Signal Generators are usually very expensive and not easily affordable for the average user.
A cheap generator can still be built using a Wideband Frequency Synthesizer like the ADF4351, which can output frequencies from 35-4400 MHz. The internal VCO runs from 2.2 to 4.4 GHz, the range from 35-2200 MHz is generated by an RF divider and therefore no sinewave without additional filtering. But as long as you are not using it as signal source for a network analyzer, it should be ok.
Evaluation Boards are available from China for under $20 delivered. They are equipped with a 25 MHz reference crystal, differential output, a connector for an external reference and a 3.3V regulator for both digital and analog supply.
The board can be controlled with pyadf435x which provides a command-line tool, a python library and the microcontroller firmware for programming the synthesizer. The physical connection is established via USB to an STM32F103 "Blue Pill" which sets the registers on the ADF4531.
With 100 MHz set, the output is about 800 Hz too low as shown in the Picture. The measurement has been taken by an HP 53131A counter with external GPSDO reference.
Phase noise can be reduced by adding additional 1000 uF capacitors to the AVDD and DVDD rail after the voltage regulator. Even lower phase noise can be achieved by using separate ultra low noise voltage regulators for AVDD and DVDD. More details and measurements are available here and here.