And if one is left with only internal noise, why not simply use a nice clean source like a Zener diode?
Quite solidly agreed. Internal noise beats external noise, and there is no point in using a radio receiver when what you want is internal noise.
It may have been misleading to refer to a radio receiver chip: the noise generated by the chip will be entirely internal. It doesn't depend on any external signal "sneaking in"; the noise comes from the effective resistance of the first amplifier stage. Nothing is being "received", in some sense; it's just a big hunk 'o gain. A Zener diode would be fine, but the design is a bit more complex, especially for wide bandwidths---you need to mess with biasing voltages, speedy op-amps, interface to CMOS, and all that jazz. You'd need external power (or a DC-DC converter). I wanted a super-simple design that people could just plonk down on a PC board. Two chips and some bypass capacitors. Peter