Short note: Microwave circulators

A circulator is a multi-port (usually 3- or 4-port), non-reciprocal microwave device in which power entering any port is transmitted only to the next port in a fixed rotation (1 → 2 → 3 → 1) and not to the others. Non-reciprocity is produced by a magnetized ferrite placed at the junction.

           port 2
             |
          +--+--+
          | /-> |
 port 1 --| | F | --- port 3
          | <-/ |
          +-----+
     F = biased ferrite disc
     1 -> 2 -> 3 -> 1

S-matrix of an ideal 3-port circulator:

       | 0  0  1 |
 [S] = | 1  0  0 |
       | 0  1  0 |

• All ports matched (Sᵢᵢ = 0), lossless (unitary) and non-reciprocal (S21 ≠ S12).
• This agrees with the rule that a matched, lossless 3-port must be non-reciprocal.

Working principle: In the Y-junction circulator, the ferrite disc biased by a dc magnetic field splits the field into two counter-rotating modes with different propagation constants (because the ferrite permeability differs for the two senses of rotation). The disc size and bias field are chosen so that the two modes add at the next port and cancel at the remaining port. A 4-port circulator can be made with two magic tees and a non-reciprocal (Faraday rotation) phase shifter.

Performance parameters: insertion loss (typ. 0.3–0.5 dB), isolation (20–30 dB), VSWR and bandwidth.

Applications:

• Duplexer in radar and transceivers: transmitter → antenna → receiver using one antenna.
• Isolator: a circulator with port 3 terminated in a matched load, used to protect sources from reflections.
• Reflection-type (negative resistance) amplifiers such as tunnel diode and parametric amplifiers.
• Separating incident and reflected signals in measurement systems.