21.5-to-33.4 GHz voltage-controlled oscillator using NMOS switched inductors in CMOS

Abstract

To demonstrate the applicability of NMOS switched variable inductors in the millimeter wave frequencies, a 21.5 to 33.4 GHz wide tuning range LC voltage-controlled oscillator (LC-VCO) with frequency tunable output buffers that uses variable inductors is reported. The measured phase noise at 10 MHz offset of VCO fabricated in a 65 nm bulk CMOS process varies from ${-}117$ to ${-}109$ dBc/Hz. The oscillator core consumes 4 or 6 mA from a 1.2 V power supply. These correspond to a record 43.3% tuning range. ${rm FOM}_{rm T}$ ranges from ${-}191.7$ to ${-}181.9$ dBc/Hz. With tunable output buffers, the measured signal output power is above ${-}15$ dBm across the entire frequencies.

Full text

478 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 7, JULY 2014
21.5-to-33.4 GHz Voltage-Controlled Oscillator
Using NMOS Switched Inductors in CMOS
Jing Zhang, Student Member, IEEE, Navneet Sharma, and Kenneth K. O, Fellow, IEEE
Abstract—To demonstrate the applicability of NMOS switched
variable inductors in the millimeter wave frequencies, a 21.5 to
33.4 GHz wide tuning range LC voltage-controlled oscillator (LC-
VCO) with frequency tunable output buffers that uses variable in-
ductors is reported. The measured phase noise at 10 MHz offset
of VCO fabricated in a 65 nm bulk CMOS process varies from
to dBc/Hz. The oscillator core consumes 4 or 6 mA
from a 1.2 V power supply. These correspond to a record 43.3%
tuning range. ranges from to dBc/Hz.
With tunable output buffers, the measured signal output power is
above dBm across the entire frequencies.
Index Terms—CMOS, LC voltage-controlled oscillators
(LC-VCOs), millimeter-wave integrated circuit, switched in-
ductor, wide-tuning.
I. INTRODUCTION
ELECTRO-MAGNETIC waves in the millimeter and sub-
millimeter wave frequency ranges have been utilized in
rotational spectroscopy to detect and identify gas molecules [1]
for safety and security applications. Advances of the high fre-
quency capability of CMOS have made it possible to consider
CMOS as a means for implementing the electronics for these
spectroscopy systems, in which a signal generation circuit oper-
atingover90GHzwitha % frequency tuning range is a key
component. This letter reports a wide tuning range voltage con-
trolled oscillator fabricated in 65 nm CMOS which can be used
in conjunction with a chain of frequency multiplier to generate
the required signals above 90GHz.ByusingNMOSswitched
differential inductors [2]–[4] up to the millimeter wave frequen-
cies against the conventional wisdom, the circuit generates sig-
nals at frequencies between 21.5 and 33.4 GHz without any fre-
quency tuning gaps.
II. OPERATING FREQUENCY SELECTION CONSIDERATION
There is a trade-off between frequency tuning and opera-
tion frequency due to the fact that the capacitance of transistors
needed to sustain operation becomes an increasing portion of the
capacitance of LC tank that determines the operation frequency.
Manuscript received January 29, 2014; accepted April 02, 2014. Date of pub-
lication June 03, 2014; date of current version June 20, 2014. This work was
supported by the Semiconductor Research Corporation through Texas Analog
Center of Excellence under task 1836.036.
The authors are with the Texas Analog Center of Excellence & Electrical
Engineering Department, University of Texas at Dallas, Richardson, TX
75080 USA (e-mail: jing.zhang@utdallas.edu; navneet.sharma@utdallas.edu;
k.k.o@utdallas.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LMWC.2014.2317112
This is exacerbated by the fact that the quality factor of varactors
and parasitic capacitance of transistors degrade with frequency
which further increases the width of transistors needed for os-
cillation and their capacitance. Because of this, most of CMOS
signal generation circuits with fundamental oscillation frequen-
cies above 90 GHz use small varactors [5], [6] or even no varac-
tors [7]. Recently, magnetic frequency tuning techniques based
on transformers that only require small varactors have been re-
ported and demonstrated over 40% tuning range [8]. However,
the design complexity of on-chip transformers quite often re-
sults frequency tuning gaps.
These limitations could be mitigated by generating signals at
lower frequencies at first, where wide-tuning can be achieved
relatively easier, and then frequency multiply to generate the
signals at the desired frequencies. Since frequency multiplica-
tion adds loss, it is critical to generate the signal at as high of
frequency as possible. As part of an investigation to determine
the optimal operation frequency, a wide tuning oscillator oper-
atingupto GHz has been implemented and characterized.
Even at GHz, implementing an LC-VCO with a frequency
tuning range approaching 50% is not trivial.
III. CIRCUIT DESIGN AND IMPLEMENTATION
Fig. 1(a) shows the circuit schematic of the proposed
wide-tuning LC-VCO. It consists of a cross-coupled NMOS
pair , a PMOS current source , a 3-bit bi-
nary-weighted accumulation-mode MOS varactor bank to
reduce VCO gain while increasing frequency tuning, and
NMOS switched differential inductors [3], [4] for frequency
tuning extension. A tunable output buffer was also implemented
to isolate the VCO core from external loads, as well as to flatten
the signal power level over the operation frequency range.
A. NMOS Switched Differential Inductors
Fig. 1(b) shows the layout of the NMOS switched variable
inductors. It is formed by cascading four inductor loops with
three NMOS switches (SW1, SW2 and SW3) between adjacent
sections. The simulated switch on-resistance is .Inthis
layout, the effective tank inductance looking into the differential
port mostly relies on the self-inductance of each section rather
than their mutual inductances, which makes it straightforward
to design and implement. Moreover, since the switches are on
differential nodes, the effective series resistance added to the
tank inductance is only a half of the on-resistance of NMOS
switches [2], which helps to mitigate the Q degradation.
The number of inductor bands is determined by the de-
siredfrequencytuningratio and the maximum-to-
minimum tank capacitance ratio .These
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ZHANG et al.: 21.5-TO-33.4 GHZ VOLTAGE-CONTROLLED OSCILLATOR USING NMOS SWITCHED INDUCTORS IN CMOS479
Fig. 1. (a) Circuit schematic of the proposed wide-tuning VCO with tunable
output buffers, and (b) layout of the NMOS switched differential inductors.
also determine the desired inductance ratio for adjacent bands
. Their relationships are
(1)
Since the oscillation frequency range target was 22.5 to
38.5 GHz, and the accumulation-mode MOS varactor used has
a maximum-to-minimum capacitance ratio of , the number
of inductor bands was set to four. The minimum required
is .Thisinconjunctionwiththe
varactor tuning ratio of 3 allows use of a larger cross-coupled
NMOS pair to provide sufficient gain for compensating the tank
loss, and frequency overlap between adjacent inductor bands.
Since the thickness of top copper layer in the process is less
than 1 m, the inductor trace was implemented using an over
1m thick aluminum pad layer stacked with the top copper
layer to increase its Q-factor. The inductors were simulated
using a 3D EM simulator, Ansoft HFSS, and simulations show
this stacking increases the inductor Q-factor from to .
The total tank Q-factor is when the switches are all off, and
reduces to when switches are on.
B. Tunable Output Buffer
Fig. 2 shows the circuit schematic of the tunable output
buffer. It consists of three inductor-loaded common-source
stages with DC-coupled input and AC-coupled output. The first
stage is for isolating the VCO. The second stage is for driving
the prescaler/divider in a phase-locked loop, and the third stage
is for driving 50 impedance of an external instrument. The
inductive loads on the first and second stages are implemented
with NMOS switched single-ended inductors [5] so that their
inductance can be tuned for a relatively flat frequency response
across the entire operation frequency range. Since these induc-
tive loads require a moderate Q-factor, single-ended NMOS
switches are used.
Fig. 2. Circuit schematic of the tunable output buffers.
Fig. 3. Die photograph of the wide-tuning LC-VCO test structure.
TAB L E I
BAND SWITCH AND BIAS SETTINGS IN MEASUREMENT
Fig. 4. Measured frequency tuning range.
The single-ended NMOS switched inductors are clearly seen
in the die photograph in Fig. 3. The two inductor loops are im-
plemented using the top copper layer and are diagonally placed
to minimize their mutual inductive coupling [4]. This reduces
the complexity in component modeling. The layout of VCO was
optimized for symmetry to minimize mismatches. The signals
are measured from one side of the buffer output with a GSG
pad, while the other side of the buffer output is terminated by a
50 on-chip resistor without a GSG pad frame to reduce the

480 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 24, NO. 7, JULY 2014
Fig. 5. Measured output power and phase noise at 10 MHz offset across all
bands.
Fig. 6. Measured phase noise plot from a 27.2 GHz carrier (Band3).
TAB L E I I
PERFORMANCE COMPARISON OF WIDE-TUNING LC-VCOS
layout area. The VCO core occupies 420 m200 m. In-
cluding buffers and pads, the area is 900 m900 m.
IV. MEASUREMENT RESULTS
An Agilent N9030A PXA signal analyzer and an Agilent
11970A harmonic mixer are used for the spectrum measurement
at 20 to 40 GHz. An Agilent E8257D signal generator was also
used to calibrate power losses from the measurement setup, in-
cluding the losses from RF probe, cables, and mixers. The bias
condition and switch settings for different bands are summa-
rized in Table I. The core consumes 4 or 6 mA, and the two
3-stage buffers consume mA in total from a 1.2 V power
supply. The current in the two higher frequency bands was in-
creased to compensate for increased loss.
Fig. 4 shows the measured frequency tuning range for all
bands. The signal frequency can be tuned from 21.5 to 33.4 GHz
without any gaps. The corresponding tuning range is 43.3%.
Fig. 5 shows the measured signal output power of the circuit
after de-embedding the losses of the measurement setup. The
power calibration has 1–2 dB of uncertainties. The signal power
is above dBm over the output frequency range. The power
variation is acceptable since for the spectroscopy, the output
power must be flat over a few MHz being analyzed at a time,
and effects of this is mitigated using FSK modulation. Fig. 5
also shows the measured phase noise at 10 MHz offset across
all bands. It varies from to dBc/Hz. Fig. 6 shows
the measured phase noise plot for a 27.2 GHz carrier (Band3).
V. C ONCLUSION
Table II summaries the performance of state-of-the-art
wide-tuning millimeter wave CMOS LC-VCO’s [9]–[12]. The
VCO reported here achieves the widest frequency tuning range
among all reported works without having frequency tuning
gaps, and with a which is comparable to that of the
others. The VCO in [8] although achieves a comparable tuning
range but it has frequency tuning gaps. It has been believed
that the NMOS switched inductors are only useful at low mi-
crowave frequencies. This work demonstrates that they can be
used at millimeter wave frequencies for ultra-wide band signal
generation. Lastly, this circuit can be cascaded with broadband
frequency multiplication circuits to generate signals over a wide
frequency range above 90 GHz for millimeter-wave rotational
spectroscopy application.
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