An X-Band low-power and low-phase-noise VCO using bondwire inductor

Abstract

In this paper a low-power low-phase-noise voltage-controlled-oscillator (VCO) has been designed and, fabricated in 0.25 μm SiGe BiCMOS process. The resonator of the VCO is implemented with on-chip MIM capacitors and a single aluminum bondwire. A tail current filter is realized to suppress flicker noise up-conversion. The measured phase noise is −126.6 dBc/Hz at 1 MHz offset from a 7.8 GHz carrier. The figure of merit (FOM) of the VCO is −192.5 dBc/Hz and the VCO core consumes 4 mA from a 3.3 V power supply. To the best of our knowledge, this is the best FOM and the lowest phase noise for bondwire VCOs in the X-band. This VCO will be used for satellite communications.

Full text

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Adv. Radio Sci., 7, 243–247, 2009
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© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Advances in
Radio Science
An X-Band low-power and low-phase-noise VCO using bondwire
inductor
K. Hu, F. Herzel, and J. C. Scheytt
IHP GmbH, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany
Abstract.
voltage-controlled-oscillator (VCO) has been designed and,
fabricated in 0.25µm SiGe BiCMOS process. The resonator
of the VCO is implemented with on-chip MIM capacitors
and a single aluminum bondwire. A tail current filter is
realized to suppress flicker noise up-conversion. The mea-
sured phase noise is −126.6dBc/Hz at 1MHz offset from a
7.8GHz carrier. The figure of merit (FOM) of the VCO is
−192.5dBc/Hz and the VCO core consumes 4mA from a
3.3V power supply. To the best of our knowledge, this is the
best FOM and the lowest phase noise for bondwire VCOs in
the X-band. This VCO will be used for satellite communica-
tions.
In this paper a low-power low-phase-noise
1Introduction
For satellite communications, such as HDTV, internet-via-
satellite and digital video broadcasting service (DVB-RCS),
a low-phase-noise VCO is a prime requirement for the fre-
quency synthesizer. Following the specifications given in
(Follmann et al., 2008), phase noise must be lower than
−110dBc/Hz at 1MHz offset from the carrier frequency.
With the integrated resonator, this specification is hard
to meet, particularly if the performance must be guaran-
teed over a wide tuning range.
−120dBc/Hz at 1MHz offset is, therefore, highly desirable.
With respect to cost, silicon compares favourably to III-V
technologies. However, it is difficult to reach the phase noise
specification with a fully integrated VCO for silicon tech-
nology due to the poor quality factor of the on-chip induc-
tor. Many studies have been presented on minimizing VCO
phase noise, e.g. Hegazi (2001); Ferndahl (2005). However,
the overall performance of these VCOs, e.g. power con-
A VCO phase noise of
Correspondence to: K. Hu
(hu@ihp-microelectronics.com)
sumption, tuning range, phase noise performance, can still
not satisfy the specifications. In order to reduce phase noise
and maintain relatively low power consumption, bondwire
inductors have been used (Craninckx et al., 1995; Kim et al.,
2008). By doing so, a figure of merit (FOM) of −190dBc/Hz
has been reported so far.
A differential CMOS VCO implemented with nMOSFETs
and pMOSFETs was presented in (Craninckx et al., 1997).
Unlike in VCOs using one transistor type only, in this cur-
rent re-using topology no mid-point in the inductor is re-
quired for biasing purpose. We adopt this topology by using
a bondwire inductor instead of an integrated coil. This ap-
proach minimizes mismatch in the VCO as only one bond-
wire is used rather than two in other topologies. Moreover,
a tail current filter is introduced to reduce the flicker noise
up-conversion from the current source. This approach sig-
nificantly improves the close-in frequency phase noise of the
VCO.
2VCO design
The VCO core employs two nMOSFETs and two pMOS-
FETs. The combination of NMOS and PMOS transistors
gives a negative resistance from both transistor pairs. Con-
sequently, to provide the same negative resistance, the com-
bined NMOS and PMOS structure can efficiently halve the
power consumption, which greatly fits the purpose of low
power design. Furthermore, by controlling the supply volt-
age, the signal swing in the VCO core can be well restricted
below the transistor breakdown voltage. It ensures a more
reliable operation, which is a very important requirement for
satellite communications (Tiebout, 2006).
Another convenient feature of this VCO topology is that
only one inductor is required for the resonator. This elimi-
nates the inductance mismatch which appears in topologies
using two bondwire inductors. The mismatch of resonator
inductors in these topologies will result in asymmetric signal
Published by Copernicus Publications on behalf of the URSI Landesausschuss in der Bundesrepublik Deutschland e.V.

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244K. Hu et al.: An X-Band low-power and low-phase-noise VCO using bondwire inductor

Figure 1. Schematic of the X-Band VCO.
se close to the carrier frequency is mainly dominated by the flicker
cillator phase noise.
Additionally, the phase noise close to the carrier frequency
is mainly dominated by the flicker noise up-conversion, i.e.
flicker noise from current source and VCO core transistors.
As a result of the odd mode virtual ground at the differential
common mode node, the flicker noise from current source
is up-converted to 2·f0and mixed down to f0(oscillation
frequency), because of the differential pairs mixing action.
The up-converted flicker noise will be delivered to the res-
onator as AM noise (Rael et al., 2000), and the AM noise
will be converted to FM noise in the varactor (Muer et al.,
2000). Thus, for the suppression of flicker noise from current
source, tail current filtering technique (Hegazi et al., 2001) is
introduced to short the 2nd harmonic at common mode node.
As a result, the close-in frequency phase noise can be largely
improved.
licker noise from current source and VCO core transistors. As a
ual ground at the differential common mode node, the flicker noise
converted to
0
2 f ⋅ and mixed down to
0f (oscillation frequency),
l pairs mixing action. The up-converted flicker noise will be
s AM noise (Rael et al., 2000), and the AM noise will be converted
(Muer et al., 2000). Thus, for the suppression of flicker noise from
filtering technique (Hegazi et al., 2001) is introduced to short the
mode node. As a result, the close-in frequency phase noise can be
g
are many studies about improving the phase noise performance for
them cannot meet both the low phase noise and the low power
The modified Leeson’s Formula (Masini et al.):
Adv. Radio Sci., 7, 243–247, 2009
3
2
1/
2
1
1 ( +

)1
f
cvm
K V
T
⋅
ω
ω
π
∆










⋅




⋅⋅⋅++⋅
, (1)
Tail current filter
Fig. 1. Schematic of the X-Band VCO.
swing across the tank and, consequently, degenerate the os-
3Bondwire modelling
As mentioned above, there are many studies about improving
the phase noise performance for VCO designs, but most of
themcannotmeetboththelowphasenoiseandthelowpower
consumption requirements. The modified Leeson’s Formula
(Masini et al., 2001):
4
structure of the bondwire inductor. The radius of the bondwire is 12.5 µm. The pad distance
and wire loop height are defined as design parameters to simulate bondwire inductance and
quality factor. In order to obtain accurate prediction results, full wave EM simulator (Ansoft
HFSS v11) is used for the bondwire simulations.

Figure 2. Bondwire model. h is wire loop height, d is pad distance.
Fig. 2. Bondwire model. h is wire loop height, d is pad distance.

h
d
Table 1. EM simulation results for bondwire.
Pad DistanceWire Radius 20µm
250µm
300µm
400µm
500µm
600µm
325pH
386pH
502pH
635pH
751pH
L(?ω) = 10 · log
?
gives a guideline for VCO optimization. Here F is the noise
figure of the transistor, ?ω is the offset frequency, ωcis the
center frequency,Ps is the power across the tank, Q is the
loaded Q of the tank, Kvis the VCO gain and Vmis the total
low frequency noise e.g. noises from DC sources or tuning
lines. It is evident from Eq. (1) that the most efficient way of
reducing VCO phase noise is to increase the loaded Q of the
resonator tank. For the LC resonator VCO, the loaded Q of
the tank can be written as:
QL· QC
QL+ QC.
In Eq. (2), QLis the quality factor of the tank inductor, and
QCis the quality factor of the tank capacitor. As a result of
using the MIM capacitors, QCis generally much higher than
QL. Thus, the loaded Q of the tank is mainly determined by
the inductor quality factor. However, due to the lossy silicon
substrate, on-chip inductor in the X-Band can only have a
quality factor around 10. A different choice for on-chip in-
ductors takes advantage of the parasitic inductance, which is
usually associated with bondwire in IC packaging. The se-
ries resistance of the bondwire is very low, which leads to
a high quality factor. Furthermore, the parasitic capacitance
value is given by the bond pads. Thus, by using optimized
bondpad size the bondwire inductor can have a very high
self-resonance frequency. Figure 2 shows the structure of the
bondwire inductor. The radius of the bondwire is 12.5µm.
The pad distance and wire loop height are defined as design
parameters to simulate bondwire inductance and quality fac-
tor. In order to obtain accurate prediction results, full wave
?2 · F · k · T
+π2
2
Ps
?Kv· Vm
·
?
1 + (1
??
2Q·ωc
?ω)2
?
·
1 +?ω1/f3
|?ω|
?
·
?ω
,
(1)
Q =
(2)
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K. Hu et al.: An X-Band low-power and low-phase-noise VCO using bondwire inductor
d quality factor for the bondwire inductor used in the X-band VCO design.
245
5
uminum bondwire with the length of 480um. With optimized bondpad size, the simulated
ality factor can reach 48 at 10 GHz. Figure 3 and Figure 4 show the simulated inductance

Figure 3. Inductance for 480 µm length bondwire (EM simulation)
Fig. 3. Inductance for 480µm length bondwire (EM simulation).

246810 121416 18020
495.0p
500.0p
505.0p
510.0p
515.0p
490.0p
520.0p
freq, GHz
L
m1

Figure 4. Quality factor for 480 µm length bondwire (EM simulation)
Fig. 4. Quality factor for 480µm length bondwire (EM simulation).
Simulation results
hown in Figure 5, the simulated result demonstrates 8 dB phase noise improvement at
Table 1 shows the simulated inductance for different bond-
wire lengths. The size of the bondpad affects the inductor
self-resonance frequency significantly, which is caused by
the parasitic capacitance between pad and substrate.
For the requirement of the X-band VCO design, a 500pH
inductor corresponds to an aluminum bondwire with the
length of 480µm. With optimized bondpad size, the sim-
ulated quality factor can reach 48 at 10GHz. Figures. 3 and
4 show the simulated inductance and quality factor for the
bondwire inductor used in the X-band VCO design.
kHz offset and 4 dB phase noise improvement at 1 MHz offset with the tail current
ring technique. The simulated phase noise for the final VCO design is -128.3 dBc/Hz at 1
z offset and -105 dBc/Hz at 100 kHz offset from a 10.2 GHz carrier. Obviously, the up-
ersion of flicker noise has been greatly reduced by introducing the tail current filtering
cture.

Figure 5. Phase noise with and without tail current filter
2468 101214 1618020
30
40
50
20
60
freq, GHz
Q
m2
Without Filter
With Filter
EM simulator (Ansoft HFSS v11) is used for the bondwire
simulations.
4Simulation results
As shown in Fig. 5, the simulated result demonstrates 8dB
phase noise improvement at 100kHz offset and 4dB phase
noiseimprovementat1MHzoffsetwiththetailcurrentfilter-
ing technique. The simulated phase noise for the final VCO
design is −128.3dBc/Hz at 1MHz offset and −105dBc/Hz
at 100kHz offset from a 10.2GHz carrier. Obviously, the
up-conversion of flicker noise has been greatly reduced by
introducing the tail current filtering structure.

MHz offset and -105 dBc/Hz at 100 kHz offset from a 10.2 GHz carrier. Obviously, the u
conversion of flicker noise has been greatly reduced by introducing the tail current filterin
structure.

Figure 5. Phase noise with and without tail current filter
Fig. 5. Phase noise with and without tail current filter.
Without Filter
With Filter
7
5 Measurement results


Figure 6. Chip photograph
A photograph of the VCO chip is shown in Figure 6. The VCO operates from a 3.3V supply
and consumes 4 mA current. With the 50 Ohm output buffer, the VCO delivers an output
5Measurement results
power from -1 to 2 dBm over the whole tuning range. The measured tuning frequency and
A photograph of the VCO chip is shown in Fig. 6. The VCO
operates from a 3.3V supply and consumes 4 mA current.
With the 50Ohm output buffer, the VCO delivers an out-
put power from −1 to 2dBm over the whole tuning range.
The measured tuning frequency and output power is shown
in Fig. 7.
TheVCOphasenoiseismeasuredbyAeroflexphasenoise
meter with the delay line method. As shown in Fig. 8, the
phase noise at 1MHz offset is −126.6dBc/Hz and shows a
very good matching with the simulation results. At 100kHz
offset from the 7.8GHz carrier, the measured phase noise is
−100dBc/Hz. This relative high phase noise at 100kHz is
due to the flicker noise of the MOSFETs.
The figure of merit for a VCO is defined by Eq. 3 (Tiebout,
2006), where ω0is the angular oscillation frequency, L(?ω)
is the phase noise at offset ?ω and Pdissis the dc power con-
sumption (mW) of the VCO. The measured X-band VCO has
a FOM of −192.5dBc/Hz. To the best of the authors’ knowl-
edge, this is the best FOM among published X-band VCOs
based on silicon technology.
output power is shown in Figure 7.

Figure 7. Measured output power and tuning curve
Bondwire inductor
Fig. 6. Chip photograph.
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246K. Hu et al.: An X-Band low-power and low-phase-noise VCO using bondwire inductor
Table 2. Performance of reported VCO.
FC
(GHz)
Pout
(dBm)
L@1MHz
(dBc/Hz)
Offset
(MHz)
FOM
(dB)
TechnologyTopology
H. Jacobsson/RFICS
H. Jacobsson/RFICS
Orsatti/CICC 1999
Svelto/CICC 2000
K. J. Kim/ISED 2008
This work
11.8
5.9
0.9
1.9
1.7
8.8
−7
−9
NA
Na
NA
1.92
−123
−126
−112.7
−148
−135.3
−125.3
1
1
0.1
3
1
1
−183.9
−185.5
−185.3
−187
−190
−192.5
0.5µm SiGe
0.25µm SiGe
NA
NA
0.18µm CMOS
0.25µm SiGe
4× coulpled VCO array
Coulpled Colpitts
External LC resonator
Bondwire VCO
Bondwire VCO
CC/Bondwire
7

Figure 6. Chip photograph
A photograph of the VCO chip is shown in Figure 6. The VCO operates from a 3.3V supply
and consumes 4 mA current. With the 50 Ohm output buffer, the VCO delivers an output
power from -1 to 2 dBm over the whole tuning range. The measured tuning frequency and
output power is shown in Figure 7.

Figure 7. Measured output power and tuning curve
Fig. 7. Measured output power and tuning curve.
Bondwire inductor
FOM = L(?ω) − 20 · log(ω0
?ω) + 10 · log(Pdiss
1mW)
(3)
Table 2 lists the performance of state-of-art VCOs based on
silicon technology.
6Conclusions
We have presented an X-band bondwire VCO in 0.25µm
SiGe BiCMOS technology. The bondwire was optimized
by using full-wave EM simulation. The tail current noise
is compensated by an integrated low pass filter. Due to the
use of the complementary MOSFETs, only one bondwire is
required. This improves the symmetry of the differential cir-
cuit. The presented VCO achieved a phase noise of less than
−125dBc/Hz at 1MHz offset. The tuning range is from 7.4
to 7.8GHz. The figure of merit is −192.5dBc/Hz. To the
authors’ knowledge, this is the best value reported so far for
silicon-based X-band VCOs. In a future design, the com-
plementary MOSFETs will be replaced with NPN and PNP
hetero-bipolar transistors (HBTs) in a complementary SiGe
BiCMOS technology (Heinemann et al., 2003). This will
reduce flicker noise (Niu, 2005) and improve radiation hard-
ness (Cressler et al., 1998).

The VCO phase noise is measured by Aeroflex phase noise meter with the delay line met
As shown in Figure 8, the phase noise at 1 MHz offset is -126.6 dBc/Hz and shows a
good matching with the simulation results. At 100 kHz offset from the 7.8 GHz carrier
measured phase noise is -100 dBc/Hz. This relative high phase noise at 100 kHz is due t
flicker noise of the MOSFETs.

Figure 8. Measured phase noise for 7.8GHz VCO
Fig. 8. Measured phase noise for 7.8GHz VCO.

The figure of merit for a VCO is defined by equation 3 (Tiebout, 2006), where
Acknowledgements. This work was supported by the European
Space Agency (ESA) and the German DLR (Deutsches Zentrum
f¨ ur Luft- und Raumfahrt). The authors thank the IHP technology
team for the fabrication of the test chip.
0
ω is
angular oscillation frequency, ()
L
ω∆is the phase noise at offset ω
∆ and
diss
P
is the dc p
consumption (mW) of the VCO. The measured X-band VCO has a FOM of -192.5 dBc
To the best of the authors’ knowledge, this is the best FOM among published X-band V
based on silicon technology.
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0
() 20 log(ω∆−) 10 log(
ω
+
∆
)
1
diss
P
mW
FOML
ω
=⋅⋅
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