What Is a Circulator?
A circulator is a non-reciprocal 3-port ferrite device. Power flows in one direction around the ports: Port 1 → Port 2, Port 2 → Port 3, Port 3 → Port 1. The reverse path has high isolation. This property arises from the gyromagnetic resonance of biased ferrite material.
Circulator S-Parameter Matrix
An ideal circulator has the following S-matrix (clockwise circulation 1→2→3→1):
Port 1 Port 2 Port 3
S = [ 0 0 1 ]
[ 1 0 0 ]
[ 0 1 0 ]
All ports are matched (S11=S22=S33=0) and isolation is perfect. Real devices have finite isolation and insertion loss.
Key Performance Specs
| Parameter | Ideal | Practical |
|---|---|---|
| Insertion loss | 0 dB | 0.3–0.8 dB |
| Isolation | ∞ dB | 18–30 dB |
| Return loss | ∞ dB | 20–30 dB |
| Bandwidth | — | Octave typical |
| Power handling | — | 1 W to 100 W |
Isolator: A 2-Port Circulator
An isolator is a circulator with port 3 terminated in a matched load. It passes signals in the forward direction (Port 1 → Port 2) with low loss but blocks reverse signals (Port 2 → Port 1) with high isolation. Isolators protect oscillators and PAs from load reflections.
| Isolator Parameter | Spec |
|---|---|
| Forward insertion loss | 0.3–0.5 dB |
| Reverse isolation | 20–30 dB |
| Input return loss | >20 dB |
Verification with RF View
For a circulator (.s3p file):
- S21 = forward insertion loss (should be low, 0.3–1 dB)
- S12 = reverse isolation (should be high, 18–30 dB)
- S31 = next forward path (1→3 should be high isolation)
- S11, S22, S33 = port match (should be <−20 dB)
For an isolator (.s2p file): S21 is forward loss, S12 is reverse isolation. RF View's batch processor lets you compare circulators from different manufacturers or temperature conditions side by side.
Applications
- PA output protection (isolator between PA and antenna)
- Duplexer replacement in high-power radar (circulator)
- Reflection coefficient measurement bridges
- Bi-static radar transmit/receive switching