Tony, have you read the theory of operation for the 83590A? In particular the frequency tuning section?
The band nos. correspond to the harmonic multiplication that occurs in the YTM.
Band 1,? ?2.0-7.7GHz? no multiplication, 2.0-? ? ? ? ? ? ? ? ? 7.7GHz output. The YTM is not used ,? ? ? ? ? ? ? ? YIG only.
Band 2,? ?3.5-6.75GHz YIG tuning,?
? ? ? ? ? ? ?x2 multiplication, YTM output 13.5GHz.
Band 3,? ?4.5-6.67GHz. YIG tuning,?
? ? ? ? ? ? ?x3 multiplication, YTM output?
? ? ? ? ? ? ?12.5-20GHz.
Band 0 would be for 10MHz to 2.4GHz output on several plugins. It is a down converted band.
Band 4 would be for 18.6-26.5GHz output on a few plugins. As you surmised.
There is band overlap so that when you have a wider sweep near a band edge the sweeper can pick the upper or lower tuning band, so that it only has to sweep one band, not across 2 bands. It can sweep across all three (or 4 bands, or more if needed). There is a slight pause in the sweep to allow for band edge crossing delays and settling time, when you sweep across band edges.
Don Bitters?
