5.7CMOS Differential LC Oscillator with Suppresd
Up-Converted Flicker Noi
Aly Ismail, Asad A. Abidi
Electrical Engineering Department, University of California, Los Angeles, CA
New CMOS oscillators with on-chip resonators are able to meet
the demanding requirements of modern communications trans-ceivers. For example, it is a challenge to meet GSM specifications on pha noi at offts of 3MHz and 20MHz from the oscilla-tion frequency. Filtering white noi in the tail current [1] allows low power CM OS VCOs to meet the specifications. However, this technique cannot be extended to other standards such as Japane PDC, where narrow channel spacing requires low pha noi at 50kHz offt. At low offts, upconverted flicker (1/f) noi dominates. The unique physical mechanisms respon-sible for this upconversion [2] must be dealt with differently. This paper prents a 1.5GHz CM OS differential VCO who clo-in p
ha noi is 20dB lower than in a conventional differ-ential oscillator across the full tuning range. At 50kHz offt, the oscillator’s pha noi of –105dBc/Hz is low enough for the PDC receiver.
Flicker noi can upconvert around the carrier frequency in many ways, but in practical oscillators two are most important. We illustrate them at work in the conventional differential LC oscillator (Fig. 5.7.1).
1. In the current-limited regime, the tail current governs
the steady-state oscillation amplitude [3]. Therefore,
1/f fluctuations in the tail current source (M3) produce
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low frequency random AM. The random AM envelope
modulates the effective capacitance of the tuning var-
actor, converting AM into FM [4]. The FM sidebands
appear as clo-in pha noi.
2. Flicker noi in the differential pair (M1, M2), modeled旅游管理毕业论文
by an equivalent noi voltage associated with one
transistor in the pair, is injected into the LC resonator
as current noi at baband and at the 2nd harmonic
[2]. This cannot account for clo-in pha noi.
文字故事However, the flicker noi also modulates the 2nd har-
monic voltage waveform at the tail every half period,
春日山居寄友人inducing a noisy current in C
tail
. After commutation through M1, M2, this current mixes down to the oscil-
lation frequency and prents a fluctuating capacitance
across the LC resonator. The resulting random FM
upconverts 1/f noi into clo-in pha noi [2].
大学生职业规划大赛Figure 5.7.2 shows an oscillator circuit that suppress both mechanisms. Instead of an FET current source, a polysilicon resistor which is substantially free of 1/f noi defines the tail current. The current is regulated by gmenting a fixed resistor, and lectively shorting the gments with digital bits derived from a replica rvo loop. A small tail resistor can load the LC tank and lower Q when the oscillation forces M1 or M2 into the triode region [1]. This problem is alleviated by an inductor in ries with the bias-tting resistor, which rais the tail imped-ance at the oscillation frequency and its harmonics. A large capacitor across the resistor shunts wideband resistor noi. Flicker noi in M1, M2 is modeled as a fluctuating offt voltage that unbalances the differential pair. A fluctuating unbalance is
responsible for the noisy current in C
tail
that after commutation ultimately modulates the oscillation frequency. However, bal-ance may be restored by decoupling the sources of M1 and M2 with a capacitor, C
c
. Decoupling should be understood in the con-text of an oscillator, which operates in large-signal periodic steady state. The classic emitter-coupled multivibrator [5] gives a uful analogy. Periodic relaxation in that circuit creates a large fundamental frequency voltage in anti-pha at the two emitters. In our oscillator the LC tank ts the frequency, and switching of M1 and M2 produces a 2nd harmonic voltage of almost constant amplitude at the source nodes. In addition, there is periodic relaxation across the decoupling capacitor, which creates a fundamental frequency component at the source voltages with amplitude that depends on C
c
and the bias current. If C
c
is too large, the 2nd harmonic dominates the fundamental, and the circuit behaves like the differential pair; if C
c
is too small, the oscillator does not start up. At the optimum the mag-nitude of the fundamental is comparable to the 2nd harmonic, signifying sufficient decoupling to enable the voltage across C
c
to track the 1/f noi that unbalances M1 and M2. The noi no longer modulates the 2nd harmonic voltage across the two source capacitors, and thus the cond mechanism of upconversion described above is suppresd.
A prototype oscillator (Fig. 5.7.3) is fabricated in the Jazz Semiconductor BC35M process using 0.35µm FETs. The circuit oscillates nominally at 1.5GHz, and its frequency can be tuned from 1.43 to 1.64GHz with an NMOS varactor (20 x 10/0.35µm) in parallel with a 4-bit binary switched capacitor array (80fF unit capacitance). The tank inductor is fabricated as a differen-tial spiral in 3µm thick Metal 3; over the 8 ohm-cm substrate, inductor Q is 9 at 1.5GHz. The tail resistors are realized in unsilicided polysilicon, and the control switch FETs are laid out in a ring gate geometry to lower drain junction capacitance. The oscillator operates from 2.7V and bias at 6mA. Packaged chips are measured on a calibrated RDL NTS-1000A Pha Noi Analyzer.
Figure 5.7.4 plots the measured pha noi for a typical chip at the nominal oscillation frequency of
1.5GHz. It is compared with the measured pha noi in an identical chip who C
c
is short-ed, that is, where M1 and M2 merge into the standard differen-tial pair. This rves as a reference oscillator to normalize depen-dencies on fabrication process. Both oscillators are biad at the same current. The flicker noi corner in the new oscillator lies at about 8kHz offt, whereas in the reference oscillator it lies at about 1M Hz. In the flicker noi-dominated region, the new oscillator’s pha noi is lower by 20dB. At 50kHz offt, pha noi is lower by 15dB. As the varactor control voltage is swept from V
DD
to ground, the pha noi remains almost constant (Fig. 5.7.5). Across the entire discrete tuning range, the clo-in pha noi changes by only about 2dB (Fig. 5.7.6). The degree of suppression is limited by C英文动画电影
tail
, which in this prototype is deter-mined by interconnects.
The wideband suppression technique prented in this paper is expected to enable u of FET VCOs in applications where previ-ously they were excluded by excessive upconversion of 1/f noi. Acknowledgements
The authors thank E. Youssoufian, P. Mudge, G. Hatcher, M. Reddy and S. Lloyd, all of Skyworks.
References
[1] E. Hegazi, H. Sjöland, and A. A. Abidi, “A Filtering Technique to Lower LC Oscillator Pha Noi,” IEEE J. of Solid-State Circuits, vol. 36, no. 12, pp. 1921-1930, 2001.
[2] J. J. Rael and A. A. Abidi, “Physical Process of Pha Noi in Differential LC Oscillators,” in Custom IC Conf., Orlando, FL, pp. 569-572, 2000.
9月27日[3] A. Hajimiri and T. H. Lee, “Pha Noi in CM OS Differential LC Oscillators,” IEEE J. of Solid-State Circuits, vol. 34, no. 5, pp. 717-724, 1998.
[4] S. Levantino, C. Samori, A. Bonfanti, S.L.J. Gierkink, A.L. Lacaita, and V. Boccuzzi, “Frequency Dependence on Bias Current in 5GHz CMOS VCOs: Impact on Tuning Range and Flicker Noi Upconversion,”IEEE J. of Solid-State Circuits, vol. 37, no. 8, pp. 1003-1011, 2002.
模仿法[5] A. B. Grebene, Analog Integrated Circuit Design, Van Nostrand, New York, 1972.
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Figure 5.7.1: Differential LC Oscillator, showing sources of 1/f noi and associated waveforms.
Figure 5.7.2: New differential oscillator with noiless current sources and capacitor-coupled sources.
Figure 5.7.3: Circuit diagram of prototype VCO.
oscillator that us conventional differential pair.
