Rigol DM3058e: Using relative mode to set a zero reference while reading dBm?

[cvanc]

I might be missing the obvious but I can't figure out the keystrokes to make this work and the manual is no help either.

I'm doing some analog audio work, so I'm reading AC volts in dBm mode (example: 400Hz at +4dBm) and want the Rigol to show 0.00dBm at that level so I can directly read gain or loss in dB as I adjust my circuit.

Anyone who has a Fluke 8060 knows how easy it is to do this.  But I can't make it work with the Rigol.

When I engage relative mode the display jumps to what seems like a nonsense value (-70 something).

And the manual is.. well the less said the better, it only made me more confused   

What the heck am I missing?  Thanks.

[joseph nicholas]

The obvious answer is: this function wasn't implemented.  But I could be wrong.

[DaJMasta]

Don't have one and can't test, but is there a dBc or dB mode?  dBm is a dB scale referenced to 1mW, so moving the reference point for a relative measurement would just be a dB or sometimes dBc measurement... I don't think it would be meaningful to have a dBm based scale with a relative reference, because then it would just be a dB*that value* scale.

So my point relative to your question is: maybe it's under some other dB mode?  I think the SCPI commands are supposed to mimic fluke and agilent meter commands, so if it were available over GPIB, I'd expect it exists in there somewhere.