For different purposes, like tuning a FM radio, I looked for a signal generator to provide a 100 MHz FM modulated signal. The Rigol DG800- and DG900-series of function generators ranges from a one-channel 10 MHz to a two-channel 100 MHz model. BUT the hardware is identical of the generators, it seems to be a matter of software setup.! Some clever people found a way to tweak an affordable DG811 to perform as an expensive DG992, it is described in this blog: eevblog_Rigol_DG800_tweak . I made the tweak work in this way to upgrade a Rigol DG821 to a DG992 model:
Hardware needed: A PC with Windows 7 or newer. A PC with an USB socket, able to run Linux commands. An older USB-stick, USB 2.0 with min. 4 GB capacity formatted with FAT32. An USB A-B cable.
Software needed: Rigol Ultra Sigma v. 00.01.06.01 installed on the Windows PC. Firmware files available via Rigol official download pages and file links in the eev blog mentioned above. NOTE and save the generator original serial number via the Utility -> System info menu .
- Use Windows Diskpart to format the USB-stick with FAT32. I used the Linux command: mkfsdos /dev/sdxy -s 8 -F 32 . Replace sdxy with the Linux USB-stick device name.!
- Via the eev blog, get the file: DG800_900_downgradableFW1.08.01.zip . It contains the file SardineUpdate.GEL , that is copied to the USB-stick.
- Insert the USB-stick into the USB socket at the rear side of the generator. Confirm the firmware change when the pop-up appears on the generator screen. Remove the USB-stick when the generator reboots. After rebooting you will get a system info screen like this:
The following steps 4..8 are to set Model and Serial number in the internal generator configuration:
4. Delete the SardineUpdate.GEL file from the USB-stick.
5. Via the eev blog, get the file: DG800_sardinha.bin. Start Linux to insert the contents of the file DG800_sardinha.bin at USB-stick sector 2000000. The file contains the binary value of: 0B0A3B2E5F4CECBE ( in hexadecimal notation ). Start Linux with the USB-stick inserted ( in this case device /dev/sda1 ) and use the following Linux command: sudo dd if=DG800_sardinha.bin of=/dev/sda1 bs=512 seek=2000000
6. Insert the USB-stick into the USB socket at the rear side of the generator, and connect a USB A-B cable between the generator and the Windows PC. Start the Rigol Ultra Sigma program:
7. Right-click the yellow line and choose SCPI Panel Control. Enter :PROJ:MODE DG992 and hit the Send Command button. Enter :PROJ:SN the original serial number and hit the Send Command button. The original serial number is exactly the string you noted in the beginning. In the following pictures the serial number of my function generator is masked out ( just DG8A is shown ). 
8. Repower the generator and verify the generator model and serial number.: 
The remaining steps are to upgrade the firmware to the latest revision:
9. Via the eev blog, get the file: Finish_Upgrade_2.04.zip . It contains the file SardineUpdate.GEL , that prepares for the upgrade to the firmware version 2.04. Copy the SardineUpdate.GEL to the USB-stick.
10. Insert the USB-stick into the USB socket at the rear side of the generator. Confirm the firmware change when the pop-up appears on the generator screen. Remove the USB-stick when the generator reboots.
11. Find the file dg900_firmware_00.02.04.00.00.rar from Rigol software download page. It contains the file SardineUpdate.GEL , that is copied to the USB-stick.
12. Insert the USB-stick into the USB socket at the rear side of the generator. Confirm the firmware change when the pop-up appears on the generator screen. Remove the USB-stick when the generator reboots.
13. Find the newest dg800 firmware available ( .zip or .rar file ) from Rigol software download page. It contains the file SardineUpdate.GEL , that is copied to the USB-stick.
14. Insert the USB-stick into the USB socket at the rear side of the generator. Confirm the firmware change when the pop-up appears on the generator screen. Remove the USB-stick when the generator reboots.
15. The Rigol function generator is now upgraded to the latest firmware version and works as a model DG992, 100 MHz and two channels: