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.