200316A.pdf

APPLICATION NOTE
APN1006: A Colpitts VCO for Wideband
(0.95–2.15 GHz) Set-Top TV Tuner Applications
Introduction
Modern set-top DBS TV tuners require high-performance, broadband 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 coverage 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 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. VCO Model Built for Open Loop Analysis in Libra Series IV
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APPLICATION NOTE • APN1006
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 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. These parasitic capacitances are included in the model as
C5, valued at 0.6 pF. This value may change depending on the
layout of the board.
DC bias is provided through resistors R1 and R4, both 3 kΩ,
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 (CSER) was modeled as an
SRC network, including associated parasitics; it was selected at
1000 pF to avoid affecting the resonator (Q).
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 (RL1) through the 2 pF coupling capacitor, modeled as a series SLC1 component.
The microstrip line (TL1) 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 = VOUT/VIN) 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 measured. 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 modeling over a purely experimental study.
The Colpitts feedback capacitances (CDIV1 = 1 pF and CDIV2 =
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.
Figure 2. Libra Test Bench
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APPLICATION NOTE • APN1006
Figure 3. Default Test Bench
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.
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.
Figure 4. SMV1265-011 Libra IV SPICE Model
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APPLICATION NOTE • APN1006
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, defines nonlinear junction capacitance of the diode)
V
1
M
Grading coefficient (with VJ and M, 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
NBV
Reverse breakdown ideality factor
-
1
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 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:
CJO
CV =
+ CP
M
V
(1 + R )
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
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)
CJO
(pF)
M
VJ
(V)
CP
(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
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.
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APPLICATION NOTE • APN1006
1.0
100
2.4
0.8
10
0.6
0.4
1
0.2
Series Resistance (Ω)
Capacitance (pF)
Approximation
Measured
2.6
2.2
2.0
1.8
1.6
RS_PWL
1.4
0
0.1
0
5
10
15
20
25
30
Varactor Voltage (V)
Figure 5. SMV1265 Capacitance vs. Voltage
Figure 5 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
RS Measured
1.2
0
5
10
15
20
25
30
Varactor Voltage (V)
Figure 6. SMV1265 Resistance vs. Voltage
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 6, 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.
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).
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
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APPLICATION NOTE • APN1006
VCO Design Materials, Layout, and Performance
Table 3 shows the bill of materials used.
Figure 7 shows the VCO circuit diagram.
Designator
VCC = 5 V
Icc = 9 mA
3.3 k
VTUNE
5.6 nH
320 x 30 mils
NE68519
560 p
1p
SMV1265-011
2p
3k
3k
SMV1265-011
300 p
9.1 k
RF Output
1.62 p
200
Figure 7. VCO Circuit Diagram
Part Type
Footprint
C1
0603AU561JAT9 (AVX)
0603
C2
0603AU2R0JAT9 (AVX)
0603
C3
0603AU561JAT9 (AVX)
0603
C4
0603AU201JAT9 (AVX)
0603
C5
0603AU1R0JAT9 (AVX)
0603
C6
0603AU1R6JAT9 (AVX)
0603
D1
NE68519 (NEC)
SOT-419
L1
LL1608-F5N6S (TOKO)
0603
R1
CR10-332J-T (AVX)
0603
R2
CR10-912J-T (AVX)
0603
R3
CR10-201J-T (AVX)
0603
R4
CR10-302J-T (AVX)
0603
R5
CR10-302J-T (AVX)
0603
V1
SMV1265-011 (Skyworks)
SOD-323
V2
SMV1265-011 ( Skyworks)
SOD-323
Table 3. Bill of Materials
720 MIL
Figure 8 shows the PCB layout. The board is made of standard FR4
material 60 mils thick.
720 MIL
Figure 8. PCB Layout
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APPLICATION NOTE • APN1006
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
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.
7
For the availability of the listed materials, please call our applications engineering staff.
POUT_EXP
6
© Skyworks Solutions, Inc., 1999. All rights reserved.
FEXP
5
POUT_MODEL
4
3
FMODEL
POUT (dBm)
Frequency (GHz)
Figure 9 shows both the measured performance of this circuit and
the simulated results, obtained with the above model. The simulated 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 complicated modeling of the components, board parasitics, and
discontinuities. However, for most engineering purposes, the circuit
performance prediction indicated here should be satisfactory.
2
1
0
0
5
10
15
20
25
30
Varactor Voltage (V)
Figure 9. Measured and Simulated
Frequency vs. Varactor Voltage
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.
VVAR
Frequency
POUT
(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
Table 4. Tabulated Measurement Data
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APPLICATION NOTE • APN1006
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