APN1007 - Skyworks Solutions, Inc.

APPLICATION NOTE
APN1007: Switchable Dual-Band 170/420 MHz
VCO for Handset Cellular Applications
Introduction
Modern multiband cellular handsets use multiple voltage control
oscillator (VCO) functions to accommodate the many down/up
conversions in the intermediate frequency (IF) portion. Using separate VCOs would cause a substantial increase in cost and size of
the radio frequency (RF) section. This component overload can be
resolved by implementing band switchable VCOs.
In many commercial switchable VCOs, the reactive elements in
the tank circuit are switched. This function is usually performed
with PIN diodes. The disadvantage of this solution is that in the
closed circuit state (“diode on” state) there is VCC current flowing
through the diode. This VCC current carries electrical noise which
directly modulates the VCO frequency. Therefore, the noise spectrum may grow significantly beyond the PLL filter range. This type
of switching also limits the switching to within 10–15 percent of
the center frequency, due to the strong effect of PIN diode series
resistance on the tank circuit losses, increasing phase noise.
In this paper, we describe a new switchable VCO solution which
employs switching between separate tank circuits. This reduces
the effect of PIN diode series resistance on VCO noise performance and results in virtually unlimited switching range and
individual optimization of each tank circuit.
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APPLUICATION NOTE • APN1007
170_420_Dual_Resonator
Figure 1. VCO Model
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APPLUICATION NOTE • APN1007
The VCO Model
In the circuit schematic in Figure 1, a traditional Colpitts structure, the varactors are connected as shunt capacitors. Tank
inductors, L4 and L1, providing DC bias to the varactors, are
shunted to ground at the common point with capacitor X6.
Inductors L4 and L1 are modeled as lossy elements (with Q = 25
at 100 MHz) in parallel with capacitors C5 and C4 of 0.38 and
0.28 pF respectively. This is typical for multilayer inductors of
style 0603 (60 x 30 mil footprint), (TOKO Coils and Filters catalogue). The tank inductor values of 12 and 56 nH were optimized
to fit the desired 170 MHz and 420 MHz frequency bands.
Capacitor X6 is modeled as a series RLC network with the length
of transmission line TL3, appropriated to its position on the layout
(see later). Series capacitative reactances, X3 and X4 are also
modeled as lossy series RLC networks, with their appropriate
layout-specific transmission lines, TL4 and TL2. Shunt capacitors,
C7 and C6, are due to the effects of multiple components pads.
The DC bias resistance, SRL2, was chosen relatively small, 300
Ω, to avoid significant thermal noise generation.
The PIN diodes were modeled as parallel RC networks, PRC1 and
PRC2, with switching resistances RSW_L and RSW_H, in the low
band and high band branches respectively. The appropriate
biasing resistors are shown as shunt elements to ground, R2, R1
and R3, respectively. The truth table showing the values of RSW_L
and RSW_H for the appropriate low/high switching is shown in
Table 1.
RSW_L
RSW_H
State
3Ω
3000 Ω
Low band
3000 Ω
3Ω
High band
The Colpitts feedback capacitances, CDIV1 = 20 pF and CDIV2 =
15 pF, were optimized to provide a reasonable power response
over the switching range.
The NEC NE68519 transistor was selected for its high gain and
low noise performance. The output is supplied from the emitter
load resistance, RL1 through the 20 pF coupling capacitor, modeled as a series SLC1 component.
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 balances the input (loop) power to provide zero
gain for a zero loop phase shift. Once the oscillation point is
defined, the frequency and output power may be measured.
We don't recommend using the OSCTEST2 component for the
closed loop analysis, since it may not always converge and does
not allow clear insight into the understanding of VCO behavior.
This is considered an advantage of modeling over a purely
experimental study.
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.
SMV1142-011 and SMV1408-011 SPICE Models
SPICE models for the SMV1142-011 and SMV1408-011 varactor
diodes defined for the Libra IV environment, are shown in Figure
4 and Figure 5 with a description of the parameters employed.
Table 1. Truth Table
Capacitor X1 improves the low band matching of the tank circuit,
insignificantly affecting high band performances of the tank,
because of the high resistance of PRC1 in the low band state.
This component may be removed for narrower frequency
switching.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • sales@skyworksinc.com • www.skyworksinc.com
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APPLUICATION NOTE • APN1007
Figure 2. Libra Test Bench
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APPLUICATION NOTE • APN1007
Figure 3. Default Bench
Figure 4. Spice Model for SMV1142-011
Figure 5. Spice Model for SMV1408-011
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APPLUICATION NOTE • APN1007
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, (CJO and M) defines nonlinear junction capacitance of the diode)
V
1
M
Grading coefficient (with VJ and M, (CJO and VJ) 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
1
NBV
Reverse breakdown ideality factor
-
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 2. Model Parameters
Table 2 describes the model parameters. It shows default values
appropriate for silicon varactor diodes that may be used by the
Libra IV simulator.
According to the SPICE model in Figures 4 and 5, the varactor
capacitance, CV, is a function of the applied reverse DC voltage,
VR, and may be expressed as follows:
CJO
( 1 + VVAR
VJ
)
M
CJO
(pF)
SMV1142-011
SMV1408-011
+ CP
This equation is a mathematical expression of the capacitance
characteristic. The model is accurate for abrupt junction varactors
(like the SMV1408). The model is less accurate for hyperabrupt
junction varactors because the coefficients are dependent on
applied voltage. To make the above equation work better for the
hyperabrupt varactors, the coefficients were optimized for the
best capacitance vs. voltage fit as shown in Table 3 and Figure 6.
Note: In the Libra model shown in Figure 6, CP is given in picofarads, while CGO is given in farads to comply with the default
unit system used in Libra.
M
VJ
(V)
CP
(pF)
13.4
1
2.2
0
0.7
1.8
21
25
68
0.13
0.6
1.8
LS
(nH)
14
SMV1142-011
approximation
12
10
8
SMV1142-011
SMV1408-011
approximation
6
4
SMV1408-011
2
0
0
1
2
3
4
Varactor Voltage (V)
Figure 6. Capacitance vs. Voltage
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RS
Ω)
(Ω
Table 3. Optimized Coefficients for Capacitance vs. Voltage
Capacitance (pF)
CV =
Part
Number
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APPLUICATION NOTE • APN1007
VCO Design, Materials, Layout and Performance
Figure 7 shows the VCO circuit diagram.
J3
1
C4
100 pF
R4
VSW_Low
1.5 k
J4
D2
1
CCC
SMV1139-011
VCC +3 V
100 pF
C8
10 pF
L2
56 nH
P2
R6
SMP1320-011
3k
R1
J1
300
L1
12 nH
P1
8 pF
Q1
NE68519
C9
100 pF
C1
1
C6
SMP1320-011
VTUNE
C10
100 pF
470 pF
D1
C2
SMV1408-011
20 pF
C3
R3
R7
1.5 k
6.8 k
20 pF
J5
1
R5
C7
RF
100
15 pF
C11
C5
100 pF
30 pF
R2
1.5 k
J2
1
VSW_High
Figure 7. VCO Circuit Diagram
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APPLUICATION NOTE • APN1007
Table 4 shows the bill of materials used.
The PCB layout is shown in Figure 8. The board was made of
standard, 30 mil thick, FR4 material.
Designator
Value
Part Number
Footprint
Manufacturer
C1
8 pF
0603AU8R0JAT9
0603
AVX
C2
470 pF
0603AU471JAT9
0603
AVX
C3
100 pF
0603AU101JAT9
0603
AVX
C4
100 pF
0603AU101JAT9
0603
AVX
C5
100 pF
0603AU101JAT9
0603
AVX
C6
20 pF
0603AU200JAT9
0603
AVX
C7
15 pF
0603AU150JAT9
0603
AVX
C8
10 pF
0603AU100JAT9
0603
AVX
C9
100 pF
0603AU101JAT9
0603
AVX
C10
20 pF
0603AU200JAT9
0603
AVX
AVX
C11
30 pF
0603AU300JAT9
0603
CCC
100 pF
0603AU101JAT9
0603
AVX
L1
12 nH
LL1608-F12NS
0603
TOKO
L2
56 nH
LL1608-F56NS
0603
TOKO
R1
300
CR10-301J-T
0603
AVX
R2
1.5 k
CR10-152J-T
0603
AVX
R3
1.5 k
CR10-152J-T
0603
AVX
R4
1.5 k
CR10-152J-T
0603
AVX
R5
100
CR10-101J-T
0603
AVX
R6
3k
CR10-302J-T
0603
AVX
R7
6.8 k
CR10-682J-T
0603
AVX
D1
SMV1408-011
SMV1408-011
SOD-323
Skyworks
D2
SMV1142-011
SMV1142-011
SOD-323
Skyworks
P1
SMP1320-011
SMP1320-011
SOD-323
Skyworks
P2
SMP1320-011
SMP1320-011
SOD-323
Skyworks
Q1
NE68519
NE68519
SOT-419
NEC
Figures 9 and 10 show the measured performance of this
circuit and the simulated results obtained with the model in
Figure 8.
In both low and high band states there is good compliance for
frequency response. Some of the difference of the measured
data and the simulation is probably due to the 5–10 percent
variation of circuit capacitances and inductances from their
nominal values.
The low band simulated power response was in fair agreement
with measured performance, assuming measurement
uncertainty of ±1 dB. The power response difference of
up to 4 dB for the high band may be due to underestimated
circuit component losses. For example, a decrease of inductor
Q-quality from 30 to 20 decreases output power about 1 dB;
an increase in resistance of switching diode from 3 to 6 Ω
will decrease power to about 2 dB.
Table 4. Bill of Materials
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APPLUICATION NOTE • APN1007
25 mm
30 mm
Figure 8. PCB Layout
450
190
Frequency
8
Frequency
Measured
points
Power
2
150
0
140
-2
Frequency (MHz)
Frequency (MHz)
4
4
430
2
420
Measured
points
410
0
Power
-2
400
Output Power (dBm)
170
Output Power (dBm)
6
160
6
440
180
-4
390
-6
380
130
0
0.5
1.0
1.5
2.0
2.5
3.0
Varactor Voltage (V)
Figure 9. Low Band Measured and Simulated Results
0
0.5
1.0
1.5
2.0
2.5
3.0
Varactor Voltage (V)
Figure 10. High Band Measured and Simulated Results
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APPLUICATION NOTE • APN1007
Figure 11 shows the phase noise in the high band state vs. frequency offset. It shows better than -95 dBc/Hz at 10 kHz offset.
This measurement was made using the PN9000 Phase Noise Test
Set, courtesy of Aeroflex Comstron, Plainview, NY
(www.aeroflex.com).
Phase noise for both bands was measured with the HP8564E
spectrum analyzer. At 10 kHz offset the noise was -95 dBc/Hz for
both bands. It was expected that phase noise would be better at
the low band. The poorer measured phase noise at the low band
may be attributed to the spectrum analyzer method where the
internal noise may be too close to the measurement level. It may
also be related to the wide-bandwidth matching requirement of
the VCO feedback circuit, which makes it difficult to satisfy noise
optimums in both bands simultaneously. Our design compromise
was to balance noise performance between both bands. In any
event, -95 dBc/Hz is considered acceptable for digital cellular
applications.
Figure 11. High Band Phase Noise vs. Frequency
List of Available Documents
VCO Related Application Notes
1. HF Switchable VCO Simulation Project Files for
Libra IV
1. Varactor SPICE Models for RF VCO Applications
2. HF Switchable VCO Circuit Schematic and PCB Layout for
Protel EDA Client, 1998 Version
3. HF Switchable VCO PCB Gerber Photo-plot Files
2. A Colpitts VCO for Wideband (0.95–2.15 GHz) Set-Top TV Tuner
Applications
3. A Balanced Wideband VCO for Set-top TV Tuner Applications
© Skyworks Solutions, Inc., 1999. All rights reserved.
(For the availability of the listed materials, please call our applications engineering staff.)
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APPLUICATION NOTE • APN1007
Copyright © 2002, 2003, 2004, 2005, Skyworks Solutions, Inc. All Rights Reserved.
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