This article will review adjacent-channel-power (ACP) measurements. ACP is a measure of the nonlinear characteristics of a device under test (DUT) and indicates the amount of spectral regrowth occurring in adjacent channels. This article will explain how the measurement is performed and how to optimize the measurement for speed, repeatability, and dynamic range. This article will also explore new techniques for making this measurement with the fastest and most repeatable results.

ACP History

ACP measurements have been performed for many years. Originally, ACP measurements were used for narrowband analog modulated signals and measured the ratio of the upper and lower channels to the total power transmitted. The total power transmitted was defined as the carrier plus the majority of power in the upper and lower channels. Today, an ACP measurement is defined as the ratio of one or more upper and lower intervals of power to the total carrier power across the bandwidth of the channel.

Cellular communications systems have relied on ACP measurements to ensure that power  radiated into an adjacent channel is limited—to ensure that the signal-to-noise ratio (SNR) in the adjacent channel does not interfere with communications in that channel. Cellular standards such a W-CDMA, cdma2000, and LTE all have defined methods and limits for ACP measurements. These standards go even further and even provide more descriptive names for ACP measurements. For example, cdma2000 adopted Adjacent Channel Power Ratio (ACPR) and W-CDMA adopted Adjacent Channel Leakage Ratio (ACLR) as more specific names for their versions of ACP measurements. GSM and EDGE have similar requirements that use what are called Output RF Spectrum (ORFS) measurements to ensure that power radiated into adjacent channels does not exceed certain levels. Most modern spectrum analyzers have programmed, predefined settings for various standards that allow for quick measurement setups.

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For narrowband analog modulated signals, the phase noise present in the local oscillator (LO) accounted for most of the power present in the adjacent channels in these systems. Today, with the introduction of wide-bandwidth signals, the power in the adjacent channel(s) can potentially be from a combination of several factors, including phase noise, intermodulation distortion (IMD), and the noise floor of the system. Similarly, these factors influence the dynamic range a spectrum analyzer can achieve in making an ACP measurement.

The ACP result can simply be stated as Eq. 1:

Understanding Adjacent Channel Power Measurements In Spectrum Analysis, Eq. 1

where is the power in the adjacent channel and is the power in the channel.

Figure 1 is the screen image of an Agilent N9020A MXA signal analyzer performing an ACLR measurement of a W-CDMA signal. The ACLR measurement is defined as the relative power in the adjacent and alternate channels to the amount of power in the carrier measured in a 3.84-MHz bandwidth. The spacings of the adjacent and alternate channels to the carrier are 5 and 10 MHz, respectively. The power levels in the channels are calculated using the integrated power method as shown in Eq. 2, where the power level of each trace point (in logarithmic dBm) in the channels is converted to a linear milliwatt (mW) value and the values are summed together with the appropriate integration parameters as shown in Eq. 2:

Understanding Adjacent Channel Power Measurements In Spectrum Analysis, Eq. 2

Compensation in power is then applied to account for the root-raised cosine filtering used in W-CDMA.

Understanding Adjacent Channel Power Measurements In Spectrum Analysis, Fig. 1

Dynamic Range

The spectral regrowth in wide-bandwidth signals in adjacent and alternate channels is primarily composed of coherent and noncoherent distortion products. Coherent products are normally comprised of third- and fifth-order nonlinear distortion products that develop within the DUT. Noncoherent products are noiselike in nature and come from phase noise associated with the system’s LO and/or the noise floor of the DUT.

A spectrum analyzer is not immune to spectral regrowth. The amount of distortion produced , however, can be greatly affected by the settings of the instrument. The third- and fifth-order distortion products that are internally generated in the spectrum analyzer are a function of the analyzer’s mixer level, where the mixer level can be calculated using Eq. 3:

Understanding Adjacent Channel Power Measurements In Spectrum Analysis, Eq. 3

Increasing the internal or external attenuation will decrease the internally generated spectrum analyzer coherent distortion products by reducing the level present in the mixer. Increasing attenuation has an adverse effect on the spectrum analyzer, however, by increasing its noise floor. This is why it is challenging to optimize spectrum analyzer dynamic range in ACP measurements.

Dynamic Range Optimization

Before attempting to choose optimum spectrum analyzer setting for dynamic range, it is first necessary to understand the dynamics associated with the various distortion products involved in the measurement. Third-order distortion products will appear primarily in the adjacent channel whereas fifth-order distortion products dominate in the alternate channels. As the mixer level is reduced by increasing attenuation, fifth-order distortion products will drop at a much faster rate than third-order distortion products (5:1 versus 3:1). This will cause the coherent distortion products to quickly fall to levels at or below the noise floor of the spectrum analyzer in the alternate channel , while the third-order distortion products will still be above the noise floor in the adjacent channel. Increasing attenuation will now negatively affect the ACP performance in the alternate channel, increasing the noise floor.

The best method to deal with the tradeoff between reducing the coherent distortion in the adjacent channel while not compromising the dynamic range in the alternate channel is to use noise corrections or noise floor extensions. First, increase the attenuation to reduce the internally generated distortion products in the adjacent channel to a point that ACP results do not change as attenuation is increased. At this attenuation level, there is an assurance that the DUT’s ACP performance in the adjacent channel is what is being measured. Noise corrections or noise floor extensions can then be used to reduce the noncoherent noise that will be present in the alternate channel that comes from the noise floor of the spectrum analyzer.

If phase noise is the limiting factor, most modern spectrum analyzers allow a user to optimize phase noise offset either close in or far from the carrier when measuring standards such as Tetra, where phase noise dominates the measurement.

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