FAQ: Understanding ADL5380 IQ Demodulator EVM Performance Plots

Document created by Rakesh on Jan 22, 2010Last modified by AndyR on Jan 31, 2012
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Figure 83 and Figure 84 of the ADL5380 datasheet show EVM curves for demodulated WiMAX and WCDMA respectively.

 

 

Figure 83.bmpFigure 84.bmp

 

 

Q.

Why is there an EVM difference between the 10MHz BW upstream WiMAX signal at 2.6GHz and a WCDMA signal at 1900MHz? The difference is about 10dB, -50dB vs. -40dB.

 

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A.

The WiMax signal has a bandwidth of 10MHz the WCDMA bandwidth is 3.84 MHz. In both cases, the IQ Demodulator outputs are ac-coupled, creating a high-pass corner. In the case of the wider band WiMax signal, a smaller fraction of the signal's energy is filtered out compared to the WCDMA carrier. In addition, there is no sub-carrier at the center of a WiMax carrier. So there is less energy to be lost at the center of the carrier.  This makes the WiMax modulation scheme conducive demodulation via Zero-IF direct conversion. This is not the case for WCDMA which is a spread spectrum signal not composed of sub carriers.

 

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Q.

In Figure 83 (WiMax), the EVM degrades at approximately -5 dBm; in Figure 84, the EVM remains flat up to a much higher level. Why is this?

 

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A.

As the input power to the IQ demodulator increases, increased distortion degrades EVM. An OFDMA WiMAX signal has an 11dB Crest Factor. So although the modulated power is -5dBm, the carrier will have peaks that can be as large as +6 dBm. In contrast to this, a reverse link WCDMA carrier (i.e. from mobile to basestation) will have a much lower crest factor, resulting in a flat EVM profile up to higher input power levels.

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