Yes, something must supply the minimum magnetic field to activate the
oscillator - but the current determines the field, so it's
necessarily part of the drift characteristic. It also depends on
whether the YIG sphere is heated - usually to 80 deg C or so, to help
stabilize it.
Without any other frequency control, you can expect it to stay within
about +/- 10 MHz of the frequency predicted by its tuning curve. It
is very linear with magnetic field, hence tuning current, until the
core material begins to saturate at high flux levels, so the tuning
current needs to be increased to compensate.
The most common type is one-octave 2-4 GHz, used in spectrum
analyzers from the 1970s on, and also the first microwave band of
many generators. Another common one is the 2-6.X GHz also used for
the same purposes, but covering a wider direct range. The next bands
up are typically 4-8, 6-12, 8-12, and 12-18 GHz, used in generators,
at least in older gear. You can expect to find about +10 to +18 dBm
output power range, and fairly flat with frequency.
The minimum setup is one or two power supplies, plus a variable coil
current driver up to one amp or so. You don't need the heater to run
it. Don't bother with the FM coil - it's not needed for basic
experimenting, and it's easy to burn out if you screw up. The main
coil is more robust, but it can be burned out with too much current,
so don't go too far beyond the maximum tuning current spec. It can
also shock you from inductive kickback, so the coil needs clamping
too. It's best study the circuits of existing sweepers and such to
get ideas on how it's all done.
Your best bet is to get an old HP8620 or Wiltron 610 sweeper with
some plug-ins. They are simple enough to dig into the guts and
modify/experiment. These are regular old analog sweepers with no
synthesis - look at their specs to see what kind of stability can be expected.
Ed
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