I dug out and tried the original external preamp on the RTSA (Tek RSA3408A) and then fitted the 346A at the input. I then flipped between hot and cold states. See the screenshots below. You can see there is a flat response from 10MHz down to maybe 3MHz and then there is some obvious loss in noise level below 2MHz. This is essentially the same test I did a few years ago.
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This is about the same as I saw when I did this last time. I probably also (further) tweaked the ENR table in the noise figure analyser menu after I did all this. I can't remember now as I did it quite a few years ago when I first got the noise source and the associated analyser that can measure noise figure. I think this was back in late 2019.
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The first plot shows the cold state and the noise floor is really flat. The noise floor is just as flat for just the raw analyser (without the preamp) but obviously at a much lower level. The typical noise floor is about 20dB lower without the preamp.
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The second plot shows what happens when I turn on the HP 346A. It looks like the droop in noise level starts below about 3MHz and this gets worse by 1MHz.
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I just spotted that I should have calibrated the analyser just before running these plots as it's possible to reduce the dc spike with a calibration. However, this doesn't really affect the results.
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If anyone is interested, I can share the design of the BJT stage although it might be better to wait until I tweak it for (very) low port VSWR.
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The MMIC stage is just a Minicircuits eval board but I need to change the biasing choke and optimise the biasing for best performance.
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It may be the case that the result will change once I tweak the preamp for lower VSWR and a flatter response but I doubt it will change a lot.
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The input VSWR of the analyser is impressively low even when set to its most sensitive range because it is set to act as a baseband analyser in the plots below. So it is a bit like a 40MHz soundcard with a 50R input.
