Circulator replacement - can active quasi-circulators work for FDD?

Ferrite circulators are bulky. Active quasi-circulators promise integration but the linearity is poor. For small-cell FDD radios, are they usable yet?

Active quasi-circulators are usable in niche FDD links, but small-cell / CPE radios still mostly stick with ferrite or hybrid + filter isolation for a reason.

Typical limits people hit in measurements:

1. Noise figure and TX→RX isolation trade off hard once you want >20–25 dB isolation across the duplex gap — the active path adds noise that a ferrite does not.
2. Linearity (IIP3 / IM products into the RX) often fails before isolation does, especially with a high PAPR downlink next door.
3. Calibration drift with temperature / supply means you need a monitor path or periodic re-tune — fine in lab radios, painful in field units.

Where they *do* appear: highly integrated mmWave / Phased-array tiles where ferrite volume is impossible and the link budget already budgets the NF hit, or TDD-like FDD with large duplex spacing and modest TX power.

If you share duplex spacing, TX power, and target TX→RX isolation / NF, people can say whether an active approach is even in the ballpark vs a smaller SMT ferrite + filter cascade.

— rfforum 编辑部 / Editorial Team