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How_to_Measure_the_Noise_Floor_of_Your_Spectrum_Analyzer.php    5621 Bytes    27-06-2024 16:40:24

Measure the Noise Floor of Your Spectrum Analyzer

Noise • How To ...

Measure the Noise Floor of Your Spectrum Analyzer

That's an easy measurement ... you just need a "Terminator" and some time

✈ Why would one do that ?

The Noise floor is an important parameter in Spectrum Analyzers and similiar devices. It determines the lowest possible signal level that these systems can measure. And knowing your tools is the key to successfully use them.

✈ How can we push it to the absolute Limit ?

The Noise floor depends on many settings, the user can make to configure the Spectrum Analyzer. Some settings are made automatically for protection purposes. These have to be overridden manually.

Everything in the path worsening the Noise Figure shall be switched off. This is for example the attenuator. You mayst need to switch it manually to 0 dB.

Everything in the path improving the Noise Figure shall be switched on. This mayst be any (low noise) preamplifier in the device.

✈ How to interpret the measurement ?

Measure the Noise Floor of Your Spectrum Analyzer

The above value (at the frequency of interest) must be corrected for the bandwidth in order to have the Noise Power per 1 Hz Bandwidth. As we used a RBW (Resolution Bandwidth) of 300 kHz, a value of P = 10 * log(300'000) = 54.77 must be subtracted. Looking at 500 MHz, this will yield : -102.88 dBm - 54.77 dB = - 157.65 dBm. This is the lowest possible signal level which we can measure with this device.

✈ What can we do to go below that value ?

As we used up all attenuator stages (by setting them to 0 dB), we mayst continue by adding an external (low noise) amplifier. This will reduce the overall Noise Figure, as we put it at the beginning of our signal chain. (See Friis formulas for noise).

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t1 = 6586 d

t2 = 279 ms

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