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Sample DB down mathmatics?

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Sample DB down mathmatics?

Postby fineshot1 » Thu Jun 29, 2017 11:31 am

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?

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Re: Sample DB down mathmatics?

Postby vk3khz » Thu Jun 29, 2017 2:44 pm

As far as power loss is concerned, for every -10dB (the minus means loss) the power is 10%
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

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Re: Sample DB down mathmatics?

Postby Al » Thu Jun 29, 2017 4:51 pm

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.
Last edited by Al on Thu Jun 29, 2017 4:53 pm, edited 1 time in total.

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Re: Sample DB down mathmatics?

Postby MSS-Dave » Thu Jun 29, 2017 4:53 pm


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.

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Re: Sample DB down mathmatics?

Postby RFguy » Thu Jun 29, 2017 5:36 pm

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.

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Re: Sample DB down mathmatics?

Postby fineshot1 » Fri Jun 30, 2017 5:35 am

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.

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