A low-phase-noise 5-GHz CMOS quadrature VCO using superharmonic coupling

Abstract

A new concept for quadrature coupling of LC oscillators is introduced and demonstrated on a 5-GHz CMOS voltage-controlled oscillator (VCO). It uses the second harmonic of the outputs to couple the oscillators. The technique provides quadrature over a wide tuning range without introducing any increase in phase noise or power consumption. The VCO is tunable between 4.57 and 5.21 GHz and has a phase noise lower than -124 dBc/Hz at 1-MHz offset over the entire tuning range. The worst-case measured image rejection is 33 dB. The circuit draws 8.75 mA from a 2.5-V supply

New capacitive coupled superharmonic quadrature LC-VCO

“…1 and the common mode node pairs in these circuits are denoted with (a, a ), (b, b ), and (c, c ). As can be easily verified, presence of a 180 • phase difference between the current and/or voltage signals at the common mode nodes of each of the core oscillators, results in 90 • phase differences in the first harmonic components of the output signals [3,4,5,6,7,8,9]. The 180 • phase difference can be provided with a transformer.…”

“…An inverting transformer, as shown in Fig. 1 (a), enforces 180 • phase difference between the potentials of the common source nodes (V a , V a ) [4]. However, the on-chip transformer occupies large chip area and its limited quality factor can cause degradation of the phase-noise.…”

“…However, the on-chip transformer occupies large chip area and its limited quality factor can cause degradation of the phase-noise. coupling ( [4,5]), (c) with direct coupling [7].…”

“…Depending on their mechanism of operation, LC-QVCOs can be categorized in two main groups: first harmonic QVCOs [2] and superharmonic QVCOs [3,4,5,6,7,8,9]. In most of the QVCOs in the former group, oscillation frequency deviates from the LC-tank resonance frequency, leading to phase-noise degradation [11].…”

Very low noise current- shaped optimally coupled CMOS LC quadrature VCO

“…Although S-QVCOs can exhibit very good FOMs (Figure of Merit), they are incapable of providing very low phase noise, because for large oscillation amplitudes, transistors operate in the linear region for most of their working cycle. Another class of QVCOs employs second harmonic to couple two VCOs [4]. While these QVCOs show very good phase noise, superharmonic coupling is not firm enough and generally suffers from large phase error.…”