I have seen enough in the video to work out an approximate frequency plan for how this sig gen works. Obviously I could do a better guess if I had access to the hardware but this looks like it is heavily based on a Hybrid DDS PLL.
i.e. there is a bank of 3 VCOs that feed a PLL chip and the reference for this PLL chip appears to be a DDS. The big square chip that Dave thought was a DAC is more likely to be a fast DDS. This looks like it gets clocked at about 650MHz by the 2600MHz Zycomm VCO plus a divide by 4 prescaler.
There is also a BFO downconverter that gets its main LO from this fixed /4 signal at approx 650MHz. This BFO presumably provides the lowest range on the generator across 9kHz to the lowish VHF region.
The strange little isolated region in the middle of the PCB looks to be the ALC detector that sets the level accuracy. It presumably is also part of the AM system and so it needs to work very precisely across the whole modulation envelope. So maybe Rigol had to crudely ovenise it to get respectable performance from it across various external temperature changes.
There is an Analog Devices DAC near the big DDS chip. I think it was an AD5449. I guess this is used for the modulation and LF generator output?
The block diagram below is just guesswork and it really needs to be refined and corrected by someone with access to the real hardware so they can correct the frequency ranges etc. But it's (probably) worth posting up as it is because it is a useful starting point at least
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