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Re: SNR calculations (was Re: setup.exe missing from FTP site)


Linda Walsh wrote:

> >   I prefer to express that as an SNR of -18dB :-)
> ---
>      Could you please show your work!
> 
>      I arrived at -9dB. Work:
>          For each doubling/halving of power one adjusts by ~(+/-)3dB.
>              (note: "3" is approximation of 10*log(2) or,
>                     to 7 decimal places, 3.010900)
>                 so 1 unit / 2 units of signal would be -3dB,
>                    1 unit / 4 units: -6dB,
>                    1 unit / 8 units: -9dB
> 
>      From my limited understanding, -18dB would be a reduction of 6 factors of
> 2, or a 1:64 signal/noise ratio.

Typically you use dB to compare the amount of power in a signal relative
to a fixed reference; in the case of electronics a common reference is
1mV, or 0 dBmV. Since power in a signal is proportional to V^2, when you
compute dBmV from a voltage level it's 20*log(v) due to the power of 2.

But when you just want to use dB to represent a unitless ratio, then I
agree with you it's simply 10*log(ratio).

Brian


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