A Novel Arbitrary Terminated Unequal Coupler With Bandwidth-Enhanced Positive and Negative Group Delay Characteristics

A novel and generalized planar coupler with four independent arbitrary terminated resistances, unequal power division, and wideband positive and negative group delays (NGDs) is proposed. The proposed asymmetrical circuit configuration includes six-section of transmission lines (TLs) and two coupled-line sections with series resistors. Six-section TLs have inherited the free impedance-transformation capability of the four independent arbitrarily terminated resistances. The combination of series resistors and short-circuited coupled-line sections enhances the NGD bandwidth and improves the flatness of NGD response. The analytical closed-form equations with reasonable practical ranges of the four terminated resistances are provided and discussed. For experimental demonstration, the proposed theory and predicted performance are verified by two fabricated examples (examples A and B) based on the microstrip technology. The measured results indicate that the coupler is able to realize good matching, excellent isolation, required NGD bandwidth, and acceptable NGD flatness. Example A (B) with different specified terminated impedances achieved an NGD bandwidth of 540 MHz (550 MHz) and 330 MHz (320 MHz) of flat NGD response with a center group delay of -0.48 ns (-0.49 ns) and ±8% NGD fluctuation that is acceptable to be considered as a wideband flat NGD response.