Specifications

58
Sensitivity
One of the primary uses of a spectrum analyzer is to search out and measure
low-level signals. The limitation in these measurements is the noise generated
within the spectrum analyzer itself. This noise, generated by the random
electron motion in various circuit elements, is amplified by multiple gain
stages in the analyzer and appears on the display as a noise signal. On a
spectrum analyzer, this noise is commonly referred to as the Displayed
Average Noise Level, or DANL
1
. While there are techniques to measure
signals slightly below the DANL, this noise power ultimately limits our ability
to make measurements of low-level signals.
Let’s assume that a 50 ohm termination is attached to the spectrum analyzer
input to prevent any unwanted signals from entering the analyzer. This passive
termination generates a small amount of noise energy equal to kTB, where:
k = Boltzmann’s constant (1.38 x 10
–23
joule/°K)
T = temperature, in degrees Kelvin
B = bandwidth in which the noise is measured, in Hertz
Since the total noise power is a function of measurement bandwidth, the value is
typically normalized to a 1 Hz bandwidth. Therefore, at room temperature, the
noise power density is –174 dBm/Hz. When this noise reaches the first
gain stage in the analyzer, the amplifier boosts the noise, plus adds some of
its own. As the noise signal passes on through the system, it is typically high
enough in amplitude that the noise generated in subsequent gain stages adds
only a small amount to the total noise power. Note that the input attenuator
and one or more mixers may be between the input connector of a spectrum
analyzer and the first stage of gain, and all of these components generate
noise. However, the noise that they generate is at or near the absolute
minimum of –174 dBm/Hz, so they do not significantly affect the noise level
input to, and amplified by, the first gain stage.
While the input attenuator, mixer, and other circuit elements between the
input connector and first gain stage have little effect on the actual system
noise, they do have a marked effect on the ability of an analyzer to display
low-level signals because they attenuate the input signal. That is, they reduce
the signal-to-noise ratio and so degrade sensitivity.
We can determine the DANL simply by noting the noise level indicated on
the display when the spectrum analyzer input is terminated with a 50 ohm
load. This level is the spectrum analyzer’s own noise floor. Signals below this
level are masked by the noise and cannot be seen. However, the DANL is not
the actual noise level at the input, but rather the effective noise level. An
analyzer display is calibrated to reflect the level of a signal at the analyzer
input, so the displayed noise floor represents a fictitious, or effective noise
floor at the input.
The actual noise level at the input is a function of the input signal. Indeed,
noise is sometimes the signal of interest. Like any discrete signal, a noise
signal is much easier to measure when it is well above the effective (displayed)
noise floor. The effective input noise floor includes the losses caused by the
input attenuator, mixer conversion loss, and other circuit elements prior to
the first gain stage. We cannot do anything about the conversion loss of the
mixers, but we can change the RF input attenuator. This enables us to control
the input signal power to the first mixer and thus change the displayed
signal-to-noise floor ratio. Clearly, we get the lowest DANL by selecting
minimum (zero) RF attenuation.
Chapter 5
Sensitivity and Noise
1. Displayed average noise level is sometimes
confused with the term “Sensitivity”. While related,
these terms have different meanings. Sensitivity is
a measure of the minimum signal level that yields a
defined signal-to-noise ratio (SNR) or bit error rate
(BER). It is a common metric of radio receiver
performance. Spectrum analyzer specifications are
always given in terms of the DANL.