A Colpitts VCO for Wideband (0.95–2.15 GHz)Set-Top TV Tuner Applications
APN1006
Introduction
Modern t-top DBS TV tuners require high performance,broadband voltage control oscillator (VCO) designs at a competitive cost.T o meet the goals, design engineers are challenged to create high performance, low-cost VCOs.
The Colpitts oscillator is a traditional design ud for many VCO applications.Designing a broadband Colpitts oscillator with coverage from 1–2 GHz requires the lection and interaction of an appropriate varactor diode for its resonator.This application note describes the design
of a broadband Colpitts VCO that incorporates the SMV1265-011 varactor diode.This varactor diode was specifically developed at Alpha for this application.The VCO design, bad on Libra Series IV simulation, shows good correlation 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.
单雄信
VCO Model Built for Open Loop Analysis in Libra Series IV山莴苣
Application Note
This circuit schematic, which is a simple Colpitts structure, us a ries back-to-back connection of two SMV1265-011 varactors instead of a single varactor.This connection allows lower capacitance 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 capacitances.The 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 1and R 4, both 3 k Ω, which may affect pha noi, but allows the exclusion of chokes.This reduces costs and the possibility of parasitic resonances — the common cau of spurious respons 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 ries capacitance (C SER ) was modeled as a SRC network, including associated par
asitics;it was lected 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 respon over the tuning range.The values may also
be re-optimized for pha noi if required.
The NEC NE68533 transistor was lected to fit the required bandwidth performance.Note:The circuit is very nsitive 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, modeled as a ries SLC 1component.
The microstrip line (TL 1) simulates the design layout which may be incorporated in the resonator.
个人特长有哪些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 pha shift.Once the oscillation point is defined, the frequency and output power can be measured.U of the OSCTEST2 component for the clo loop analysis is not recommended, since it may not converge in some cas, and doesn’t allow clear insight into understanding the VCO behavior.The properties are considered an advantage of modeling over a purely experimental study.
Figure 3 shows the default bench.The variables ud for more convenient tuning during performance analysis and optimization are listed in a “variables and equations”
英语自我介绍简短component.
SMV1265-011 SPICE Model
Figure 4 shows a SPICE model for the SMV1265-011varactor diode, defined for the Libra IV environment, with a description of the parameters employed.
足付Voltage Range
C JO M V J C P (V)
(pF)(V)(pF)0–2.522.5 2.0 4.000.002.5–6.521.025.068.000.006.5–1120.07.314.000.9011–up
20.0
1.8
1.85
0.56
usable varactor voltages is gmented into a number of subranges each with a unique t of the V J , M, C JO , and C P parameters as given in the T able 2.
Parameter
Description
不言而喻
Unit Default IS Saturation current (with N, determine the DC characteristics of the diode)A 1e-14R S Series resistance
Ω0N Emission coefficient (with IS, determines the DC characteristics of the diode)-1TT Transit time
S 0C JO Zero-bias junction capacitance (with V J and M, defines nonlinear junction capacitance of the diode)F 0V J Junction potential (with V J and M, defines nonlinear junction capacitance of the diode)V 1M Grading coefficient (with V J and M, defines nonlinear junction capacitance of the diode)-0.5E G Energy gap (with XTI, helps define the dependence of IS on temperature)
EV 1.11XTI Saturation current temperature exponent (with E G , helps define the dependence of IS on temperature)-3KF Flicker noi coefficient -0AF Flicker noi exponent
-1FC Forward-bias depletion capacitance coefficient -0.5B V Rever breakdown voltage
V Infinity I BV Current at rever breakdown voltage A 1e-3ISR Recombination current parameter A 0NR Emission coefficient for ISR -2IKF High-injection knee current A Infinity NBV Rever breakdown ideality factor -1IBVL Low-level rever breakdown knee current A 0NBVL Low-level rever breakdown ideality factor
-
1T NOM Nominal ambient temperature at which the model parameters were derived °C
27FFE
Flicker noi frequency exponent
1
T able 1 describes the model parameters.It shows default values appropriate for silicon varactor diodes which may be ud by the Libra IV simulator.
According to the SPICE model in Figure 4, the varactor capacitance (C V ) is a function of the applied rever DC voltage (V R ) and may be expresd as follows:
用典的作用This equation is a mathematical expression of the capacitance characteristic.The model is accurate for abrupt junction varactors (Alpha’s SMV1400 ries);however, the model is less accurate for hyperabrupt junction varactors becau the coefficients are dependent on the applied voltage.T o make the equation fit the hyperabrupt performances for the SMV1265-011 a piece-wi approach was employed.Here the coefficients (V J , M, C JO , and C P ) are made piece-wi functions of the varactor DC voltage applied.Thus, the whole range of the
The subranges are made to overlap each other.Thus,if a reasonable RF swing (one that is appropriate in a practical VCO ca) 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
()
Figure 5 demonstrates the quality of the piece-wi 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 ud in the “variables”component of the schematic bench.
M = pwl (V VAR 0 2 2.5 2 2.500009 25 6.5 256.50009 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 6.5 686.50009 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 06.50009 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 6.521 6.50009 20 11 20 11.0009 20 40 20)*1012
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 e 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 ries resistance is strongly dependent on the rever voltage applied to varactor junction.The value of ri
es resistance (R S )measured at 500 MHz is shown in Figure 6, with a piece-wi approximation of R S also given.
The piece-wi function may be ud as follows:
R S = pwl (V VAR 0 2.4 3 2.4 4 2.3 5 2.2 6 2 7 1.858 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 ries resistance was defined as dependent on rever 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 rever voltage.
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