[G0HZU_JMR]

See below for some plots of my old homebrew external preamp box. This was what I used for many years with my Advantest TR4172 and HP 8566B spectrum analysers for noise figure measurements. I'm lucky to own a newer analyser now that has an internal preamp and it can also mimic a noise figure analyser as it has the relevant hardware and software options fitted. So this preamp rarely gets used.

It's old and a bit battle scarred but this preamp was designed for about 3dB noise figure and flat 32dB gain from 1MHz to about 1GHz. It has been repaired and tweaked several times over the years and it does work very well in my opinion.

A key requirement for a noise figure preamp is that it should have low input VSWR and high reverse isolation. This preamp has ultra low VSWR as you can see in the plots below. The noise figure is about 2.7dB across 10MHz to 1GHz. The reverse isolation isn't shown below but it is about 55-60dB.

It isn't necessary to have something this good but you should be able to make an analyser preamp using a couple of low cost MMIC amps from Minicircuits. In your case I think you only need about 25dB gain in the preamp. I needed more because the HP8566B doesn't have a preamp. The Advantest TR4172 does have a preamp but its noise figure was too high. I also wanted very low input VSWR for the preamp as this minimises mismatch uncertainty effects.

Note that the analyser source power is set really low at about -35dBm to prevent any compression effects. So the traces are a bit noisy.