As in most nonlinear distortion measurements, such as measurements of harmonics, third-order intercept (TOI), and second harmonic inversion (SHI), care must be taken that internally generated intermodulation products do not cancel the distortion products from the DUT. This can occur when internally generated distortion products are close to the same amplitude as the DUT distortion products but 180 deg. out of phase. When large improvements can be observed to ACP results with slight changes to the input attenuation or input level, this is the most likely cause. When the amplitudes of the DUT and internally generated distortion products are equal, the uncertainty in the measurement ranges from +6 dB to –infinity. The smallest uncertainty in measurement results can be maintained when spectrum analyzer distortion products are several dB below the DUT distortion products. This may require using a higher-performance spectrum analyzer in some cases to eliminate these uncertainties.

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The Agilent X-Series signal analyzers have a built in Adjust for Minimum Clip feature that can be used to automatically set the mixer level based upon the input signal level. The mixer level is optimized to provide the optimum dynamic range without excessive measurement uncertainty.

Speed Versus Repeatability

Most modern spectrum analyzers use an average or root-mean-square (RMS) detector for measuring the power in the channels when performing a swept-tuned ACP measurement. The variance observed will be given as Eq. 4:

Understanding Adjacent Channel Power Measurements In Spectrum Analysis, Eq. 4

In most cases, the channel bandwidth, resolution bandwidth (RBW), and span are fixed based upon the ACP standard being measured. Sweep time will need to be increased if more repeatable results are needed.This will have an adverse effect on the overall measurement time.

Agilent Fast Power Option

The Agilent X-Series signal analyzers have a Fast Power option that allows a user to rapidly perform power measurements such as channel power, adjacent channel power, occupied bandwidth, and X-dB measurements (Option FP2). First, the power measurements are accelerated in hardware by performing real-time overlapped Fast-Fourier-Transform (FFT) computations in field-programmable-gate-array (FPGA) code. The results of each FFT are RMS averaged together to provide the real-time spectrum in which the power results are computed. This is a SCPI-only feature that allows users to perform measurements such as ACP directly within the hardware. As an example, a W-CDMA ACLR result can be requested by means of programming while in EVM measurement mode. For such a measurement, a total measurement time of 3 ms can be achieved with an acquisition time of 1 ms.

The Fast Power method improves repeatability because all the channels are measured simultaneously in a much wider bandwidth. As an example, the repeatability of the swept ACLR measurement in Fig. 1 can be calculated. The measurement setup uses a sweep time of 3 ms, an RBW of 100 kHz, and a channel bandwidth of 3.84 MHz in a span of 25 MHz. This swept measurement requires an overall time of 13 ms for completion. The repeatability of the swept measurement can be calculated by using Eq. 4:

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

It is now possible to compare the repeatability and measurement time of the Fast Power method using a 1-ms acquisition time and total measurement time of 3 ms to the swept example. For the Fast Power method, the following relation can be used for noise bandwidth (NBW):

Understanding Adjacent Channel Power Measurements In Spectrum Analysis, Eq. 5

Therefore, the Fast Power repeatability is:

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

In summary, a Fast Power ACLR measurement can be made four times faster and with repeatability improved by nine times over a swept measurement.

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

Figure 2 depicts various repeatability values for acquisition times for swept-tuned ACP and Fast Power ACP measurements. The blue and red lines are the theoretical results for swept-tuned ACP and Fast Power ACP measurements, respectively, from Eq. 4.

Swept tuned measurements will always provide higher dynamic range because a narrow bandwidth analog intermediate-frequency (IF) filter will be used. This narrow IF bandwidth filter provides a much lower peak power that will be observed at the analyzer’s analog-to-digital converter (ADC) compared to when the entire bandwidth is measured at once, as in the Fast Power case. If dynamic range is a major requirement, a swept-tuned ACP measurement should be used. If measurement speed and repeatability are needed in an automated test environment where maximum dynamic range is not the dominating factor, the Agilent X-Series signal analyzer’s Fast Power option is far superior to other methods.

Summary

This article has covered a broad set of issue related to making ACP measurements. As modulation standards evolve, other measurements, such as the spectral emission mask (SEM) measurements, will allow users to gain more insight into the linearity of their DUT. Measurements made in conjunction with linear distortion measurements, such as error vector magnitude (EVM) measurements, will ensure that the end users of this equipment enjoy better communications and a clean use of the limited spectrum.

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