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Spectrum analysis is essential for understanding the frequency-domain characteristics of components, circuits, and systems, but these instruments and their measurements are not foolproof. In fact, eight common mistakes plague the accuracy and effectiveness of spectrum-analyzer measurements—errors that can lead to improperly adjusting a device under test (DUT) or shipping a device to a customer that has not met its required specifications. Luckily, some simple guidelines can be followed to ensure that the spectrum analyzer is being used properly and performing to expectations.

Many of the mistakes made when using a spectrum analyzer have to do with using the wrong equipment, or else using the analyzer’s controls incorrectly. The eight common errors mentioned above are as follows:

  1. Using the wrong detector.
  2. Using the wrong averaging type.
  3. Measuring the analyzer’s own internally generated distortion products.
  4. Incorrect mixer level for EVM measurements.
  5. Not using single sweep when remotely controlling the analyzer.
  6. Not synchronizing measurements with *OPC.
  7. Turning the display off and using binary data types when transferring data for speed.
  8. Feeding too much power to the input of the spectrum analyzer.

Such errors are quite innocent and easy to make. The first one (using the wrong detector) can lead to wrong results simply by not matching the detector to the needs of the measurement. Modern spectrum analyzers operate with a variety of different detectors, for different signal types—including peak, sample, average, and normal type detectors. Using the wrong detector type can produce incorrect results, potentially leading to incorrectly adjusting a device under test (DUT) or missing a present-but-undetected signal.

Picking A Detector

Selecting the proper detector for a spectrum analyzer is a simple-enough task when some general rules are followed. A sample detector, for example, provides a single sample for each trace point on the analyzer display. If the display is set for 1001 trace points (#Pnts), each trace point will represent a single sample evenly spaced across the span of the instrument in the frequency domain. The interval in frequency bandwidth between trace points will be given by the frequency span divided by the number of trace points, or SPAN/(#Pnts-1). A sample detector is effective for measuring noiselike signals.

Sample detection

When measuring continuous-wave (CW) signals, however, the analyzer’s resolution bandwidth (RBW) must be set wider than the trace interval. If the RBW is too narrow, a CW signal amplitude measured with a sample detector may appear too low or be missed altogether (Fig. 1). Most spectrum analyzers will automatically select the sample detector when trace averaging is applied, so it is possible to unknowingly be using the sample detector while measuring CW signals.

In contrast, a peak detector maintains the highest amplitude value in each measurement interval and displays this value in the trace point. A peak detector is effective for measuring CW signals, but can provide incorrect levels when measuring noiselike signals, unless it is a “max hold” type measurement where the analyzer is being used to read worst-case maximum power.

An average detector averages the power between two trace points and displays the mean power that has been averaged on a linear scale, such as in milliwatts (mW). Such a detector is well suited for noiselike signals, but is also effective for correctly showing the amplitude of a CW signal, provided that the RBW is at least as wide as the trace interval. As with the sample detector, an average detector can show too low a reading for the amplitude of a CW signal if the RBW is set too narrow.

A normal detector, in most cases, is the default detector for a spectrum analyzer. A normal detector always shows the correct amplitude for a CW signal regardless of the RBW selected relative to the trace interval. It is also effective when measuring noiselike signals. It does this by displaying the peak value of a signal that rises and falls in level during an odd trace point and shows the minimum value of the signal during the even trace point. This causes the peak-to-peak value of a noiselike signal to be accurately represented on the analyzer’s display.

For trace intervals where a signal only rises or falls, the peak value will be displayed. This occurs when a CW signal is swept through the trace and the amplitude is retained. A normal detector should not be used when integrating noise power—such as for channel-power or adjacent-channel-power measurements—since the alternating peaks and minimums will improperly represent the distribution of power in a noiselike signal.

In general, unless there is certainty about the type of detector to use for a particular measurement, it is best to use the default detector selected by the spectrum analyzer. And if there is some uncertainty, the peak detector can be used for measuring CW signals and the average detector selected for noiselike signals.

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