skyworks solutions, inc. ? phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com 200316 rev. a skyworks proprietary information products and product information are subject to change without notice. july 21, 2005 1 APN1006: a colpitts vco for wideband (0.95?2.15 ghz) set-top tv tuner applications application note introduction modern set-top dbs tv tuners require high-performance, broad- band voltage control oscillator (vco) designs at a competitive cost. to meet these goals, design engineers are challenged to create high-performance, low-cost vcos. the colpitts oscillator is a traditional design used for many vco applications. designing a broadband colpitts oscillator with cov- erage from 1?2 ghz requires the selection and interaction of an appropriate varactor diode for its resonator. this application note describes the design of a broadband colpitts vco that incorpo- rates the smv1265-011 varactor diode. this varactor diode was specifically developed at skyworks for this application. the vco design, based on libra series iv simulation, shows good correla- tion between measured and simulated performance. this application note includes a board layout and materials list. vco model figure 1 shows the vco model built for open loop analysis in libra series iv. figure 1. vco model built for open loop analysis in libra series iv
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. 200316 rev. a application note ? APN1006 2 this circuit schematic, which is a simple colpitts structure, uses a series back-to-back connection of two smv1265-011 varactors instead of a single varactor. this connection allows lower capaci- tance at high voltages, while maintaining the tuning ratio of a single varactor. the back-to-back varactor connection also helps reduce distortion and the effect of fringing and mounting capaci- tances. these parasitic capacitances are included in the model as c 5 , valued at 0.6 pf. this value may change depending on the layout of the board. dc bias is provided through resistors r 1 and r 4 , both 3 k ? , which may affect phase noise, but allows the exclusion of chokes. this reduces costs and the possibility of parasitic resonances ? the common cause of spurious responses and frequency instability. the resonator inductance was modeled as a lossy inductor (with q = 25 at 100 mhz) in parallel with a capacitance of 0.25 pf. this is typical for a multilayer inductor of style 0603 (60 x 30 mil) footprint (toko coils and filters catalog). the inductor value of 5.6 nh was optimized to fit the desired 1?2 ghz frequency band. the dc blocking series capacitance (c ser ) was modeled as an src network, including associated parasitics; it was selected at 1000 pf to avoid affecting the resonator (q). the colpitts feedback capacitances (c div1 = 1 pf and c div2 = 1.62 pf) were optimized to provide a flat power response over the tuning range. these values may also be re-optimized for phase noise if required. the nec ne68533 transistor was selected to fit the required bandwidth performance. note: the circuit is very sensitive to the transistor choice (tuning range and stability) due to the wide bandwidth requirement. the output is supplied from the emitter load resistance (rl 1 ) through the 2 pf coupling capacitor, mod- eled as a series slc 1 component. the microstrip line (tl 1 ) simulates the design layout which may be incorporated in the resonator. figure 2 shows the libra test bench. in the test bench, we define an open loop gain (ku = v out /v in ) as a ratio of voltage phasors at 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 not converge in some cases, and doesn?t allow clear insight into understanding the vco behavior. these properties are considered an advantage of mod- eling over a purely experimental study. figure 2. libra test bench
application note APN1006 skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com 200316 rev. a skyworks proprietary information products and product information are subject to change without notice. july 21, 2005 3 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. figure 3. default test bench figure 4. smv1265-011 libra iv spice model smv1265-011 spice model figure 4 shows a spice model for the smv1265-011 varactor diode, defined for the libra iv environment, with a description of the parameters employed.
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. 200316 rev. a application note APN1006 4 voltage range c jo mv j c p (v) (pf) (v) (pf) 0?2.5 22.5 2 4 0 2.5?6.5 21 25 68 0 6.5?11 20 7.3 14 0.9 11?up 20 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 a 0 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 ()
application note APN1006 skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com 200316 rev. a skyworks proprietary information products and product information are subject to change without notice. july 21, 2005 5 figure 5 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 6, 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 5. smv1265 capacitance vs. voltage figure 6. smv1265 resistance vs. voltage
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. 200316 rev. a application note APN1006 6 figure 8 shows the pcb layout. the board is made of standard fr4 material 60 mils thick. table 3 shows the bill of materials used. table 3. bill of materials designator part type footprint c 1 0603au561jat9 (avx) 0603 c 2 0603au2r0jat9 (avx) 0603 c 3 0603au561jat9 (avx) 0603 c 4 0603au201jat9 (avx) 0603 c 5 0603au1r0jat9 (avx) 0603 c 6 0603au1r6jat9 (avx) 0603 d 1 ne68519 (nec) sot-419 l 1 ll1608-f5n6s (toko) 0603 r 1 cr10-332j-t (avx) 0603 r 2 cr10-912j-t (avx) 0603 r 3 cr10-201j-t (avx) 0603 r 4 cr10-302j-t (avx) 0603 r 5 cr10-302j-t (avx) 0603 v 1 smv1265-011 (skyworks) sod-323 v 2 smv1265-011 ( skyworks) sod-323 3 k 320 x 30 mils 300 p 5.6 nh smv1265-011 smv1265-011 560 p 3 k 9.1 k 3.3 k v cc = 5 v i cc = 9 ma ne68519 1 p 1.62 p 200 2 p rf output v tune figure 7. vco circuit diagram vco design materials, layout, and performance figure 7 shows the vco circuit diagram. 720 mil 720 mil figure 8. pcb layout
application note APN1006 skyworks solutions, inc. phone [781] 376-3000 fax [781] 376-3100 sales@skyworksinc.com www.skyworksinc.com 200316 rev. a skyworks proprietary information products and product information are subject to change without notice. july 21, 2005 7 table 4 shows tabulated measurement data. in voltage ranges of 1?27 v, the usable frequency coverage was estimated from 0.98?2.15 ghz. 030 25 20 15 10 5 varactor voltage (v) 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 frequency (ghz) 0 1 2 3 4 5 6 7 p out (dbm) f exp f model p out_exp p out_model figure 9. measured and simulated frequency vs. varactor voltage table 4. tabulated measurement data figure 9 shows both the measured performance of this circuit and the simulated results, obtained with the above model. the simu- lated tuning curve (frequency vs. voltage) is in excellent agreement with measured data, proving the effectiveness of the piece-wise approximation technique. the measured power response, shows some differences from its simulation, but is within the same range. a possible reason for the discrepancy could be the effect of higher harmonics. to simulate this would require significantly more com- plicated modeling of the components, board parasitics, and discontinuities. however, for most engineering purposes, the circuit performance prediction indicated here should be satisfactory. v var frequency p out (v) (ghz) (dbm) 0.5 0.95 5.7 1 0.974 5.5 2 1.018 5.4 4 1.184 4.7 8 1.68 3.2 12 1.886 5.2 14 1.932 4.9 18 2.008 5 22 2.076 3.9 25 2.12 3.5 30 2.188 2.2 list of available documents 1. colpitts wideband vco simulation project files for libra iv. 2. colpitts wideband vco circuit schematic and pcb layout for protel eda client, 1998 version. 3. colpitts wideband vco gerber photo-plot files 4. a colpitts vco for wideband (0.95?2.15 ghz) set-top tv tuner applications. (current document). 5. 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.
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. 200316 rev. a application note APN1006 8 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.
|
