With respect to what signal is this 50MHz being measured and given in dB?
If you measure voltage you write how many times the measured value is higher than 1V. So 1V is 1 times higher then 1V and 10V is 10 times higher then 1V.
The same is when you want to write the result in dBV - you take measured value, compare it to 1V reference - how many times it is higher and then convert into dB, but the unit of result contains not only dB, but also the value you compared your value with.
So:
1 V = 20log(1V/1V) dBV = 0 dBV
10 V = 20log(10V/1V) dBV = 20dBV.
If you use dBuV then the reference you compare to is 1uV.
If you use dBuV/m then the reference you compare to i 1uV/m.
Consider also that basic unit is Bel, but all industry decided to use dB (1dB = 0.1B - like 1dm = 0.1m, and 1cm=0.01m)
And to not mix you too much I will not go into detail, but it is worth to remember that to convert into dB you not always use 20log, but sometimes 10log.