dB is usually used as a relative measure of power, so it all depends upon if the dB value on each side of the equation has the same relative base value.
My Kenwood TS-530S service manual specifies a signal generator as a required piece of test equipment. The specification is written as follows.
STAND SIGNAL GENERATOR (SSG)
1) Frequency range: 1.8 to 30 MHz
2) Output: -20 dB/0.1uV ~120dB/1V
3) Output Z = 50Ω
Later sections of the TS-530S service manual specify “SSG” settings as NNdB, where NN is the desired value. A typical setting seen in the manual would be 60dB.
My signal generator is a Marconi 2018. It’s RF output is specified as:
0.2 uV to 2 V e.m.f.; c.w. and f.m.
0.2 uV to 1 V e.m.f.; when a.m. is selected.
By keyboard entry – units may be
(i) uV, mF, V, e.m.f. or p.d. or
(ii) dB relative to 1 uV, 1 mV, 1 V, e.m.f. or p.d. or
It is specific as expecting 50 ohm impedance, same as the TS-530S.
However, there is no mention of -20 dB/01.uF or ~120dB/1V in the Marconi manual, so I needed to figure out how to covert from Kenwood TS-530S service manual settings to Marconi settings. First, I decided to use the default dBm setting on the Marconi, since that is a pretty standard usage for dB in the RF world.
I’m interpreting the -20dB/0.1 uV value in the TS-530S spec as meaning that 0.1 uV should be interpreted as -20dB.
You can manually do the following equations, but I find these sort of jobs are more easily done by finding and using an online calculator. Using the online calculator at http://www.eeweb.com/toolbox/rf-unit-converter, I entered 50 Ohm impedance and .1 uV Voltage. This returned a value of -127dBm Power. This means that at -20dB(TS-530S value), the dBm Value is -127, a difference of -107.
Leaving the 50 Ohm impedance, I entered 1000000 in the uV Voltage box, which returned 13 dBm. This means that at 120dB(TS-530S value), the dBm Value is 13. Once again a difference of -107.
This consistent difference in value makes sense, since dB is a relative value. So now I knew that I could simply subtract 107 from any value in the TS-530S service manual in order to know what value to program into the Marconi 2018. A typical signal generator setting in the TS-530S manual is 60 dB. I would calculate the Marconi signal generator value by subtracting 107, getting a -47. So yes, -47dB can be equal to 60 dB.