What is DANL?

DANL or displayed average noise level is a measure used to define the sensitivity of a spectrum analyzer. The DANL specifies all the internal noise of the spectrum analyzer referenced to 1 Hz and represented in dBm/Hz. The DANL is frequency dependent and it increases with frequency. Spectrum analyzers use preamplifiers to reduce DANL. The instrument is unable to measure any input signal that is below this level.

Displayed Average Noise Level (DANL) is a specification used to define the sensitivity of a spectrum analyzer. It represents the internally generated noise of the instrument, measured at its input, with the input terminated in 50 ohms. DANL is the average noise power displayed on the screen when no signal is applied to the analyzer input.

In simple terms, DANL defines the lowest signal level that a spectrum analyzer can measure. Any signal below the DANL will be indistinguishable from the instrument’s own noise floor.

DANL is typically specified in:

• dBm (at a specified resolution bandwidth) - Example: -135 dBm @ 1 kHz RBW
• dBm/Hz (normalized to a 1 Hz bandwidth) - Example: 160 dBm/Hz

When specified in dBm/Hz, the value is normalized to a 1 Hz bandwidth, allowing sensitivity comparisons independent of RBW.

Relationship Between DANL and Resolution Bandwidth (RBW)

DANL depends on the resolution bandwidth setting. A narrower RBW reduces displayed noise, improving sensitivity. Noise power increases with bandwidth according to:

Where:

• -174 dBm/Hz is the thermal noise floor at room temperature (290 K)
• RBW is the resolution bandwidth in Hz

For example:

If DANL = -160 dBm/Hz at 1 kHz RBW, then

This is why datasheets often specify DANL at the narrowest RBW to show best-case sensitivity.

Frequency Dependence of DANL

DANL is frequency dependent and typically degrades as frequency increases. At higher frequencies, front-end losses, mixer conversion loss, and internal amplifier noise figure generally worsen, resulting in a higher (less negative) displayed noise floor. As a result, a spectrum analyzer may have a better DANL at lower frequencies and a poorer DANL at the upper end of its operating range. This is why DANL is often specified over different frequency ranges in instrument datasheets.

Effect of a Preamplifier

Adding a preamplifier reduces the overall noise figure of the spectrum analyzer, which lowers the DANL and improves sensitivity. By amplifying the incoming signal before it reaches the mixer and IF stages, the preamp reduces the relative contribution of internal noise sources further down the signal chain. This allows the analyzer to detect weaker signals. However, using a preamplifier can reduce the instrument’s maximum input power capability and dynamic range, making it more susceptible to overload when measuring strong signals.

DANL vs Noise Figure

DANL and noise figure are related but not identical specifications. Noise figure describes how much noise a receiver adds relative to an ideal noiseless receiver at a given temperature, typically referenced to the theoretical thermal noise floor of −174 dBm/Hz at room temperature. DANL, on the other hand, represents the actual noise level displayed by the spectrum analyzer for a given resolution bandwidth. While noise figure is a fundamental receiver parameter, DANL is a practical measurement of the instrument’s sensitivity as seen on the screen. The two can be related mathematically, but they describe different aspects of performance.