The proper balance: Overview of microstrip wideband balance circuits with wideband common mode suppression

With the development of wireless communication technology, radiofrequency (RF) circuits and integrated circuits are becoming more complex and packing more functionality and signals into an ever closer space, with a high level of electromagnetic interaction between circuit nodes and interference/crosstalk from substrate coupling and free space [1]. Balanced/differential circuit technology has become more important in modern communication systems because of good commonmode rejection that leads to relatively high immunity to environmental noise when compared with the single-ended technology [1]-[3], as shown in Figure 1(a) and (b). For balanced circuits, the differential mode should have excellent out-of-band rejection and high selectivity for the desired frequency band, while the common mode should be suppressed over a wider frequency band. In the past few years, many microstrip balanced filters for single band and dual band with common-mode suppression have been realized [4]-[10], with different balanced networks, balanced driven antennas, balanced amplifiers, and mixers [11]-[18].