You can see the noise floor is a fairly flat -154dBm/Hz across the AF band.
Yeah - bipolar technology, using rf-transistors (with very low intrinsic base resistance) makes for a good noise matching at low impedances like 50 ohms – and a low 1/f corner frequency.
High impedance FET input stages are noisy under such conditions. On the other hand, high impedance inputs are much more versatile. We can adapt them to any impedance we like by means of a pass through terminator (at least at low frequencies).
Meanwhile I’ve experimented a bit further and detected at least two flaws in my previous noise measurement:
1. The input was AC coupled by accident, which of course increases LF-noise significantly.
2. The input had a 50 ohm through terminator fitted, but since this scope is sensitive to the source impedance, an additional 50 ohm end terminator should be used to complete the 50 ohms setup.
Now look at the screenshot attached.
Pico4262_Noise_5MHz_D50kHz
I’ve tried to resemble your settings as close as possible but still kept the total FFT bandwidth at 5 MHz in order to keep the high frequency noise out of the LF region. Display units are dBm now for better comparability. Frequency step is 38.15 Hz, which is equivalent to a RBW of 112 Hz with the Flat-Top window – so noise levels will read slightly higher than in your setup.
With a noise level of -134.7 dBm this is very comparable to your RSA 3408A above some 30 kHz.
At 1 kHz, the FET input goes up by 17.8 dB to -116.9 dBm, but obviously stops at -114 dBm with this RBW.
So I’m confident to claim that the Pico 4262 has the same low noise in a 50 ohm system, as long as you keep the input DC-coupled and stay above 30 kHz.
EDIT: I have updated my original posting, where you can also see the updated noise density plot.