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IEEE MICROWAVE AND WIRELESS TECHNOLOGY LETTERS 1
A 206–220-GHz Compact Fundamental Oscillator
With up to 7-dBm Output Power and 7.4% Peak
DC-to-RF Efficiency in a 130-nm SiGe Technology
Arjith Chandra Prabhu , Graduate Student Member, IEEE, Janusz Grzyb , Marcel Andree , Member, IEEE,
Holger Rücker , and Ullrich R. Pfeiffer , Fellow, IEEE
Abstract— This letter presents a 206.5–220.5-GHz fundamental
differential Colpitts oscillator in a cascode topology implemented
in a 130-nm SiGe HBT technology with ft/fmax of 350/450 GHz.
Base inductors at the common-base (CB) stages are used to
provide an inductive load at the output of the common-emitter
(CE) stage, hence, boosting the output power by 27%. The
resonant tank is embedded straight on top of the devices,
reducing layout parasitics and resulting in a compact and
efficient oscillator core layout. The oscillator provides a peak
output power of 7 dBm at 208 GHz, 7.4% peak dc-to-RF
efficiency at 220 GHz, and 6.55% tuning range (TR). At peak
efficiency, the oscillator delivers an output power of 5.7 dBm
with 50-mW dc power consumption and a phase noise (PN) of
−90.2/−110 dBc/Hz at 1-/10-MHz offset, respectively. To the best
of the authors’ knowledge, the presented oscillator has the best
PN figure-of-merit (FoM) of −182.1 dBc/Hz at 1-MHz offset in a
SiGe/CMOS technology above 200 GHz. It occupies a total area
of 0.086 mm2, including the RF pad, and an ultracompact core
size of 0.0049 mm2.
Index Terms— Colpitts, dc-to-RF efficiency, frequency gener-
ators, fundamental oscillator, mm-Wave, output power, phase
noise (PN), PN figure-of-merit (FoM), power boosting, SiGe,
ultracompact.
I. INTRO DUC TIO N
FREQUENCY generators at mm-Wave frequencies play a
crucial role in high data-rate wireless communication [1],
[2],[3], radar systems [4],[5],[6], high-resolution terahertz
(THz) imaging systems [7],[8], and spectroscopy [9],[10].
In coherent systems applied in laser holography [11] and
high-resolution radar imaging [5],[12], not only amplitude but
also phase information are extracted, allowing distance mea-
surements with an accuracy in the µm range [13]. Extracting
accurate phase information typically requires locked sources
based on a phase-locked loop (PLL) feedback system. How-
ever, implementing a PLL at frequencies beyond 200-GHz
poses a challenge due to the lack of frequency dividers at these
frequencies. Another approach for signal locking is based on
Received 31 January 2025; accepted 19 February 2025. This work was
supported by the European Research Council (ERC) through the European
Union’s Horizon 2020 Research and Innovation Programme under Grant
101019972. (Corresponding author: Arjith Chandra Prabhu.)
Arjith Chandra Prabhu, Janusz Grzyb, Marcel Andree, and
Ullrich R. Pfeiffer are with the Institute of High-Frequency and
Communication Technology, University of Wuppertal, 42119 Wuppertal,
Germany (e-mail: chandra@uni-wuppertal.de).
Holger Rücker is with the IHP—Leibniz-Institut für Innovative
Mikroelektronik, 15236 Frankfurt (Oder), Germany.
Digital Object Identifier 10.1109/LMWT.2025.3546162
Fig. 1. Comparison of SOTA efficiency and output power of oscillators in
SiGe (blue) and CMOS (red) technologies operating from 180 to 300 GHz.
the injection locking method [14],[15],[16],[17] which tends
to be area-consuming or inefficient. An alternative approach
for high-resolution 3-D imaging is based on light-field ray-
tracing [18] utilizing multipixel focal plane arrays (FPAs) of
incoherent sources resulting in a compact and efficient imaging
system.
High power frequency sources beyond fmax/2 become
challenging due to the transistor degraded performance and
increased back-end losses. Typically, such sources are imple-
mented using an oscillator at lower frequencies followed by
a frequency multiplier chain [19],[20],[21]. Here, peak
output powers of 9.6 dBm at 270 GHz occupying an area of
0.92 mm2with 1.38% efficiency [19] and 8 dBm at 240 GHz
occupying an area of 0.28 mm2with 1.47% efficiency [21]
are reported. Nevertheless, due to the high area and low
efficiency, such multiplier chains are unsuitable as sources
in large-scale FPAs for THz light-field applications. A good
alternative is different oscillator topologies, subdivided into
harmonic, push–push/triple-push, and fundamental oscillators
that are generally compact and efficient at the cost of output
power. In more detail, an efficiency of up to 15.3% and
6.5-dBm output power at 195 GHz [22] that drops to 12.3%
with 5.17-dBm output power at 215 GHz [23], 2.76% with
−2.74-dBm output power at 293 GHz [24] are reported,
while in [25] a 301-GHz push–push oscillator [26] with an
efficiency of 2.8% and 2.85-dBm output power is presented.
The corresponding state-of-the-art (SOTA) trend in dc-to-RF
efficiency and output power of oscillators in SiGe and CMOS
technologies beyond 200 GHz is shown in Fig. 1.
This letter presents an efficient and compact 206–220-GHz
fundamental differential Colpitts oscillator in a 130-nm SiGe
HBT technology, which utilizes a 10 pH base inductor to boost
the output power by 27% by providing an inductive load to the
output of the common-emitter (CE) device. The innovative lay-
out reduces the parasitic effects by implementing the resonant
inductor and capacitor directly above the transistors resulting
in a compact oscillator core. The oscillator delivers the highest
© 2025 The Authors. This work is licensed under a Creative Commons Attribution 4.0 License.
For more information, see https://creativecommons.org/licenses/by/4.0/
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2 IEEE MICROWAVE AND WIRELESS TECHNOLOGY LETTERS
Fig. 2. Schematic of the ×4 ( Ae=4×0.07 µm×0.9 µm) fundamental
differential Colpitts oscillator with output matching and biasing networks.
output power compared to the SOTA above 200 GHz with
values up to 7 dBm, a maximum peak efficiency of 7.4%,
and occupies a total area of 0.086 mm2. The corresponding
efficiency and power are well above the trend line shown in
Fig. 1.
II. CIR CUI T DESIGN
The fundamental oscillator is designed in a 130-nm SiGe
BiCMOS HBT technology with a ft/fmax of 350/450 GHz
offering a 12-µm-thick seven-metal aluminum back-end with
two thick top-metal layers [43] and complying to the standard
DRC rules of the technology. All inductors and transmission
lines (TLs) have been implemented on the 3-µm Top-Metal2
(TM2) layer with Metal3 as the ground plane. The schematic
of the fundamental oscillator is shown in Fig. 2consisting of
an x4 (Ae=4×0.07 µm×0.9 µm) differential CE Colpitts
topology followed by a common-base (CB) stage to provide
isolation. The bases of the CE and CB devices are biased
using current mirror and voltage divider circuits, respectively.
The Colpitts oscillator, including RF pads, is full wave 3-D
electromagentic (EM) simulated with parasitic interconnects
in Ansys HFSS.
A. Oscillator Core
A load–pull simulation was used to analyze the output of the
CE stage, resulting in an impedance of 15 +30 jto enhance
the fundamental voltage and current swings. To achieve this,
the Lcasc inductor was used to transform the input impedance
of the CB stage Zin_CB from 30 +15 jto 15 +30 j
thereby maximizing the voltage and current swings as shown
in Fig. 3. As the oscillation frequency decreases with increas-
ing Lcasc, both Ceand Lbare adapted to maintain a constant
frequency of 220 GHz. Although an Lcasc of 28 pH improves
the current swing by 26% and doubles the voltage swing,
a 10-pH inductor was implemented to ensure a compact
oscillator core size of 70 ×70 µm and provide the required
load impedance of 15 +30 jat the output of the CE device,
and improve the voltage and current swing by 34% and 14%,
respectively.
Fig. 3. Ideal core large signal simulation of the magnitude of 220-GHz
fundamental voltage (left) and current (right) swing at the input of CB device
(across nodes AB) for a varying Lcasc.
Fig. 4. (a) 3-D EM model of the compact 70 ×70 µm oscillator core with the
resonance inductors implemented on TM2. (b) Closed-up highlighting MIM
capacitors implemented on Metal5 (M5) and TM1 buried under inductor Lb.
The 3-D EM model of the 70 ×70 µm compact oscillator
core is shown in Fig. 4consisting of symmetrical differential
inductors Lb,Le, and Lcasc . The emitter inductor Le, extracted
from EM simulations, has a differential inductance of 210 pH
achieving a self-resonance frequency (SRF) of 338 GHz. The
ground under Leis removed to improve the inductance and
quality factor. The differential resonant tank inductor Lbis
implemented directly above the differential CE transistors
having a value of 26 pH, and an SRF of 447 GHz. A 15-
fF resonant tank MIM capacitors Ceare buried under Lbas
shown in Fig. 4. The complete EM simulated oscillator core,
including output matching and the 10-pH Lcasc , shows 30%
improvement in voltage swing and 27% improvement in output
power compared with the core without Lcasc .
B. Output Matching
An optimal differential impedance Zopt of 50 +65 j
after modeling the interconnects up to Top-Metal1 (TM1) at
the oscillator’s output was found through load–pull providing
an ideal output power of 7.8 dBm with a shift in oscillation
frequency to 217 GHz. The outputs have been separated into
two single-ended configurations instead of a balun to reduce
further losses, frequency detuning, and maintain compactness.
The output matching consists of a differential 220-pH shunt
inductor Lcwith 318.7-GHz SRF. A 25-µm-long, 65-sym-
metrical series TLs, and two 70-µm-long, 70-shunt lines
along with the RF pads are implemented on each side along
with large 490-fF dc blocking series capacitors to realize an
impedance of 42 +66 jclose to Zopt .
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CHANDRA PRABHU et al.: 206–220-GHz COMPACT FUNDAMENTAL OSCILLATOR WITH UP TO 7-dBm OUTPUT POWER 3
TABLE I
PER FORM ANCE COMPARISON OF FUND AMEN TA L OSCIL LATORS I N SILICON-BAS ED TECHNOLOGY
Fig. 5. Measurement setup and chip micrograph of the fundamental
differential Colpitts oscillator.
Fig. 6. Measured and simulated differential output power (left) and dc-to-RF
efficiency (right) across the TR of 220.5–206.5 GHz and operated at Vcc/Vcasc
of 3.5/3.2 V.
III. M E AS URE MEN T RES U LTS
Fig. 5shows the measurement setup, including the chip
micrograph. The two single-ended outputs enable simultane-
ous frequency and power measurement with 50-termination
on each side using a J-band WR-03 vector network analyzer
(VNA) extension module connected to an E44404 spectrum
analyzer from Agilent and a VDI-Erickson PM4 power meter
with WR-03 to WR-10 taper. All relevant losses, such as
probe/waveguide losses and conversion gain (CG) of the VNA
extension modules, were calibrated and de-embedded. The
oscillator is operated at a nominal collector (Vcc) and CB base
(Vcasc) bias of 3.5 and 3.2 V, respectively. Fig. 6presents
the measured output power and efficiency across a tuning
range (TR) of 206.5–220.5 GHz, demonstrating excellent
agreement with the simulated results, with an oscillation
frequency variation of <2%. Fig. 7shows the downconverted
output spectrum at 210.7 MHz with a peak of −16.5 dBm
and the corresponding PN of −90.2/−110 dBc/Hz and PN
Fig. 7. Measured PN and spectrum of the oscillator operating at 220-GHz
biased at Vcc/Vcasc /Vcm of 3.5/3.2/1.06 V.
figure-of-merit (FoM) of −182/−181 dBc/Hz at 1-/10-MHz
offset, respectively. At 220 GHz, the oscillator has an output
power of 5.7 dBm, dc-to-RF efficiency of 7.4%, and dc power
consumption of 50 mW.
IV. CON C LU SIO N
In this work, an efficient 206.5–220.5-GHz fundamental
Colpitts oscillator implemented in a 130-nm SiGe HBT tech-
nology with a peak output power of 7 dBm around 208 GHz
and peak dc-to-RF efficiency of 7.4% at 220 GHz is reported.
The implemented 10-pH inductance at the CB device pro-
vides an inductive load to the CE device and boosts the
output power by 27%. This design has the best PN FoM of
−182/−181 dBc/Hz at 1-/10-MHz offset as compared to the
SOTA for silicon-based oscillators above 200 GHz presented
in Table I. Therefore, based on its ultracompact 0.0049 mm2
core size, the presented oscillator is suitable for large-scale
source arrays for high-resolution THz light-field imaging.
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