Duplexer isolation: limits of instrumentation
IntroductionWith 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 analyserA 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.