How to use it
- Choose how you know the signal and the noise: as levels in dBm, as powers in watts or as RMS voltages in volts.
- Enter both values in the same form.
- Enter the channel bandwidth in MHz to see the Shannon capacity; the SNR itself does not depend on it.
Formulas
- SNR(dB) = 10·log₁₀(Ps ÷ Pn) = 20·log₁₀(Vs ÷ Vn) = Ls(dBm) − Ln(dBm)
- Linear power ratio = 10^(SNR ÷ 10); voltage ratio = its square root
- Shannon–Hartley: C = B·log₂(1 + SNR linear); C ÷ B is the spectral efficiency limit in bit/s/Hz
Worked example
A Wi-Fi client measures the access point at −50 dBm and the noise floor at −90 dBm on a 20 MHz channel.
- SNR = −50 − (−90) = 40 dB.
- Power ratio 10^4 = 10,000, voltage ratio 100.
- Capacity = 20 × log₂(10,001) = 20 × 13.288 = 265.757 Mbit/s, a spectral efficiency limit of 13.288 bit/s/Hz.
Tips
- Measure noise in the same bandwidth as the signal. A noise floor quoted per hertz must be multiplied by the bandwidth (add 10·log₁₀ B in dB) first.
- dBm is absolute, dB is relative. Two dBm levels subtract to dB; a dB value cannot be turned back into dBm without a reference.
- 3 dB is a factor of 2 in power. Doubling the distance in free space costs about 6 dB of signal.