A new robust capacitively coupled second harmonic quadrature LC oscillator

Abstract

A study of some reported superharmonic LC quadrature voltage-controlled oscillator (LC-QVCO) is performed in which it is shown that robustness of the quadrature oscillation varies depending on the coupling configuration. Next, a new superharmonic LC-QVCO is proposed in which the common source node in either of two identical cross-connected LC-VCOs is coupled via a capacitor to the node common between the two varactors in the LC-tank of the other LC-VCO. As a result of connecting common mode nodes, the currents flowing through the two coupling capacitors are comprised of only the even harmonics. In the proposed coupling configuration there exists a closed loop through which the second harmonic signals circulate. A qualitative argument is presented to justify the robustness of the quadrature nature of the proposed QVCO by applying the Barkhausen phase criterion to the second harmonic signals in the loop. Since the coupling devices are only two capacitors, no extra noise sources and power consumption are added to the core VCOs. A Monte-Carlo simulation showed that the phase error of the proposed QVCO caused by device mismatches is no more than 1°. Also, generalizing this method to several numbers of VCOs in a loop, multiphase signals can be generated. The proposed circuits were designed using a 0.18-lm RF CMOS technology and simulation results are presented.

CMOS second-harmonic quadrature voltage controlled oscillator using substrate for coupling

“…In configuring this circuit the analysis of the superharmonic QVCOs presented in [16] was considered. In [16] it is suggested that the presence of a loop through which the second harmonic signals can circulate leads to the generation of quadrature signals. Based on this idea, the common mode nodes of the two identical core LC-VCOs denoted in Fig.…”

“…receivers and transmitters operating based on the direct modulation and demodulation, respectively [1,2]. Different methods for generating quadrature signals have been introduced so far, some of which are: poly-phase RC-CR filters [2], master-slave flip-flops [2,3], ring-oscillators [4], and coupled LC-oscillators [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. QVCOs based on coupled LC-oscillators are popular in RF circuits because of their relatively low phase noise.…”

“…QVCOs based on coupled LC-oscillators are popular in RF circuits because of their relatively low phase noise. The LC-QVCOs are mainly built by coupling two identical cross-connected LC-VCOs which here will be called the ''core'' VCOs [7][8][9][10][11][12][13][14][15][16][17][18][19]. The circuits of the cross-connected LC-VCOs have a symmetric topology with two outputs having 180°phase difference with respect to each other which are frequently used in RF circuits [6,20].…”

“…2nd, 4th …) to the other one, giving rise to the name ''superharmonic'' QVCOs [12][13][14][15][16][17][18][19], which sometimes are also called ''second-harmonics'' as it is the dominant harmonic injected. As mentioned before, the core oscillators are symmetric circuits and therefore nodes and branches on the axis of symmetry of these circuits contain only even harmonics signals.…”

“…In one subgroup the 180°phase difference is enforced between the second harmonic signals in the two core VCOs due to the coupling network e.g. a transformer [14,22]; or due to the presence of a loop through which the second harmonic signals circulate [16].…”