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816 views
01:28 May-11-2001

elgazery

Consultant
National Institute for Standards,
Egypt,
Joined May 2003
10
Difference between dB and dBM

Please
there are any one can discuss me the difference between dB and dBM units for measurement attenuation.
Thank you in advance.
looking forward to recieve your valuable discussion.
elgazery@sit.nis.sci.eg


 
04:16 May-11-2001

jean paul van Cuyck

Director
A2MS, Applied Acoustics, Materials and Systems,
France,
Joined Nov 1998
13
Re: Difference between dB and dBM : Please
: there are any one can discuss me the difference between dB and dBM units for measurement attenuation.
: Thank you in advance.
: looking forward to recieve your valuable discussion.
: elgazery@sit.nis.sci.eg
.
dB or dBW relativ to 1W
dBm for dB relativ to 1 milliwatt. 0 dBm= 1 mW = -30dBW



 
05:43 May-11-2001

Dr. Sotirios J. Vahaviolos

R & D, -
MISTRAS Group, Inc.,
USA,
Joined Jan 2000
15
Re: Difference between dB and dBM : : Please
: : there are any one can discuss me the difference between dB and dBM units for measurement attenuation.
: : Thank you in advance.
: : looking forward to recieve your valuable discussion.
: : elgazery@sit.nis.sci.eg
: .
: dB or dBW relativ to 1W
: dBm for dB relativ to 1 milliwatt. 0 dBm= 1 mW = -30dBW
.
Dear Mr.Elgazery
Its interesting,but these terms are used interchangeably at times, erroneously.The term dBm is used by
communications engineers and it is absolute.Most Power meters commercially available have this scale.
It is : dBm=10log(base 10)(P1/P2) at two different Power points.Power is read always across a 50 Ohm
resistor.IEEE has made this a standard and 1mW=0dBm(-40dBm is 100nW accross 50 Ohm and +20dBm is 100mW).
As far as I know dB(not DB)refers simly to gain(and loss/attenuation as -dB).It is different this time
as the equation is not Power but a simple ratio: dB=20log(base 10)(Gain or attenuation).One can have
a reference and above that reference he is talking in positive dBs(Gain) and below in negative dBs(loss
or attenuation).Such scales can then be modified as we have done for Acoustic Emission where we talk in
dBae and our refence is the "perfect" sensor giving us ONLY 1microVolt(!) Peak output noise.Anyway a
good goal.So for 40dBae a sensor/Amplifier(40dB Gain)output across a 50 Ohm resistor is 10mV.
Sotirios



 


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