Noise Floor

Noise floor is the measure of the noise density (dBm/Hz), or the noise power, in a signal of 1 Hz bandwidth. Noise can be classified into several types: shot noise, thermal noise, flicker noise, burst noise, quantization noise, and avalanche noise. In the NI 5610 upconverter module, thermal noise is the dominant noise source. Thermal noise is caused by random fluctuations produced by thermal agitation of bound charges.

The noise due to a matched resistive load can be expressed as the following relationship:

N_i = kTB watts

where

k is Boltzmann's Constant (k = 1.38 x 10^-23J/K)

T is the resistor temperature in Kelvin

B is the bandwidth in Hz.

If B is set to 1 Hz, then N_i is equal to the output noise density in watts/Hz. For the system shown in the following figure, the output noise floor is the combination of the input noise multiplied by the gain or loss of the system plus the internal noise of the system, N_n.

The internal noise of a system is therefore:

The noise factor (F) of a system is a measure of how noisy a component or system is. The noise factor is defined as the following relation:

If N_i is equal to kTB, then

N_o = FGkTB (watts)

N_o(B = 1) = 10log(1000(FGkT)) (dBm/Hz)

Therefore, the output noise floor of the NI 5610 is not only dependent on the noise generated by the internal components, but it is also dependent on the noise generated by the source and by system settings. The noise floor of the NI RF signal generator is dominated by the NI 5610 upconverter module. This behavior is because the NI 5610 upconverter utilizes a superheterodyne architecture that locates a significant amount of gain in the broadband output section. As a result, care should be taken in nonfrequency-selective applications (such as testing broadband devices with broadband detectors), as noise power is integrated. Integrated noise power can be calculated using the following formula:

N_o = N_o(dBm/Hz) + 10logB

where

B is equal to 2.7 GHz (if no external filter is used)

N_o is the noise floor spectral density

With the NI RF signal generator set to minimum power output, the noise power density at the output is -158 dBm/Hz. The following table shows typical output noise power density for several RF signal generator output power level settings: