Comparing RF Signals (Or Noise Floors) Using Different RF Spectrum Analyzers
People often ask why the strength of a transmitted signal is higher using one device compared to another — or why the noise floors differs. The answer is simple — it is not valid to compare absolute values from different devices. The reason is that a variety of factors affect the measurement — e.g. the types of antennas used by the devices, how well tuned the antennas are to the frequency range being scanned, the location, position and orientation of the antennas, the resolution bandwidths used during the measurements, etc. So, unless all these factors are identical for the two instruments, then comparing absolute measurements has little value. The measurement that you make using a particular device is a relative value and only has meaning when using that particular device.
So — what to do? The answer is to instead measure the SNR (signal-to-noise) value — that is, the delta (or difference) between a signal’s peak and the noise floor. SNR (rather than absolute signal power) should be used as the evaluation or test metric when comparing different instruments.
For example, using one device the peak may be -65 dBm and the noise floor -110 dBm, which results in a SNR of 45 dB. Using another
device, the peak may be -50 dBm and the noise floor -95 dBm, which results in a SNR of 45 dB. So, here you can see that even though the absolute
values can differ significantly, when using the SNR then you get the correct result and can conclude that both devices are working properly.