Substrate effects in monolithic RF transformers on silicon

Abstract

Abstract-The effect of substrate RF losses on the characteristics of silicon-based integrated transformers is studied experimentally by using a substrate transfer technique. The maximum available gain is used to evaluate the quality of transformers similarly to that of active devices. The silicon substrate has a pronounced effect on the quality factor and mutual resistive coupling factor of the primary and secondary coils, thereby degrading the maximum available gain of the transformer. A highly structured patterned ground shield is shown to improve the maximum available gain of a transformer at high frequencies, while at low frequencies, it has little effect on the maximum available gain and even degrades the quality factors of the transformer coils. It is shown that the low-frequency degradation of the coil quality factors relates to local eddy currents in the patterned metal shield.Index Terms-Eddy-current losses, etching, ground shield, integrated transformer, maximum available gain, mutual coupling, periodic ground pattern, quality factor, substrate effect, substrate transfer.

Experimental study of integrated tunable transformer

“…The phase shift achieving maximum Q p is near that achieving the maximum G MSG for the tunable transformer. This results indicate that maximizing the Q p leads to a higher G MSG , which agreed with the simple analysis of maximum available gain discussed in [8]. This, however, can be disadvantage of tunable inductors [5,6] because the power coupled between the inductor and other components [9] can be larger than conventional inductors when the quality factor of the tunable inductors is maximized.…”

“…The G MSG , Z in and the Q P of the transformer are tuned by varying the relative phase of power flowing through the transformer and the coupled inductor. Note that G MSG , a figure of merit for a potentially unstable component [7], is a useful measure to characterize the tunable transformer performance rather than maximum available gain used for passive RF elements [8]. Figure 2 shows a test setup to measure the characteristics of the tunable transformer.…”

Surface-passivated high-resistivity silicon as a true microwave substrate

“…The measured coupling factor was for this particular transformer. It is important to note that transformer losses in the passband of these structures are caused both by losses in the metal windings and in the conductive silicon substrate [22], [24]. A more practical figure-of-merit for use by the circuit designer than the factor is the loss under conjugate matching conditions at both the primary and secondary windings.…”

Comparative investigation on various on-chip center-tapped interleaved transformers

“…The power-transfer efficiency is defined as the ratio of the power delivered to the load to the input power. As indicated by Ng [5], such a ratio is dependent on the termination impedance and a more favorable choice of figure-of-merit for the integrated transformers' efficiency is its maximum available gain G max , given by…”