In my opinion, the only HPAK spectrum/signal analyser that can be expected to work (very) well with the noise figure option and a 5dB ENR HP 346A noise source is the classic E444xA from about 20 years ago.
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The ESA analysers like the E4407B don't have a fully digital IF and the DANL with the preamp is a few dB worse than the E444xA. This all adds to the uncertainty. I definitely don't recommend using the ESA analysers for noise figure stuff unless you opt for the 346B noise source with its 15dB ENR and you don't expect to ever measure any amplifiers with very low noise figures.
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The more modern PXA, MXA and EXA analysers have a very good digital IF but the RF converter ahead of the digital IF isn't in the same class as the converter in the old E444xA analysers. The DANL of the PXA/MXA/EXA is quite a bit worse and the noise floor rises quite a bit by 3GHz or so. So these analysers are disappointing too. They are better than the ESA in this respect but still not close to the performance of the E444xA when it comes to noise figure measurements.
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The E444xA is the only one that can compete with the dedicated noise figure analysers. In some aspects of its performance it is slightly superior to the dedicated analysers and in others it slightly loses out. Either way, there is very little difference between them although the E444xA preamp only covers up to 3 GHz. So this limits the frequency range that it can deliver good noise figure measurements.
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