[1000mp] dB etc

[Mike Gilmer - N2MG]

While the results are correct, there are a couple of 
misstatements...

>> We can easily convert dBu to dBm (dB above 1mV)

o  dBm as used here, and is typically, dB above a 
   milliWATT (not volt)

>> All of these assume that the measurements are 
   across the same impedance

o  The conversion from dBu to dBm (where -107 was used)
requires that the impedance be 50 ohms, not just 
the "same".  For example, if the 50uV (34dBu) were 
developed across 100ohms, and then converted to power, 
one would get -76dBm for a conversion factor of -110.

73 Mike N2MG



--- Ronald Long <rlong3@columbus.rr.com> wrote:
> Commenting on various recent posts:
> 
> +8dBu is 2.5uV
> 
> With a calculator dBu = 20log(uV) where uV is
> microvolts and dBu is dB above 1uV
> 
> So dBu = 20log(2.5) = +8dBu
> 
> We can easily convert dBu to dBm (dB above 1mV) by
> adding -107 to the dBu result
> 
> Thus for 50uV (commonly S9)
>       first dBu = 20log(50) = 34
>       then dBm = -107 + 34 = -73dBm as has been
> posted.
> 
> Another example, if minimum detectable signal is
> 0.2uV
> it can be converted to dBm as follows
>       first dBu = 20log(0.2) = -13.98
>      then dBm = -107 + (-13.98) = -120.97dBm
> 
> All of these assume that the measurements are across
> the same impedance, 
> usually 50 ohms when referring to signal generators
> and receiver inputs.

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