Duplexer isolation: limits of instrumentation

Duplexer isolation: limits of instrumentation

Introduction

With high isolation duplexers, about 100 dB, conventional spectrum analysers and tracking generators are beyond their capability of about 80 dB. The rejection of a high isolation duplexer can be checked using a RF signal generator and spectrum analyser.

High isolation duplexers.

I have a couple of sets of four cavity 70 cm pass reject duplexers. The two cavities give about 70 dB rejection, which is ok but more would be better for RX. A third cavity can be added for greater isolation.

Each cavity is individually tuned with a spectrum analyser and tracking generator. The two cavity duplexer can then be assembled and tested. A nice plot with about 70 dB rejection at 438.1 MHz and a pass of 433.1 MHz.

Adding the third cavity results in noise at the rejection notch. The spectrum analyser cannot show the full depth of the notch; it does not have enough dynamic range.

RF generator and spectrum analyser

A RF generator can be used instead of a tracking generator as it has a greater output at the TX or reject frequency.

When connected with two cavities there is quite a spike.

 However, when the third cavity is added the spike almost disappears with the extra 35 db rejection.

Conclusion

A RF generator can supplement a spectrum analyser and tracking generator when aligning high isolation duplexers.