Spectrum analyzers are often used to measure low-level signals. They may be known signals that must be characterized or unknown signals that must be found. In either case, knowing how to enhance the sensitivity of a spectrum analyzer can greatly aid the process. Certain spectrum-analyzer settings are optimum for measuring low-level signals, and these will be reviewed, along with how to use noise corrections and noise floor extensions to maximize a spectrum analyzer’s sensitivity.
A spectrum analyzer’s sensitivity can be found in the listing of specifications for a given instrument, usually as the parameter displayed average noise level (DANL) or as noise figure (NF). The DANL specification is the amplitude of the noise floor of the spectrum analyzer over a given frequency range with the input terminated in a 50-Ω load and with 0-dB input attenuation. The specification is normally given in units of dBm/Hz. In most cases, any averaging is performed on a logarithmic scale. Averaging causes a reduction in the noise floor by 2.51 dB. This reduction essentially differentiates DANL from NF. For example, for a -151 dBm/Hz DANL specification in a 1-Hz resolution bandwidth (RBW), it should be possible to lower the analyzer’s noise floor to at least this level using the settings provided in the specification. It should be noted that a continuous-wave (CW) signal with the same amplitude as the analyzer’s noise floor will measure 2.1 dB above the noise floor due to the summation of the noise and the CW signal. Similarly, a noise-like signal will appear 3 dB above the noise floor.
A spectrum analyzer’s noise floor has two components: the noise figure, denoted NFSA, and the thermal noise energy. The amplitude of the thermal noise energy is given by the product of three parameters, kTB, where:
k = Boltmann’s constant (1.38 x 10-23 J/°K;
T = temperature (in degrees Kelvin); and
B = bandwidth in which the noise is measured (in Hz).
This is the amount of thermal noise energy present at the input of the spectrum analyzer when the input port is terminated in a 50-Ω load. In most cases, the bandwidth is normalized to a 1-Hz bandwidth and, at room temperature, 10log(kTB) is calculated as -174 dBm/Hz. The DANL specification in a 1-Hz RBW is then given by:
DANL = -174 dBm/Hz + NFSA - 2.51 dB (1)
NFSA = DANL + 174 dBm/Hz + 2.51 (2)
[Note: for cases where root-mean-square (RMS) averaging is used in the DANL specification, the 2.51 dB can be omitted from the calculations.]
For example, a -151 dBm/Hz DANL specification equates to a NFSA value of 25.5 dB.