For a low-band Spectrum Analyzer, the input signal will necessarily be applied to the IF port, which is DC-coupled. The RF and LO ports are transformer-coupled, and typically operate at microwave frequencies.
The normal range of the 7L14 is from 10kHz to 1.8GHz. The 1st
Local Oscillator runs at 2.095 to 3.895GHz. When running at
2.095GHz, frequencies around 0Hz are modulated by the switching of the
diodes and the associated transformers to become 2.095GHz signals which
are tuned in by the IF (Intermediate Frequency) system which is centred
on this frequency. As the 1st LO is stepped up in frequency, the
lower sideband of the input signal, where 0 Hz is stepped up to the 1st
LO frequency, is the one of interest. For example if the 1st LO
is running at 2.195GHz, then input signals in the 0.1GHz range, in the
lower sideband, are now at the 2.095GHz frequency which is tuned in by
the 1st IF system.