Hm. Do you operate the Schottky diode with a huge backward voltage,
so that it breaks down and is really used as an avalanche diode? That's very un-Schottky-ish :-)
That would have a lot of dissipation for the small junction, even if the small junction would
make it wideband.
Schottky diodes when used as a Schottky ring mixer for example do not generate much noise;
the noise figure is not much worse than the mixing loss, maybe 7 dB, most of which is loss.
I did measure my diodes in forward direction to create bias voltages.
You need an output attenuator anyway to get an acceptable return loss. That is much more
important than a flat spectrum. You can calibrate away the varying spectrum, but a bad return
loss makes the source unusable even as an indicator, For sub-dB noise figures you won't
get far with a 15 dB source. That's why I want to have my Agilent 15 dB source calibrated
with that Rosenberger PC3.5 precision attenuator.
DJ9BV RIP has written an excellent article on that in DUBUS some 20 years ago.
Found one on the net, there are more:
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http://f1chf.free.fr/hyper/article%20DJ9BV%20from%20F6DRO.pdf >