I recently discovered HP 8780A generator, one of the first vector generators made by HP, that is still in many ways more powerful than modern ones, but has not been discussed here before. It has several unique modulation capabilities and can be found for less than $200, roughly one thousandth of its original price. It generates a signal near 10 GHz and then mixes it down, allowing for wide frequency and amplitude modulation.
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Here are a few examples of its capability:
Wide vector modulation bandwidth >300 MHz. Uniquely, it has a parallel interface for clockless digital vector modulation in addition to analog vector modulation.
Wide frequency modulation up to 250 MHz pp, even at low frequency. For example, frequency can be changed quickly by a factor of 6, from 10 to 60 MHz.
Simultaneous frequency, phase, scalar amplitude, vector amplitude and burst modulation, if one desires.
Here are some example screen shots:
Vector amplitude modulation at 300 MHz of a 600 MHz signal. For comparison, also 30 MHz modulation of a 60 MHz signal is shown.

Frequency modulation from 10 to 60 MHz and from 77 to 322 MHz, measured with HP modulation domain analyzer.

Burst modulation of 1 GHz signal at 100 MHz.

Measurements of burst extinction ratio. Here it is a little weak with only 50 dB extinction and some transient tail. The main signal is 500 mVrms here.

The generator also has additional hidden features with "special" commands. It survived until Agilent days, there are application notes summarizing its capabilities. It is a good option for a low-cost "agile" signal source, while new ones from Keysight still cost 1000 times more.