Adjacent channel power (ACP) measurement measures the power leakage from the carrier channels into the neighboring frequency channels, commonly referred to as offset channels. The following figure shows a typical carrier channel and offset channel configuration.

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 and integration bandwidth (IBW). 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 channel center frequency is specified relative to the center frequency of the closest carrier. The placement of offset channels from the nearest carrier is defined by the ACP offset frequency definition.

Offset Frequency Definition

The frequency offset of the offset channels from the closest carrier is defined by the ACP offset frequency definition. The offset frequency is defined in one of the following ways:

  • Carrier Center to Offset Center—The offset frequency is defined from the center of the closest carrier to the center of the offset channel.
  • Carrier Center to Offset Edge—The offset frequency is defined from the center of the closest carrier to the nearest edge of the offset channel.

The following image describes the offset frequency definitions:

Offset Sideband

Two offset channels are configured on either side of the carriers when you set the ACP Offset Sideband to Both. Only one offset channel is configured when you set the ACP Offset Sideband to either Neg or Pos. A negative sideband creates a lower offset channel to the left of the leftmost carrier. A positive sideband creates an upper offset channel to the right of the rightmost carrier. You can independently configure the IBW, RRC filter, power reference, and relative attenuation for each offset channel.

Power Reference

Power reference refers to the carrier channel relative to which the offset channel power is measured. The power reference is one of the following carriers:

  • Closest carrier—Power in the carrier with center frequency closest to the offset channel center frequency
  • Highest carrier power—Highest power among all the active carriers
  • Composite carrier—Total power of all the active carriers
  • Specific carrier—Power in the carrier specified by the carrier index
Note If the power reference carrier is set to passive mode, the measurement does not compute the offset channel power.
Note Use relative attenuation to compensate for external attenuation for each offset channel.

RBW and Sweep Time

When you set the ACP RBW Auto property to True, the measurement sets the RBW using the following ratio:

Min (Carrier IBW, Offset Channel IBW):RBW3dB = 100, where RBW3dB is a value between 1 Hz and 1 MHz

When you set the ACP Sweep Time Auto property to True, the measurement sets the sweep time to the following values:

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

Refer to the Spectral Measurements topic for more details on 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.

Measurement Method

ACP measurement supports two measurement methods based on the requirement of measurement speed and dynamic range.

When you set the ACP Meas Method to Normal, the measurement acquires the signal using a wideband front-end filter. This measurement acquires the spectrum with a large span and perform FFT-based processing.

The following steps help improve the dynamic range of the measurement:

  • The RF input attenuation must be set such that the mixer operates at optimal operating level. Refer to the dynamic range chart of the device to know the optimum mixer level. Mixer Level = Reference Level - RF Attenuation
  • The reference level must be adjusted to the power level of the test signal. Doing so sets the signal at the A/D converter (ADC) to full scale, which results in the best SNR and SFDR performance.
  • Narrowband spectrum analysis mode allows dynamic range beyond that of the ADC.
  • Re-ranging the reference level when measuring the distortion amplitude allows the best dynamic range.

When you set the ACP Meas Method to Dynamic Range, the measurement uses hardware features, such as the IF filter and IF gain. The measurement acquires the signal using narrowband IF filters available on the analyzer. Analog IF filters used for each offset channel may vary based on the device in use. Narrow bandwidth IF filters are used to acquire a spectrum closer to the carrier channel to filter out the high power seen from the carrier in the adjacent channels. A wideband IF filter is used to acquire a spectrum farther from the carrier to reduce measurement time. The carrier channels are not filtered.

The measurement resets the ranges for each offset channel based on the reference level set for the measurement and the analog IF filters to increase the power seen by the ADC of the digitizer.

Note Using narrowband IF filters restricts span per acquisition and increases the time taken to sweep and acquire the total span required by the measurement. This method restricts the RBW based on the narrowband filter used by the measurement. The following table lists the analyzers and the filters used by the measurement.
Supported Analyzer IF Filters Max RBW
NI PXIe-5665 (3.6 GHz) 300 kHz 300 kHz
NI PXIe-5665 (14 GHz) 300 kHz, 5 MHz 300 kHz
NI PXIe-5668 300 kHz, 5 MHz 300 kHz
Note   Dynamic Range method is not supported when NI PXIe-5668 is used with NI PXIe-5698 preamp.

Noise Compensation

Noise compensation enables the measurement of the inherent noise floor of the signal analyzer for the RF path used by the measurement, and uses this result to compensate the signal measurement. For a given set of measurement configurations and the state of the signal analyzer, noise floors are constant. Because measuring the noise floor adds to the measurement time, the noise floor results are cached so that the measurement can use it later. When the signal analyzer or measurement parameters change, the noise floor measurement is reinitiated to return valid measurements.

Note The standard-specific ACP 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.
Note Noise compensation is not supported when NI PXIe-5668R is used with NI PXIe-5698 preamp.

Custom ACP

Custom ACP enables you to perform ACP measurement on channels with custom settings. Set ACP Channel Configuration Type to Custom to enable Custom ACP. This allows you to change the following parameters in order to configure custom channels. These parameters are valid only for Custom ACP mode.

  • Subblock Offset specifies the offset of the subblock measurement relative to the subblock center as computed in the Standard ACP mode.
  • Subblock Integration Bandwidth specifies the integration bandwidth for subblock measurement.
  • Carrier Integration Bandwidth specifies the integration bandwidth for carrier measurement.
  • Number of Offsets specifies the number of configured offset channels.
  • Offset Frequency specifies the offset frequency where the offset channel should be measured. This offset is with reference to the subblock/carrier shifted by Subblock Offset.
  • Offset Integration Bandwidth specifies the integration bandwidth for offset channel measurement.
  • Offset Sideband specifies which sidebands of the offset channel should be measured.

In Custom ACP mode, the following parameters are configured as defined in the standard specifications:

Uplink Downlink
Offset Frequency Definition Subblock Center to Offset Center Carrier Center to Offset Center
Power Reference Composite Subblock Closest Carrier