Considerations for board design

Figure 6 Example of the placement of an ESD protection diode for an antenna that should be avoided
Figure 6 Example of the placement of an ESD protection diode for an antenna that should be avoided

Even the optimum ESD protection diode does not provide any beneficial effects unless a printed circuit board is properly designed.
The following considerations apply to the board design:

  1. Connect an ESD protection diode directly to a board trace leading to the DUP. Do not connect an ESD protection diode to a divergent path from the antenna line because such a trace has undesired inductance, increasing the impedance of the path to the ESD protection diode.
  2. The above guideline also applies to GND. Connect an ESD protection diode to the GND plane.

16-QAM/64-QAM

Modern telecommunication systems use 16-QAM, 64-QAM or other modulation methods in order to satisfy the requirement for high-speed communication. Quadrature amplitude modulation (QAM) transmits information by modulating both the amplitude and phase of two carrier waves. In QAM, the constellation points are usually arranged in a square grid as shown in Figure 7 instead of a concentric grid. Although QAM provides high spectral information density, it is susceptible to noise and waveform distortion.

Modulation methods are broadly divided into amplitude-shift keying (ASK), phase-shift keying (PSK), frequency-shift keying (FSK), and quadrature amplitude modulation (QAM).

Accompanying a shift from analog data to digital data, many communication standards have adopted a modulation method called quadrature phase-shift keying (QPSK). QPSK uses a constant amplitude level and four phase levels. It transmits information as one of four symbols, each representing two bits of data. To further increase the spectral information density, migrations to higher QAM channel counts such as 16-QAM and 64-QAM are in progress. 16-QAM uses four amplitude levels along two orthogonal directions and transmits information as one of 16 symbols, each representing four bits of data. 64-QAM uses eight amplitude levels along two orthogonal directions and transmits information as one of 64 symbols, each representing six bits of data.

QPSK
QPSK
16-QAM
16-QAM
64-QAM
64-QAM

Figure 7 Modulation methods

Figure 8 Waveform distortion and harmonics
Figure 8 Waveform distortion and harmonics

Harmonic distortion

Harmonic distortion is amplitude distortion caused by system nonlinearities. It occurs when the gain of an amplifier varies with input voltage and when an input waveform is clamped.
For example, a clean sine wave contains only a fundamental harmonic. Suppose that the fundamental harmonic is clamped by a circuit. This causes harmonic distortion. The harmonic frequency becomes an integer multiple of the fundamental frequency. In Figure 8, clamping generates an odd-order harmonic because the sine wave is equally clamped on the positive and negative sides. Even-order harmonics occur, depending on the waveform distortion.

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