Signal-to-Noise (SNR) Ratio calculator

This Signal-to-Noise Ratio calculator converts signal and noise voltage levels to a deciBel (dB) ratio. To use this tool simply enter the levels and select the appropriate units.




SNR (dB) = 20*Log10(S/N)

Note that since the signal and noise values are represented as voltage levels, there’s a square or 20*Log relationship to convert to power ratio.

Example Calculations

  • If the signal and noise voltage levels are the same, the SNR is 0 dB
  • For a signal level of 9V and Noise level of 3V, the SNR is 9.54 dB

Can SNR be negative?

Yes, SNR can be a negative value. This is the case when the noise is greater than the signal. Practically this means that the signal is buried in the noise.

A related question is:

Can the signal be detected if the SNR is negative?

The answer is Yes. Use this calculator to see that with 10 MHz of bandwidth and an SNR value of -10 dB, a throughput of over 1 Mbps cane be achieved.

An example of this is with Direct Sequence Spread Spectrum Systems such as Code Division Multiple Access (CDMA), a signal can be below the noise and still be detected.

Is High SNR better than Low SNR?

In general, a high SNR is always better than low SNR. However there is a limit for every modulation scheme beyond which a higher SNR makes no difference to performance.

For instance, BPSK modulation requires an SNR of 7 dB for an error rate lower than 10-5. In this case improving the SNR to 10 dB will not provide any benefit if your BER requirement is the same.

How to convert from SNR to volt?

SNR is a ratio expressed in decibel or dB. It is a relative quantity and as such does not have units. You can use this calculator to convert from dB to linear signal-to-noise voltage ratio.

How to measure SNR?

One method to measure SNR voltages is to use an oscilloscope shown in the picture below.

Siglent Technologies SDS1104X-E 100Mhz Digital Oscilloscope 4 channels Standard Decoder, Grey

The first step is to measure the RMS value of the signal and noise

Modern oscilloscopes like the one above have built-in Math functions to do this. It essentially performs this RMS computation.

Let’s call this VS+N

The next step is to measure the RMS voltage of the signal alone

In the video below, Mark Schnittker shows how to measure the signal voltage in a very noisy signal. Here are the steps:

  • Input the signal to be measured to Channel 1 of the scope
  • Input the clean reference signal to Channel 2 of the scope and trigger using this signal
  • Average the noise on Channel 1 (using 128 averages in this case)
  • You should now be able to see the sine wave that has a fixed phase relationship with the signal on channel 2
  • Measure the root-mean-square (RMS) voltage value

Let’s call this value VS

The final step is to do the math

The RMS noise voltage is VN = VS+NVS

The SNR is then given by VS/VN

This is a linear value and can be converted to the dB equivalent using the calculator on this page.