How to Tune a Duplexer Cavity Filter

How to Tune a Duplexer

What a duplexer does

To Know how to tune a duplexer you first need to understand a duplexer’s job. Generally it is to join a base transmitters Tx and receive Rx together without any damage being caused by overloading the receiver and allowing one Aerial or antenna to be used. This is achieve by way of tuned cavities with adjustments to create passing as much as possible at one frequency while rejecting the other as much as possible and the reverse of each frequency at the other port. Which is why you need to tune a duplexer to specific frequencies.

Equipment needed

Generally the minimum you should have to tune a duplexer is a spectrum analyser and a tracking generator. For better results a network Analyser is preferred.

It is possible to align a duplexer using a signal generator and a receiver, this should only be attempted by someone who can visualise what is happening from previous experience of tuning on the above mention equipment and if the frequencies are of a very similar split between them as originally tuned. NEVER attempt to adjust a duplexer with the real transmitter attached, this could cause high SWR to this transmitter causing it to fail, or the RF may arc across the insulator in the duplexer and destroy it. Essentially, tuning is a specialist job best left to someone who has the right Knowledge & equipment

Tune a Duplexer

    • First connect the signal out of the analyser to either the high or low port.
    • Connect the analyser input to the antenna port(or combined port).
    • Terminate the left over high or low port with a 50 ohm dummy load.

how to tune a duplexer pass low reject high

    • The cavities on the side you are connected to should be adjusted to get maximum rejection at the opposite frequency to the port (if high port, tune to low frequency and visa-versa).
    • Then do the same while connected to the other half of the duplexer.

how to tune a duplexer pass high reject low

Once you have done both sides you should note the amount of loss you have on the other frequency (typically 1dB but depends of duplexer type and specification).

If you have the facility on the test equipment you also need to check the SWR on the passing side at the same point. Most duplexer’s are made for a specific split between High and low (or TX and RX) and should be within this specification or the SWR may not ever be low enough.

    • If needed you should adjust for low SWR typically below 1.5:1 (remember to go back and check other readings too and adjust as necessary)

This varies on different duplexer’s so you need to find out how this is achieved on your chosen type.
All the adjustments change everything else, albeit slightly in most cases, so always check back and forth with connections and readings afterwards.
This principle works for pretty much any duplexer, be it large barrel cavities to much smaller units, all that generally changes is the size and how they are adjusted.

This is how to tune a duplexer.

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