In this chapter the design of a D-Band fundamental-wave VCO was presented. Various topologies suitable for realizing mm-wave and THZ VCOs were presented and briefly discussed.
The VCO achieved one of the highest reported output power and lowest phase noise in this frequency range. It was argued that a VCO core designed at a higher biasing current and peakfT/fMAX current density is a better design strategy than designing a VCO with lower
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8.4 Summary current and using a buffer to boost the output power. The use of this buffer mostly leads to a higher total power consumption and deteriorated phase noise performance. The designed VCO achieves a state-of-the-art FoMT of −184 dBc/Hz, with a tuning range, peak output power, efficiency and minimum phase noise of 12 GHz, 9 dBm, 6 % and −96 dBc/Hz (@ 1 MHz offset), respectively.
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CHAPTER
9
Push-Push Topology based J-Band High Efficiency VCOs
In order to successfully implement any integrated THz system, one of the key challenges includes the development of high power integrated THz sources working within the atmospheric transmission windows [44]. These signal sources are indispensable for transmitters and as well as receivers. In this chapter a novel method of maximizing the output power and efficiency of mm-wave and terahertz signal sources based on the push-push topology is presented. In this method, the common-mode impedance of a differential Colpitts oscillator operating in the odd mode is maximized by introducing a fixed-valued capacitor (Cr) at the common-base node.
This capacitor is designed to introduce a common-mode parallel resonance at the desired second harmonic, boosting the common-mode voltage swing and subsequently its output power. The proposed method is analyzed using high frequency even modeπ-model. Analytical expressions of input impedance are derived and are used for calculating the common-mode resonance frequency and the required value of Cr. Two 0.3 THz voltage controlled oscillators (VCOs) are implemented in a 130 nm SiGe BiCMOS process. It is shown that by using the proposed technique, the output power is improved by more than 6 dB, as compared to the conventional approaches. The implemented VCOs work from 292−318 GHz and 305−327 GHz, delivering a peak output power of 0.6 dBm and 0.2 dBm, with a DC-to-RF efficiency of 0.8 % and 0.9 %, and can achieve a phase noise of−108 dBc/Hz and−105 dBc/Hz at 10 MHz offset, respectively.
As compared to the prior state-of-the-art Si-based tunable signal-sources and arrays working above 270 GHz, this work shows the lowest phase noise and the best figure-of-merit, while having excellent output power, tuning range and DC-to-RF efficiency. The technique proposed in this chapter can be employed on all oscillator and frequency multiplication circuits relying on push-push based even-harmonic extraction. For the initial design the conventional design procedures can be followed and then the proposed technique can be applied in the final stages to achieve an improved performance without having a significant influence on the fundamental frequency of operation.
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9. Push-Push Topology based J-Band High Efficiency VCOs
The chapter is organized as follows, Section 9.2 provides an analysis of the proposed technique using high-frequency even-mode equivalent circuit of a Colpitts differential oscillator. Using both analytical expressions and Spectre simulations of the common-mode input impedance, the efficacy of the technique in improving the output power is explained. Section 9.3 focusses on the details of the THz VCO circuit design, parasitics, and layout issues, with emphasis on common-mode impedance design methodology. In this section, the influence of the proposed technique on tuning bandwidth, frequency pulling, and phase-noise performance is also discussed. The detailed characterizations and a thorough comparison with state-of-the-art sub-THz and THz signal sources are provided in Section 9.4.
9.1 State-of-the-Art THz Signal Sources
Fundamental signal sources based on common-base cross-coupled topology working above 300 GHz with more than 4.3 dBm output power have been demonstrated using a 250 nm InP based process with an fT and fmax of 392 GHz and 859 GHz, respectively [179]. Using the same technology, two differential oscillators working at 280 GHz were combined using rat-race and Wilkinson power combiner to achieve an output power of 10 dBm [180]. In the near future, Si-based alternatives will enable fundamental-mode circuits and systems to work even above 0.3 THz. For now, harmonic extraction continues to be an indispensable technique to obtain sufficient output power at THz. At these frequencies, the available gain from the transistors is very low and passive devices exhibit much higher losses. These fundamental limitations have led to the development of new techniques and architectures for designing high power THz frequency sources. In [181], a maximum-gain ring oscillator topology was introduced maximizing the power gain by means of appropriately designed passive matching networks.
This increase in gain helps to achieve higher oscillation frequency. However, the topology is limited to ring oscillators which is suitable mostly for CMOS based designs. To maximize the output power of the harmonics, traditional load-pull simulations were utilized, beyond which power-combining network was used to sum the power from multiple stages resulting in more DC power consumption and complexity. Building up on this approach, a cross-coupled push-push VCO working around 239 GHz with−4.8 dBm output power was reported in [171]
which replaces the passive matching network with transformer based resonators. Similarly, capacitive feedback frequency-enhancement which utilizes both negative resistance Colpitts approach and a capacitive feedback similar to that of−Gm oscillators, was reported in [165].
An approach for increasing the oscillation frequency beyond the device cut-off frequency offT was published in [182], by using a frequency selective negative resistance tank. A fundamental oscillator prototype based on buffer-feedback topology working at 300 GHz in a 65-nm CMOS technology was demonstrated by Razavi [183].
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9.1 State-of-the-Art THz Signal Sources
VB
Q1
LB
rB
VT
Cvar
2Cr
Vcc
Ibias
(a)
rπ
Cπ
Cvar
Cµ
rB
LB
βib
ZB Zπ Zµ
ZE
Zin
Cr
Zin0
(b) ZB
Zµ
Zr
Zπ0 Zin0
Zπ0 =Zπ+ (β(ω) + 1)ZE
Zin
Zeff
Zeff =Reff +jXeff =Zπ0 kZµ
(c)
Cr
Zin0 Reff
Zin
rB
LB
Ceff
Reff
rB
LB
Ceff
R0eff R0eff
(d)
Figure 9.1: (a) Simplified lumped element circuit of a differential Colpitts oscillator based push-push VCO. The dashed-blue line shows the proposed common-mode resonant capacitor Cr. (b) High-frequency equivalent even-modeπ-model of the VCO half-circuit shown in (a).
(c) Simplified equivalent circuit for calculating the input impedanceZin (withoutCr) and Zin0 (withCr). (d) Equivalent input-impedance of the circuit depicted in (c), with and withoutCr, showingReff andCeff.
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9. Push-Push Topology based J-Band High Efficiency VCOs
Table 9.1:Description and Values of The Small-Signal Equivalent Circuit Parameters
symbol description value
rπ small-signal base-emitter resistance 12 Ω
rB base spreading resistance 3 Ω
Cπ base-emitter capacitance 7 fF
Cµ base-collector capacitance 15 fF
Cvar varactor capacitance 50 fF
Cr common-mode resonant capacitance 16 fF
LB base tank inductance 20 pH
gm small-signal transconductance 420 mS
fT transit frequency 300 GHz
Ibias bias current 40 mA