I think Marko is referring to a 'digital' noise source. I've got various digital noise sources here and some are built into vector signal generators.?
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For example, an additive white gaussian noise generator (AWGN) can generate noise quite accurately. These can be homebrew or you can buy them. The AWGN feature in my sig gens can be set to have a random seed and then it produces a long random sequence of noise. It does repeat itself after a while but the repeat rate can be controlled in the menu. It's possible to define the bandwidth and the power level of the noise. A decent attenuator would be needed in order to get the level down to a very low level. A typical RF sig gen can do this with it's internal step attenuator.
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I think the main niggle with using the 346A below 10MHz is the low ENR it offers and this gets lower at frequencies below about 2 or 3 MHz. The noise floor of a typical RF/Microwave spectrum analyser will gradually rise below 10MHz and this increases the overall measurement uncertainty when trying to measure noise figure with a low ENR noise source. The noise figure analysers are only spec'd down to 10MHz and I'm only aware of one HPAK RF/microwave spectrum analyser that can work with the 346A below 10MHz with reasonably low uncertainty. Even then, it isn't an ideal combination.
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