LO feedthrough
Generally RF spectrum analyzers allow an LO frequency value as low as the first IF.
Because there is finite isolation between the LO and IF ports of
the mixer, the LO appears at the mixer output.
When the LO equals the IF, the LO signal itself is processed by the system and appears as a response on the display, as if it were an input signal at 0 Hz.
This response, called LO feedthrough, can mask very low frequency signals, so not all analyzers allow the display range to include 0 Hz.