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Sample DB down mathmatics?

Posted: Thu Jun 29, 2017 11:31 am
by fineshot1
I have a couple of Bird sampling elements with the BNC output, 30DB and 50DB down.
I am wondering if there is a math calculation online(or anywhere) one can use to get
a possible sample output with an input power(ie:100W) and get the sample output.
Am I assuming correctly of thinking 30DB down is about 10% of a given input?
Perhaps there is a chart of this somewhere, maybe?

Re: Sample DB down mathmatics?

Posted: Thu Jun 29, 2017 2:44 pm
by vk3khz
As far as power loss is concerned, for every -10dB (the minus means loss) the power is 10%
so...
1000 Watts into a 10dB Attenuator you will get 100 Watts out,
1000 Watts into a 20dB attenuator will give you 10 Watts,
1000 Watts into a 30dB Attenuator will give you 1 Watt....

In Terms of Power Gain, +10dB will amplify x 10, so an amplifier with gain of 20dB will take 1 watt input and give 100 Watts output.

The formula for Power Gain/Loss dB=10log(P1/P2)

The formula for Voltage Gain/Loss dB=20log(V1/V2)

Gain is +dB
Loss is -dB

Re: Sample DB down mathmatics?

Posted: Thu Jun 29, 2017 4:51 pm
by Al
All you need is a handheld calculator. Just do 10^(-dB/10), move the decimal point in the result two places to the right to get percent and you've got your answer. Example: 10^(-10/10) = .1, move the decimal to the right two places and you have 10%. Another example: 10^(-30/10) = .001, moving the decimal right you have .1%. OK? The ^ raises a value to a power which is the opposite of the log base 10 operator which is used to derive dB values such as you have shown on your Bird samplers.

Re: Sample DB down mathmatics?

Posted: Thu Jun 29, 2017 4:53 pm
by MSS-Dave
Or......

Google "convert watts to dbm" and there is a bunch of charts and apps available, take your pick. Example. ... 100 watts is +50 dBm. -50 dB tap = 0 dBm which equals 1 milliwatt.

Re: Sample DB down mathmatics?

Posted: Thu Jun 29, 2017 5:36 pm
by RFguy
2 basic numbers I always refer to in dB is 3dB (or -3 dB) and 10 dB (or -10 dB).

3 dB is twice the power (-3dB is 1/2 power) and 10 dB is 10 times the power (-10dB is 1/10 power).

Here we're talking minus dB.

For your example, 100 watts, -10 dB attenuation is 10 watts, -20 dB is 1 watt, -30 dB is 0.1 watt (100 mw), 40 dB is 0.01 watt (10 mw) and -50 dB is 0.001 watt (1 mw).

Other references is dBw (reference to 1 watt where 0 dBw=1 watt) and dBm (reference to 1 mw, where 0 dBm=1 mw

10 dBw = 10 mw, 20 dBw= 100 mw, 30 dBw= 1 att which also equals 0 dBw.

Re: Sample DB down mathmatics?

Posted: Fri Jun 30, 2017 5:35 am
by fineshot1
thanks for all your responses - especially Al and RFguy - that is exactly what i was looking for.
i used 100W as an example but seldom do such round even numbers show up on the bird which is
why i was looking for a quick and easy calculation.

"Gain is +dB
Loss is -dB"

This part i knew and would consider this that even a newbie would know.