A properly calibrated spectrum analyzer, where the display accurately reflects what is being applied at the input port, will exhibit unity gain, showing signals at their actual amplitudes. If a 0-dBm signal is applied to the input port, the measured/displayed signal should be 0 dBm plus or minus the accuracy of the analyzer. Adding attenuation or gain to the spectrum analyzer will change this relationship. Increasing input attenuation causes the spectrum analyzer to increase equivalent gain in the instrument’s intermediate-frequency (IF) stage to maintain a calibrated level on the display. This in turn raises the noise floor by an equivalent amount on the display, thereby maintaining the same signal-to-noise ratio. This applies to external attenuation as well. In the calculations, an RBW greater than 1 Hz is compensated by adding 10log(RBW/1 Hz). Through simple calculations (Eq. 3), it is possible to determine the spectrum analyzer noise floor at different settings of attenuation and RBW:

Noise Floor = DANL + Atten + 10log(RBW)             (3)

An internal or external preamplifier can be used to improve the noise floor of a spectrum analyzer. Typically, there will be a second DANL specification that covers the internal preamplifier case and the previous equations will apply as well. If an external preamplifier is used, the new DANL can be found by using the cascaded  noise-figure equations, with unity gain for the spectrum analyzer. Considering the system as the combination of a preamplifier and a spectrum analyzer, it can be represented by Eq. 4:

NFSYS = NFPREAMP + [(NFSA - 1)/(GPREAMP)]             (4)

Using the previous example with NFSA = 25.5 dB and a preamplifier with gain of 20 dB and noise figure of 5 dB, the overall noise figure for this system of analyzer and amplifier can be found. The values must be first converted to power ratios and those results recorded:

NFSYS = 10log[3.16 + (355/100)] = 8.27 dB          (5)

Equation 1 can then be used to determine the new DANL with an external preamplifier connected by simply replacing NFSA with the NFSYS calculated using Eq. 5. The preamplifier reduced the DANL significantly in the example, from -151 dBm/Hz to -168 dBm/Hz. The external preamplifier brings tradeoffs, however. Preamplifiers bring their own nonlinear distortion behavior to the system which can limit the capability to measure large signals. This is where an internal preamplifier is more useful because it can be simply switched in and out as the measurement needs change. This is especially true in an automated test environment.

So far, the sensitivity of a spectrum analyzer has been improved by modifications to the RBW, attenuation, and preamplifier gain/noise. Most modern spectrum analyzers offer methods for measuring the noise floor of the spectrum analyzer and then correcting for this in the measurement results—methods which have been available for some time.