Dbm to Watts?

Thread Starter

Rickreezi

Joined Aug 9, 2013
9
I am now working on a question that says,

5. A microphone has an output power of –45 dBm, the output power in Watts is;

A. 5.6 mW
B. 31.6 μW
C. 5.6 nW
D. 31.6 nW
E. None of the above

i found the formula to convert dBm to watts which is

P(Watts)= 10^(dB/10)/1000

i dont actually know exactly what is the formula saying apart from the divide by 1000 part because 1watt = 1000mV

10^(-45/10)/1000= 0.0000000316227766 Watts

but then i multiplied the Ans by various submulitples to get me an answer that are in the options. i did 0.0000000316227766 x 10^9(nanowatts)= 31.6nW

is this the correct way of getting the answer?
 

Papabravo

Joined Feb 24, 2006
21,158
dBm is dB with respect to a reference power of 1 milliwatt = .001 Watt. Your answer is correct. I always get the right answer by writing the definition and doing the algebra.
Rich (BB code):
-45 dBm = 10*log(Po/Pref), where Pref = 1 mW
-45 / 10 = log(Po /.001)
-4.5 = log(Po/.001)
Po = 10^(-4.5) * .001
Po = 3.162e-8
For me is is just easier to start with the definition and do the algebra. Your mileage may vary.
 

#12

Joined Nov 30, 2010
18,224
I've dragged my trees out of the forest dozens of times with the, "back to basics" approach recommended by Papabravo. I must agree with his method.
 
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