A circulator is a three port RF device that sends a signal from each port to the next port in one fixed direction. Most RF parts, such as cables, filters and splitters, work the same way in both directions, and the circulator does not. This page explains how a circulator routes a signal, why that is useful between a transmitter, a receiver and an antenna, which hardware types exist, and how to read its specification.

• What does a circulator do ?
• Why we need a Circulator ?
• Types of Circulators
• Typical Specification

What does a circulator do ?

Circulator is a device that flows the input energy (wave, signal) through neighbouring port only in a specific direction. If you see the hardware of circulator, there are many cases where the circulator and the divider look similar. If you compare the circulator and splitter in illustration, it would be as shown below.

As you may notice, in the circulator the input energy p1 follow through neigbouring port only on one side (p3 in this case) whereas in splitter, the input engergy(p1) devicided equally and flow though all the ports regardless of the position of the port.

A circulator and a splitter with the same three ports. The circulator sends all of the p1 input to one port, and the splitter divides it between both.

• Circulator, left: the red arrow enters at p1 and leaves at p3. The path towards p2 carries a red cross.
• Splitter, right: the red arrow enters at p1 and splits into two arrows, one out of p2 and one out of p3.

In most of circulator, any port can be used as an input port and any port can be used as output port. Which port become the output port is determined by which port is the input port. For example, let's take a look at following example. In this example, if you put the input to p1, the energy(wave, signal) goes out of p3 only (not through p2). If you put the input to p3, the energy(wave, signal) goes out of p2 only (not through p1).  If you put the input to p2, the energy(wave, signal) goes out of p1 only (not through p3). It looks as if the energy is 'circling (circulating)' in only one direction.

One circulator fed at each port in turn. The output is always the next port in the same rotation.

• (A): the input enters at p1 and leaves at p3.
• (B): the input enters at p3 and leaves at p2.
• (C): the input enters at p2 and leaves at p1.

The rotation in the picture above can be written as a scattering matrix. For an ideal circulator with this direction, S₃₁ = S₂₃ = S₁₂ = 1 and every other S parameter is 0. Now compare S₃₁ with S₁₃. A passive device without magnetized material is reciprocal, which means S_ij = S_ji. But here S₃₁ = 1 and S₁₃ = 0. So the circulator must be non-reciprocal. Most circulators get this property from a ferrite disk biased by a permanent magnet. In the magnetized ferrite, a wave that rotates one way sees a different permeability from a wave that rotates the other way. This difference makes the transfer depend on the direction.

An isolator is a circulator with one port terminated inside the housing by a matched load. A signal can then pass in the forward direction only. The reverse signal is steered into the load and absorbed. An isolator is often placed after a power amplifier, so that power reflected from a bad antenna match does not return into the amplifier.

• A circulator routes each port to the next one : with this direction p1 goes to p3, p3 to p2 and p2 to p1.
• A splitter divides and a circulator steers : the splitter sends part of the input to every other port, and the circulator sends it to one port only.
• The circulator is non-reciprocal : S₃₁ is not equal to S₁₃, which no passive reciprocal device can do. A magnetized ferrite provides this property.
• An isolator is a circulator with a terminated port : it passes the forward signal and absorbs the reverse one.

Why we need a Circulator ?

One of the most common use case of a circulator is illustrated as shown below. If you connect an antenna to a circulator (p3 in this example), you can make the transmitter signal (TX signal) and the reciever signal (RX signal) in separate port. That is, it can function as a duplexer.

A circulator between a transmitter, a receiver and one antenna.

• The Transmitter connects to p1, the Reciever to p2 and the antenna to p3. The picture spells receiver as Reciever.

You may easily understand how this can work as a duplexer if you draw the path of the energy flow as shown below.

Transmission and reception through the same circulator. The transmit signal goes only to the antenna, and the received signal goes only to the receiver.

• (A) Transmission: the signal enters at p1, turns inside the circulator, and leaves at p3 towards the antenna.
• (B) Reception: the signal from the antenna enters at p3 and leaves at p2 towards the receiver. Nothing leaves at p1.

In my personal use case, I often use the circulator to separate Tx from Rx signal from TDD signal. When you measure TDD signal in spectrum analyzer, it is hard to figure out whether the captured signal is uplink signal or downlink signal. This cause even bigger problem if you want to setup a trigger. If you don't separate TX and RX,  there would be many cases where the trigger you set for a TX signal gets triggered by RX signal and the other way around as well,

A circulator separates the two directions, not the two frequencies. That is the difference from a duplexer built from filters. A filter duplexer in an FDD system passes the TX band to the antenna and the RX band to the receiver. A circulator does the same job even when TX and RX use the same frequency. So it fits TDD systems, radar and full duplex experiments, where a filter cannot separate the two signals.

The separation is not perfect, and two paths leak into the receiver. The first path is the isolation from p1 to p2. For example, with a 30 dBm transmitter and 20 dB of isolation, 10 dBm reaches the receiver port directly. The second path is the antenna mismatch. If the antenna has a VSWR of 1.5, its return loss is 14 dB. The reflected power of about 16 dBm enters p3 and the circulator sends it to p2, exactly like a received signal. So a receiver behind a circulator usually still needs a limiter or a filter, and a good antenna match matters as much as the isolation.

• A circulator separates TX and RX by direction : it works even when both use the same frequency, where a filter duplexer cannot help.
• Isolation sets the direct leakage : with 20 dB of isolation, a 30 dBm transmitter puts 10 dBm on the receiver port.
• The antenna reflection goes to the receiver : a mismatched antenna reflects part of the TX power back into p3, and the circulator routes it to p2.

Types of Circulators

There are many different types of Circulator you can get in the market. You may get the types you would use in ordinary RF (e.g, A,B,C) and those waveguide type for super high frequeny or high power (e.g, D, E)

Circulator hardware. Coaxial units cover ordinary RF, and waveguide units cover very high frequency or very high power.

• (A): four small units, each with three coaxial connectors.
• (B): four boxed units with coaxial connectors.
• (C): one unit with only two connectors, labelled 1 and 2 and 8-18 GHz. A unit with two ports is an isolator: the third port is terminated inside.
• (D): a large waveguide circulator with rectangular waveguide flanges.
• (E): four waveguide units of different sizes.

The choice between these types follows the frequency and the power. A coaxial circulator is small and easy to connect, but the connector and the ferrite limit its power. A waveguide circulator has lower loss and handles much more power. But a rectangular waveguide passes only frequencies above its cutoff, so each waveguide size covers one band. That is why the waveguide units in (D) and (E) come in different sizes. In a phone or a small radio module, a circulator or isolator can also be a surface mount part on the circuit board.

Let's put a number on the waveguide band. A rectangular waveguide with a broad wall width a has a cutoff frequency of c/(2a) for its main mode. The common WR-90 size has a = 22.86 mm, so its cutoff is 6.56 GHz. It is normally used from 8.2 to 12.4 GHz, because close to the cutoff the loss rises, and at twice the cutoff, 13.1 GHz, the next mode can start to propagate. So a waveguide circulator for another band needs another waveguide size.

• Coaxial types cover ordinary RF : they are small and use standard connectors, as in (A), (B) and (C).
• Waveguide types cover high frequency and high power : each waveguide size covers one band, as in (D) and (E).
• A two port unit is an isolator : the third port of the circulator is terminated inside the housing.

Typical Specification

The typical specification for Circulator can easily be understood from s parameter graph as shown below. Let's assume that you want to use the circulator at the frequency range as in orange box.  In some datasheet of the device, the manufacturer would provide plot like this, but sometimes they would provide the same information just as numbers at a certain frequency.

How wide frequency span it can operate as desired circulator : Ideally it is better if it covers infinate range of frequency span as the desired circulator. In every circulator like any other RF devices, it shows the desired performance only within a certain range of the frequency.

How much of the input power/energy/signal (p1 in this example) can be transferred to the desired output port(p3 in this example) : This is indicated by s31 (red line) in this example. Ideally 100% of p1 power should be transferred to p3 (i.e, p1 = p3) but this is impossible in reality. In reality, just the higher, the better.

How little of the input power/energy/signal (p1 in this example) can be transferred to the desired output port(p2 in this example) : This is indicated by s21 (green line) in this example. Ideally it should be 0 (-Infinity in the plot) but in reality, just the little the better.

How little of the input power/energy/signal (p1 in this example) bounce back (reflected back) to the input port(p1 in this example) : This is indicated by s11 (blue line) in this example. Ideally it should be 0 (-Infinity in the plot) but in reality, just the little the better.

Graph Source : Cliick Here

S parameters of a circulator. The device works as a circulator only inside the operating range, where the isolation and the return loss dips line up with the peak of the forward transmission.

• Top: the circulator with a red arrow from input p1 to p3, a green arrow to p2 and a blue arrow reflected back at p1.
• Red dash-dot line: S31 = S23 = S12, the forward transmission. It is highest inside the operating range and falls towards the right.
• Green line with markers: S21 = S32 = S13, the leakage to the wrong port. Its deep dip is the isolation.
• Blue line: S11 = S22 = S33, the reflection at the input. Its dip marks the best match.
• The orange box marks the Operating Range. The vertical axis carries no values, so the plot shows the shape only.

Datasheets give these curves as three numbers in dB. The insertion loss is -20 x log₁₀|S₃₁|, the isolation is -20 x log₁₀|S₂₁|, and the return loss is -20 x log₁₀|S₁₁|. Let's read typical values as power ratios. An insertion loss of 0.5 dB passes 89.1 % of the power to the output port. An isolation of 20 dB lets 1 % of the power leak into the wrong port. A return loss of 20 dB reflects 1 % of the power, which is a VSWR of 1.22.

• Forward transmission should be high : the insertion loss from S₃₁ is the loss the wanted signal sees.
• Isolation should be high : a large isolation from S₂₁ keeps the signal out of the wrong port.
• Return loss should be high : a small S₁₁ means the input is well matched.
• All three are valid only in the operating range : outside it the device stops behaving as a circulator.