Amps
[Top] [All Lists]

Re: [Amps] Watts to DBM??

To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Watts to DBM??
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Sun, 27 Sep 2009 14:39:38 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
On Fri, 25 Sep 2009 10:13:59 +0100, Andy GD0TEP wrote:

>Can someone tell me what the formula is for converting Watts to DBM please,
>or DBM to Watts.

It's actually pretty simple if you know logs. All dB values are ratios to 
some specified reference. The dBm means dB with 1 mW as a reference, and dB 
is 10 times the log of the ratio. Adding dB is like adding logs, so adding dB 
is like multiplying the fundamental quantity. 

SO -- 10X the power is 10 dB, 10X the voltage is 20dB. Every time you 
multiply by 10 you add 10 dB. since 1mW is 0 dBm, 1 watt (10 x 10 x 10 x 1mW) 
is +30dBm, 10W is +40dBm, 100W is +50dBm, 1kW is +60dBm. Some other easy 
numbers -- double the power is 3dB, 4X the power is 6dB, 5X the power is 7dB. 
To divide the power, simply subtract the dB numbers. 

Note, BTW, that it's dBm. dB is the abbreviation for a deci Bel (one tenth of 
a Bel, that log) and named for Alexander Graham Bell, and the "m" means one 
milliwatt. dB are used in MANY disciplines, including acoustics (sound 
levels), audio, and telephony. Every discipline has its own reference for 
what "0dB" is. 0dBV is the voltage with resect to 1 volt. 0dBu is the voltage 
equivalent to 1mW in 600 ohms, which is 0.78 volts. To convert from a VOLTAGE 
dB to a power dB, you must know the impedance of the circuit. :)  But dBm is 
impedance-blind. That is, it talks ONLY about power, and the circuit 
impedance can be anything that works for the circuit in question. Likewise, 
dBV is impedance-blind -- it tells us only about the voltage. 

73,

Jim Brown K9YC


_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>