Isolating Bandpass Filters Using Time-Modulated Resonators

Abstract

In this paper, we demonstrate, for the first time, an isolating bandpass filter with low-loss forward transmission and high reverse isolation by modulating its constituent resonators. To understand the operating principle behind the device, we develop a spectral domain analysis method and show that the same-frequency nonreciprocity is a result of the nonreciprocal frequency conversion to the intermodulation (IM) frequencies by the time-varying resonators. With appropriate modulation frequency, modulation depth, and phase delay, the signal power at the IM frequencies is converted back to the RF frequency and adds up constructively to form a low-loss forward passband, whereas they add up destructively in the reverse direction to create the isolation. To validate the theory, a lumped-element three-pole 0.04-dB ripple isolating filter with a center frequency of 200 MHz and a ripple bandwidth of 30 MHz is designed, simulated, and measured. When modulated with a sinusoidal frequency of 30 MHz, a modulation index of 0.25, and an incremental phase difference of 45°, the filter achieves a forward insertion loss of 1.5 dB and a reverse isolation of 20 dB. The measured nonmodulated and modulated results agree very well with the simulations. Such nonreciprocal filters may find applications in wideband simultaneous transmit and receive radio front ends.

Frequency Tunable Non-Reciprocal Bandpass Filter and Diplexer Using Time-Modulated Microstrip λg/2 Resonators

“…Similar to previous works in STM-based non-reciprocal circuits ( [21], [22], [35]), the f m -f RF isolation in [36] solely relies on the indcutor L m . In contrast, this work takes advantage of the virtual ground in λ g /2 resonator to present a biasing scheme that is much more effective than [36] in terms of f m -f RF isolation, isolation bandwidth, and simplicity.…”

“…The filter and diplexer are designed on Rogers 4003C substrates with a thickness of 0.813 mm, a relative permittivity of 3.55, and a loss tangent of 0.0027. As illustrated in [35], the design of non-reciprocal filter can start with designing a conventional reciprocal filter of static specifications. The final non-reciprocal filtering performances can be achieved by applying proper time-modulation with a modulation phase difference (∆ϕ) between the adjacent resonators.…”

“…Existing works on spatio-temporal modulation (STM) based non-reciprocal components are predominantly based on modulating lumped-element resonators [21], [22], [35]. Both series As illustrated in Fig.…”

“…In addition, it is important to realize that f m is approximately equal to the inherent static RF bandwidth BW of the non-reciprocal filter [35]. A larger static BW together with higher f m can provide wider non-reciprocal bandwidth.…”

“…In [35], we demonstrate a novel non-reciprocal bandpasss filter using spatio-temporal modulation of a lumped-element coupled-resonator filter. A full-spectra domain method is developed to analyze and synthesize the time-varying network properties.…”