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 (v var 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) v j = pwl (v var 0 4 2.5 4 2.500009 68 6.5 68 6.50009 14 11 14 11.0009 1.85 40 1.85) c p = pwl (v var 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) c jo = pwl (v var 0 22.5 2.5 22.5 2.500009 21 6.5 21 6.50009 20 11 20 11.0009 20 40 20)*10 12 note: while c p is given in picofarads, c go 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). 0.1 0 5 10 15 20 25 30 1 10 capacitance (pf) 100 0 0.2 0.6 1.0 0.4 0.8 varactor voltage (v) approximation measured 030 25 20 15 10 5 varactor voltage (v) 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 series resistance ( ? ) r s_pwl r s measured since the epitaxial layer for this kind of hyperabrupt varactor has relatively high resistivity, the series resistance is strongly depen- dent on the reverse voltage applied to varactor junction. the value of series resistance (r s ) measured at 500 mhz is shown in figure 7, with a piece-wise approximation of r s also given. the piece-wise function may be used as follows: r s = pwl (v var 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 (v j ,m,c jo , and c p ) become parametric functions of the reverse voltage. figure 6. smv1265 capacitance vs. voltage figure 7. smv1265 resistance vs. voltage
application note APN1005 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 vco design and performance figure 8 shows the vco schematic. v 1 ne68119 v 2 ne68119 t1 16 x 0.4 mm t2 15 x 0.7 mm c 1 10 c 2 10 l 1 33 nh l 2 33 nh r 1 33 r 2 33 r 3 120 r 4 120 r 5 820 r 7 51 r 8 1k r 6 820 r 10 2.4k d 1 r 9 1k r 11 1000 c 3 100 v var1 v cc1 5-8 v c 4 100 c 5 100 r 12 50 a d 2 smv1265 t3 3 x 0.7 mm smv1265 c 6 1000 figure 8. vco schematic
skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com july 21, 2005 skyworks proprietary information products and product information are subject to change without notice. 200314 rev. a application note APN1005 8 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 mea- sured 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 attrib- uted 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 rec- ommended 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 ana- lyzer 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 shows the bill of materials used. designators comment footprint c 1 0603au100jat9 (avx) 0603 c 2 0603au100jat9 (avx) 0603 c 3 0603au101jat9 (avx) 0603 c 4 0603au101jat9 (avx) 0603 c 5 0603au101jat9 (avx) 0603 c 6 0603au102jat9 (avx) 0603 c 6 0603au102jat9 (avx) 0603 d 1 smv1265-011 (skyworks) sod-323 d 2 smv1265-011 (skyworks) sod-323 l 1 ll1608-f33nj (toko) 0603 l 2 ll1608-f33nj (toko) 0603 r 1 cr10-330j-t (avx) 0603 r 10 cr10-242j-t (avx) 0603 r 11 cr10-102j-t (avx) 0603 r 2 cr10-330j-t (avx) 0603 r 3 cr10-121j-t (avx) 0603 r 4 cr10-121j-t (avx) 0603 r 5 cr10-821j-t (avx) 0603 r 6 cr10-821j-t (avx) 0603 r 7 cr10-510j-t (avx) 0603 r 8 cr10-102j-t (avx) 0603 r 9 cr10-102j-t (avx) 0603 v 1 ne68119 (nec) sot-416 v 2 ne68119 (nec) sot-416 table 3. bill of materials
application note APN1005 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 varactor voltage (v) 030 25 20 15 10 5 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 frequency (ghz) simulations measured 030 25 20 15 10 5 varactor voltage (v) -8 -6 -4 -2 0 2 4 6 8 power (dbm) measured @ 7 v measured @ 5 v simulated @ 7 v figure 10. frequency tuning figure 11. power response figure 9. pcb layout
skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com july 21, 2005 skyworks proprietary information products and product information are subject to change without notice. 200314 rev. a application note APN1005 10 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 applica- tions engineering staff. ? skyworks solutions, inc., 1999. all rights reserved. v var frequency p out @ 7 v p out @ 5 v (v) (ghz) (dbm) (dbm) 0 0.788 3.5 -8.3 1 0.842 3.7 -7.6 2 0.91 3.7 -6.1 4 1.144 4.8 -2.8 6 1.492 6.5 1 8 1.714 6.4 1.8 10 1.848 6 1.2 12 1.946 5.2 -0.1 14 2.016 4.8 -0.9 16 2.066 4.4 -1.5 18 2.106 4.3 -1.8 20 2.134 4.4 -2.4 25 2.198 3.7 -3.3 28 2.225 3.5 -3.7 30 2.238 3.4 -4 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. table 4. tabulated measurement data
application note APN1005 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 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. these mate rials, including the information contained herein, are provided by skyworks as a service to its customers and may be used for informational purposes only by the customer. skyworks assumes no r esponsibility for errors or omissions in these materials or the information contained herein. skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. skyworks makes no commitment to update the materials or information and shall have no responsibility whatsoever for conflicts, incompatibilities, or other diffic ulties arising from any future changes. no license, whether express, implied, by estoppel or otherwise, is granted to any intellectual property rights by this document. sky works assumes no liability for any materials, products or information provided hereunder, including the sale, distribution, reproduction or use of skyworks products, information or material s, except as may be provided in skyworks terms and conditions of sale. the materials, products and information are provided ?as is? without warranty of any kind, whether express, implied, statutory, or ot herwise, including fitness for a particular purpose or use, merchantability, performance, quality or non-infringement of any intellectual property right; all such warranties are hereby expressly disclaimed. skyworks does not warrant the accuracy or completeness of the information, text, graphics or other items contained wit hin these materials. skyworks shall not be liable for any damages, including but not limited to any special, indirect, incidental, statutory, or consequential dam ages, including without limitation, lost revenues or lost profits that may result from the use of the materials or information, whether or not the recipient of mate rials has been advised of the possibility of such damage. skyworks products are not intended for use in medical, lifesaving or life-sustaining applications, or other equipment in which th e failure of the skyworks products could lead to personal injury, death, physical or environmental damage. skyworks customers using or selling skyworks products for use in such applications do so at their own risk and agree to fully indemnify skyworks for any damages resulting from such improper use or sale. customers are responsible for their products and applications using skyworks products, which may deviate from published specific ations as a result of design defects, errors, or operation of products outside of published parameters or design specifications. customers should include design and operating safeguards to m inimize these and other risks. skyworks assumes no liability for applications assistance, customer product design, or damage to any equipment resulting from the use of skyworks products outside of stated published specifications or parameters. skyworks, the skyworks symbol, and ?breakthrough simplicity? are trademarks or registered trademarks of skyworks solutions, inc., in the united states and other countries. third-party brands and names are for identification purposes only, and are the property of their respective owners. additional information, including rel evant terms and conditions, posted at www.skyworksinc.com, are incorporated by reference.

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