How does the noise marker feature work on a spectrum analyzer?

The noise marker algorithm works very similarly to the X-Series signal analyzers. The biggest difference is the type of detection used. All of these analyzers calculate the marker noise power using the following equation:

This value is equal to the sum of the left endpoint x1 plus the right endpoint x2 (0.05 * total length of the interval, where the endpoint is located at the center of the noise marker).

The Px value is the power ratio of the value of the marked trace data at point x to the reference value. For example, if x = -60 dB, its value is 0.000001.

Span/FreqPts (Span/Power Pts) - 1 is the spacing of the trace data points.

NBW is defined in Note 1 below.

8590 and 8560 Series

The noise marker function is accessed from the Marker Functions or Marker Menu by pressing MKR FCTN > MK NOISE ON (859XE) or MKR > MKRNOISE ON (856XE/EC).

1. The sample test will be activated.

2. To calculate the total power in a frequency interval, use the equation,

3. A correction to support the RBW filter shape factor is applied1,

4. Correction for video averaging errors is applied2,

5. The result is normalized to a 1 Hz bandwidth and displayed as dBx(1 Hz), where x depends on the selected amplitude unit, e.g. dBm(1 Hz)

To get accurate measurements, make sure the noise markers are installed so that all data points are above the noise floor. The data points take up almost 5% of one horizontal grid section or span on the 859XE, which has a fixed 401 display points, and half the area on the 856XE, which has a fixed 601 points.

1. The near-Gaussian shape of the resolution bandwidth (RBW) filter is corrected to a rectangular equivalent noise bandwidth (NBW) that passes the same noise power. The correction factor ranges from 1.128 times the 3 dB bandwidth for filters > or = 1 kHz.
2. The 2.51 dB under-response is due to errors in averaging the logarithmic values ​​of the data points and in the displayed interval of the noise signal.

X-Series Signal Analyzers

The noise marker function can be accessed from the marker or marker function menu by pressing Marker > More > Function > Marker Noise (ESA) or Marker Fctn > Marker Noise (PSA). When the noise marker is active,

1. If the detector is set to Auto (the default), mean value detection is activated.

2. The average value type is set to Power (RMS) 4,

3. Using the above equation, the total power within the frequency interval can be calculated.

4. A correction for the RBW filter shaping factor is applied,

5. The result is normalized to a 1 Hz bandwidth and displayed as dBx(1 Hz), where x depends on the selected amplitude unit, e.g. dBm(1 Hz)

Note: The average detector type was not available in earlier analyzers. The detector averages all digitized data in the frequency bin according to the selected averaging type. See note 4. In X-Series signal analyzers, if RBW < 1 kHz, sample detection is used. Since the power (RMS) average takes the square root of the sum of the squares of the voltage data (given internally by dividing by Zin in the analyzer) in the bin, it calculates the true average power. However, if the user selects log power (video) averaging, the 2.51 dB under-response error can be compensated. In X-Series signal analyzers, the user can also select voltage averaging, in which case the 1.05 dB under-response can be compensated.