Re: [linrad] dBc/Hz

[Marko Cebokli <s57uuu@xxxxxxxxxxxxxxx]

Hello Leif,

for a log detector, the correction is 2.51dB and for linear 1.05dB
(the true RMS value is 2.51dB more than the measured value)

73, Marko S57UUU

On Saturday 06 December 2003 07:40, Leif Åsbrink wrote:
> Hi All,
>
> Is there anyone who knows the correction factor????
>
> When we measure noise levels on a spectrum analyser we use
> a logarithmic detector. By means of a video filter we can average
> the detected voltage to get smooth curves. It may be a standard
> procedure to take the reading for a carrier and the sideband noise
> from the spectrum analyser screen, correct for the bandwidth and
> present the result as dBc/Hz but it is not correct. The definition
> of dB says power ratio and the result obtained from the averaged
> logarithmic detector output is not power.
>
> I can find empirically that the result obtained from a logarithmic
> average is about 6 dB lower (better) than the true result from a
> RMS detector. Obviously it is "simple" to deduce the difference
> between the RMS and the log detector results under the assumption
> of white noise (within the passband). Unfortunately I am not
> clever enough to do the mathematics myself but I think the result
> should be well known.
>
> Is there anyone who knows how many dB (with decimals) one has
> to add to correct the result from an oldfashioned spectrum analyser
> to get the true dBc/Hz value for the sideband noise?
>
>
> 73
>
> Leif / SM5BSZ



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