Note that for measurements between 150KHz and 30MHz, only the
50uH/8uF/5R filter section is required. If you are anticipating
problems around 150KHz, the second section is advised in order to
reduce measurement complications due to the nearby self-resonance of
the simpler network.
The lower frequency section is normally intended only for measurements
extending down to 9KHz, which were not covered by CISPR. If you are
playing it safe and using air-cored inductors, the extra effort
required to produce it with appropriate materials is not
insignificant.
The use of a switch, or any 'power-on' fiddling with spectrum analyser
signal input routing, is inadvisable, but unmonitored lines should be
50ohm-terminated. The actual output coupling C value in the standard
is 0.25uF. Any in-house-built networks tend to raise warning bells and
eyebrows, now-adays. Calibration stickers and certificates of
compliance......yadda......yadda......yadda.
RL