LINEAR Application Note 99 LT5528 WCDMA ACPR, AltCPR and Noise Measurements User Guide

Application Note 99

May 2005

LT5528 WCDMA ACPR, Alt CPR and Noise Measurements Doug Stuetzle INTRODUCTION ACPR (adjacent channel power ratio), Alt CPR (alternate To measure ACPR and Alt CPR, refer to the test setup channel power ratio), and noise are important perfor- shown in Figure 2. The DUT (device under test) is the mance metrics for digital communication systems that LT5528, which is a high linearity direct I/Q modulator. It use, for example, WCDMA (wideband code division mul- accepts WCDMA modulation at the baseband inputs, and tiple access) modulation. ACPR and Alt CPR are both generates a WCDMA modulated signal at the RF output. measures of spectral regrowth. The power in the WCDMA Note that a free running RF generator provides the LO carrier is measured using a 5MHz measurement band- signal. This type of generator is used because of its width; see Figure 1. In the case of ACPR, the total power superior noise performance. This is critical, as a noisy LO in a 3.84MHz bandwidth centered at 5MHz (the carrier signal may corrupt the modulator output, and conse- spacing) away from the center of the outermost carrier is quently the ACPR measurement. For the generator shown, measured and compared to the carrier power. The result the automatic level control must be switched off to avoid is expressed in d Bc. For Alt CPR, the procedure is the degrading its broadband noise floor. Also, the operating same, except we center the measurement 10MHz away frequency can drift slightly, so manual frequency correc- from the center of the outermost carrier. tion could be needed.

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1 TO 4 CARRIERS 3.84MHz WIDE SPACED 5MHz APART

3.08MHz TO 6.92MHz:

–45d Bc/30k Hz 8.08MHz TO 11.92MHz: –50d Bc/30k Hz

FREQUENCY

MHz MHz MHz

Figure 1. WCDMA ACPR Limits, Per 3GPP TS 25.104, Section 6.6.2.2.1

AN99-1