[G0HZU_JMR]

Yes, there are ways to extend the range of the test gear. Using BPF and BRF can work well.

Luckily for me, I rarely have to measure IMD terms that are required to be lower than about -80dBc. When I need to measure IMD this low, this is usually done at the IF output port of a downconverter, and this can be in the range of maybe 20MHz to 300MHz. Usually, it's in the range of 50MHz to 150MHz. I've been doing this stuff for decades and in my experience, it's fairly easy to achieve -100dBc IMD from the sig gens and combiner when using the simple test setup I showed earlier. This is all I need in order to measure -80dBc IMD with low uncertainty. The tool of choice at work for the last 20 years has been the Agilent E4440A PSA analyser and in this frequency range it has 104dB SFDR with 30Hz RBW. I'm lucky in that I've also got one of these analysers here at home.

I've never seen an issue when using a 6dB hybrid combiner when trying for -100dBc IMD in this frequency range and this is when producing tones at up to 0dBm each. It might be different if I was trying for tones at (say) 13dBm each. So far, I've never needed to do critical IMD testing with tone levels that high. I'm not sure what the TSC-2-1 can deliver in terms of IMD free test tones, but I'd expect it to be in the ballpark of -100dBc at maybe -10dBm per tone. The last time I played with a TSC-2-1 at 70MHz or 140MHz was about 30 years ago and -72dBc IMD was our usual design requirement in those days.