Isolator (circulator) tuning

How to go about Isolator (circulator) tuning

An Isolator (or what is essentially a circulator with a terminated port) is used in a TX path to stop any significant RF power going back into the transmitter from another part of the system.
The basic isolator consists of ferrite material arranged in a magnetic field so that RF energy is conducted in one direction with low loss but highly attenuated in the opposite direction.
A basic diagram is shown below

Diagram 1

isolator1

Because the Isolator is built with magnetic properties RF passes from port to port in a Clock-wise circular motion.
So, the power passes from the IN to the OUT with minimal loss.
However any RF power coming from the OUT port will go clock-wise to the LOAD (or third port if being used as a circulator)
Therefore dissipating without harm.

On this diagram the Isolator has 2 adjustments on each port, some isolators may have only one on each. (Some maybe fixed values, in which case adjustment is not possible and any specification should be kept to as regards band pass of the IN to OUT port)
The LOAD value is decided by the maximum amount of power likely to be reflected if a fault occurs.

Tuning

Isolators will normally be Factory tuned to a specification at an exact frequency and have a 2.5% bandwidth.
For retuning a Network Analyser should be used. The LOAD port should have a 50ohm load attached too.

1. Connect a signal source to the OUT port of the isolator and monitor the signal on the IN port.

2. Adjust C6 (see diagram) so that the notch is on the required frequency.
If no notch is visible adjust C5 until it is visible and has maximum attenuation, then re-adjust C6 as required.

3. Adjust Capacitors C3 & C4 for best SWR reading.

4. Reverse connections so the signal is on the IN port and monitor the OUT port.

5. Adjust C1 & C2 for minimum insertion loss and best SWR on the required frequency.

The loss should be minimal somewhere around 0.2dB and the attenuation in reverse for a single isolator is around -30dB (check specification of isolator)

** NOTE of Caution ***
Because the components of the Isolator are Magnetic, positioning to other ferrous metal objects can affect the alignment once mounted

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