Spectral Emission Mask (SEM) Overview

NI-RFmx EV-DO 2.5 Help

Edition Date: June 2018

Part Number: 375769F-01

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SEM measurements measure out-of-band emissions in the neighboring bands of the carrier. In measurements such as ACP, the emissions may remain undetected when measuring the integrated power in the band. SEM uses the spectral mask or limit you specify to measure the margin of the emission level from the limit and reports the measurement status.

The following figure shows the configuration of SEM carriers and offset segments.


Carrier Channels

Carriers are channels in which significant power is transmitted and for which power leakage in the offset channels is measured. Multiple carriers are configured relative to the RF center frequency by specifying the carrier offset. Each carrier is configured for the integration bandwidth (IBW), RBW, and RRC channel filter. Power in each carrier is measured by integrating the power in the specified IBW after applying the channel filter. The total carrier power measured is the aggregate power of all the active carriers.

Offset Segments

Offset segment start and stop frequencies are specified relative to the closest carrier offset frequency. The placement of the offset segments with respect to the nearest carrier is defined by the SEM offset frequency definition.

Offset Frequency Definition

The offset frequency definition defines the offset start and stop frequency of the offset segment with respect to the nearest carrier channel in one of the following ways:

  • Carrier Center to Meas BW Center—The start frequency and stop frequency are defined from the center of the closest carrier channel bandwidth to the center of the offset segment measurement bandwidth respectively.
  • Carrier Center to Meas BW Edge—The start frequency and stop frequency are defined from the center of the closest carrier channel bandwidth to the nearest edge of the offset segment measurement bandwidth respectively.
  • Carrier Edge to Meas BW Center—The start frequency and stop frequency are defined from the nearest edge of the closest carrier channel bandwidth to the center of the nearest offset segment measurement bandwidth respectively.
  • Carrier Edge to Meas BW Edge—The start frequency and stop frequency are defined from the nearest edge of the closest carrier channel bandwidth to the edge of the nearest offset segment measurement bandwidth respectively.

The measurement bandwidth of the offset segment is the bandwidth represented by the RBW and bandwidth integral values as shown in the following equation:
Measurement Bandwidth = RBW * Bandwidth Integral

The following image describes the offset frequency definitions:

Offset Sideband

An offset segment is configured on either side of the carrier when you set the offset sideband to BOTH. Only one offset sideband is configured when you configure a positive (POS) or negative (NEG) sideband. Negative sideband creates a lower offset segment to the left of the leftmost carrier. Positive sideband creates an upper offset segment to the right of the rightmost carrier. Each offset segment is configured for IBW, RBW, RRC filter, relative attenuation, spectral mask, and measurement failure criteria.

RBW and Sweep Time

RBW is configured for each carrier and offset segment. When you set the RBW to Auto, the measurement sets the RBW using the following ratio:

IBW:RBW3dB = 100, where RBW3dB is a value between 1 Hz and 1 MHz

When you set the sweep time to Auto, the measurement sets the sweep time using the following value:

  • k/RBW3dB when the RBW filter is FFT based, where k is a constant that depends on the FFT window
  • 10/RBW3dB for other RBW filters

Refer to the Spectral Measurements topic for more information about RBW, sweep time, and averaging.

Power Measurement

Power Units

The absolute power values measured are reported in dBm (integrated power) or dBm/Hz (power spectral density). The relative power values are not affected by the units specified.

Use relative attenuation to compensate for external attenuation for each offset channel.

Relative Power Reference

Carriers are measured relative to the total power of all enabled carriers.

Offset segment power is measured relative to either integrated power or the peak power of the closest carrier. Use the SEM Ref Type property to specify integrated power or peak power as reference.

Note Note   If the closest carrier configured is disabled, the offset segment power is not measured.

Spectral Mask

Limit Lines

The absolute and/or relative limit lines for each offset segment define the spectral mask. A limit line is defined by a start and stop power. Start power corresponds to the frequency point closer to the carrier. A level line can be configured by specifying the start power limit and coupling it to the stop power using the SEM Offset Relative Limit Mode.

Relative limits are specified relative to the integrated power or peak power of the carrier closest to the offset segment. Use the SEM Ref Type property to specify whether to use integrated power or peak power.

Margin and Measurement Status

The spectrum is corrected for external attenuation for each offset based on the specified relative attenuation. This spectrum is compared against the limit lines to report margin and measurement PASS/FAIL status. Margin, in dB, is the largest difference between the measured spectrum and the limit line.

The measurement failure criteria, also called the limit fail mask, can be one of the following criteria:

  • Absolute—Checks for emissions crossing the absolute limit line.
  • Relative—Checks for emissions crossing the relative limit line.
  • Absolute AND Relative—Checks for emissions crossing both the absolute and relative limit lines. The highest margin with respect to the higher limit mask is reported.
  • Absolute OR Relative—Checks for emissions crossing either the absolute or relative limit lines. The highest margin with respect to the lower limit mask is reported.

The measurement returns the frequency and absolute power (dBm or dBm/Hz) at the reported margin, and it also returns the power relative to the integrated power or the peak power based on the value of the SEM Ref Type property.

Note Note   The standard-specific SEM measurement does not support configuring all the properties and fetching all the results mentioned in this topic. The standard-specific measurements do not expose some properties with settings that are specified by the standard or are not relevant to the standard.

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