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The RBN uses the term "SNR" to denote its measure of the strength of a
signal.
Normally, this term is intended to denote "signal to noise" ratio, although
what is often meant by this is not the ratio of the signal to the noise,
but rather the ratio (signal + noise) to noise. But in the case of the RBN,
neither of these is related other than peripherally to the reported number.
The following information comes from VE3NEA, who is the author of the
software used to report signals on the RBN.
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The reported number is the ratio of two power levels, in dB. The numerator
represents the total received power in a bandwidth of 50 Hz. In other
words, it is the signal plus noise in a 50 Hz bandwidth. The denominator,
however, is not a simple measurement: it is calculated in the following
steps:
1. Determine the total received power in a bandwidth of (depending on the
hardware), typically 200 kHz.
2. Divide this number by the bandwidth to determine a flux-per-Hz.
3. Normalise this value to a bandwidth of 500 Hz.
The result of step 3 is used as the denominator in the calculation of the
number that is reported.
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It is worth noting, then, that (1) the notional bandwidths for the numerator
and the denominator are different; and (2) the denominator includes all the
signals that are present in a wide bandwidth. During a busy contest
weekend, these will likely vastly overwhelm the actual background noise --
and, in particular, will be a far cry from a measurement of the noise in a
50 Hz bandwidth which would accord with the conventional notion of
reporting the ration of signal to noise or (signal + noise) to noise.
VE3NEA states that the RBN does not report simply the total received flux
in a standard bandwidth because SDR software has no capability to determine
the actual signal strength at the input terminals.