Detectors and Power Spectrum Analysis

Power Spectrum Analysis measurements (Power Spectrum, Spectral Emissions Mask and Adjacent Channel Power) allow you to apply any of the following detectors to trace data (the term "bucket" refers to the multiple measurement points that make up a single display point on a trace):

Normal (Min/Max) Detector: determines the peak of CW-like signals, and it yields alternating maximums and minimums of noise-like signals. This is also referred to as "rosenfell" or "min/max" detection.
Peak (Max) Detector: determines the maximum of the signal within the bucket.
Negative Peak (Min) Detector: determines the minimum of the signal within the bucket.
Average Detector: determines the average of the signal within the bucket. The averaging method depends upon Average Type selection (voltage, power or log scales).
Sample Detector: indicates the instantaneous level of the signal at the center of the bucket represented by each display point.

Because they may not find a spectral component's true peak, neither average nor sample detectors measure amplitudes of CW (continuous wave) signals as accurately as peak or normal, but they do measure noise without the biases of peak detection.

About Detectors

For vector measurements, the output of the FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. yields as many measurement points as there are frequency points. For Power Spectrum Analysis measurements, the output of the FFT can yield more measurement points than there are frequency points. Measurement points refer to the number of points output by the FFT. Frequency points refers to the number of points displayed on the screen.

Because the FFT may produce more measurement points than can be displayed on the screen, Power Spectrum Analysis measurements allow you choose a detector to evaluate multiple measurement points into a single frequency point. This process of reducing measurement points is called detection.

The number of frequency points affects the display resolution-the higher the number of frequency points, the better the resolution, as shown in the following formula:

Resolution = (Frequency Span)/((number of frequency points) -1)

For example, if the frequency span is 4 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). and the number of frequency points is 401, the resolution (spacing between frequency points) is 10 Hz. This means the closest frequency you can resolve is 10 Hz. The start frequency determines the first frequency point, the stop frequency the last frequency point, and the remaining frequency points are spaced at 10 Hz intervals.

During Power Spectrum and Spectral Emissions Mask measurements, the analyzer measures the energy (power) at all frequencies within the displayed frequency span. Thus, the analyzer measures energy that occurs at all frequencies, but can only display energy that occurs at the frequency points. The detector adjusts the value displayed at a frequency point to include energy measured between frequency points. Thus, the detector, in effect, improves display resolution by taking into account energy between frequency points.

Viewing Detector Output

For most hardware configurations (e.g., M9391A and M9393A), any or all of the available detectors can be enabled at any given time. All enabled detectors are applied to every measurement. The output from an enabled detector can be displayed on one or more traces. The output from different detectors can be displayed simultaneously by assigning them to different traces. Multiple traces can be viewed side-by-side, or they can be overlaid onto a single set of axes. Since the process of applying a detector to the measurement results takes computation time and memory, you can make your application run faster by disabling those detectors whose output is not required.

To select which detectors are to be applied to each measurement, go to the Detectors tab on the MeasSetup menu and check the box for each detector to be enabled. Uncheck detectors that are not required. Once the desired detectors are selected, their output can be assigned to a trace using the Trace > Data menu or the drop-down menu on the right side of any trace tab.

Power Spectrum measurements with a Vector Network Analyzer can use only one detector at a time. A new measurement must be made to view the spectrum with a different detector selected. To select which detector is applied, go to the Detectors tab on the MeasSetup menu and select the detector from the drop-down list.