ACPR (adjacent channel power ratio), AltCPR (alternate channel power ratio), and noise are important performance metrics for digital communication systems that use, for example, WCDMA (wideband code division multiple access) modulation. ACPR and AltCPR are both measures of spectral regrowth. The power in the WCDMA carrier is measured using a 5MHz measurement bandwidth. For ACPR, the total power in a 3.84MHz bandwidth centered at 5MHz (the carrier spacing) away from the center of the outermost carrier is measured and compared to the carrier power. The result is expressed in dBc. For AltCPR, the procedure is the same, except we center the measurement 10MHz away from the center of the outermost carrier. To measure ACPR and AltCPR, refer to the test setup shown in Figure 2. The DUT (device under test) is the LT5528, which is a high linearity direct I/Q modulator. It accepts WCDMA modulation at the baseband inputs, and generates a WCDMA modulated signal at the RF output. Note that a free running RF generator provides the LO signal. This type of generator is used because of its superior noise performance. This is critical, as a noisy LO signal may corrupt the modulator output, and consequently the ACPR measurement. For the generator shown, the automatic level control must be switched off to avoid degrading its broadband noise floor. Also,
This document details performance metrics for digital communication systems using WCDMA modulation, specifically focusing on spectral regrowth (ACPR and AltCPR) and noise. It describes a test setup involving an HP8616A signal generator, the LT5528 device under test, and an Agilent E4436B spectrum analyzer to measure these parameters accurately.
The guide provides specific instructions for measuring ACPR and AltCPR using 5MHz bandwidths centered at various offsets from the carrier. It also outlines a procedure for measuring noise at a 30MHz offset by adding a tunable bandpass filter to reject the main signal, preventing analyzer overload while maintaining measurement accuracy.
| Measurement Bandwidth (Carrier) | 5MHz |
|---|---|
| ACPR Measurement Offset | 3.84MHz centered at 5MHz away from carrier center |
| AltCPR Measurement Offset | 10MHz away from carrier center |
| Resolution Bandwidth for Noise Measurement | 30kHz |
| Video Bandwidth for Noise Measurement | 100kHz |
It provides superior noise performance; a noisy LO signal may corrupt the modulator output and degrade ACPR measurement accuracy.
The LT5528 is a high linearity direct I/Q modulator that accepts WCDMA modulation at baseband inputs and generates a WCDMA modulated signal at the RF output.
The carrier power is measured using a 5MHz bandwidth, while the adjacent channel power is measured in a 3.84MHz bandwidth centered at 5MHz away from the center.
The suggested filters must provide greater than 20dB rejection at specific lower frequencies and greater than 80dB rejection at higher frequencies.