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Address ATE challenges for mission-mode test of wireless transceivers

31 Jul 2012  | Steven Fields, Scott Therrien, Steve Lyons

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Wireless devices have become vital tools in everyday life. They allow us to transfer voice, data, images and video with ease. The evolving quest for increasing quantities of data to be transferred to the furthest reaches has driven modulation standards to allow higher date rates. In order to produce devices that utilize new high-data-rate modulation standards, the baseband and RF modulator chipsets which comprise these devices must deliver increasing levels of performance and precision. This makes the performance of the automatic test equipment used to test the semiconductor devices in production more demanding. Combined, all of this requires new testing strategies.

Transmitting and receiving data over the air is challenging because wireless devices need to be able to decode symbols accurately in the presence of numerous uncontrolled sources of noise and interference. This is especially critical when using increased data rates for WLAN and cellular devices, as well as when trying to maintain very high data rates at the edges of the wireless range. Due to the environment in which these devices operate, it is critical that the wireless transmitter and receiver elements produce little or no impairment to the modulated / demodulated signal. For heterodyne systems that use an intermediate frequency, controlling impairments can be made easier by the use of better components and multiple down conversions that can maintain image frequency rejection.

Impairments
Heterodyne systems also can have the ability to process the in-phase (I) and the quadrature-phase (Q) signals in DSP and avoid exposing the baseband signals to analog circuitry. This generally reduces the amount of IQ impairments that are introduced into the system at the cost of increased design complexity and power consumption. Homodyne (Zero-IF) conversion systems on the other hand are less expensive, less complex and require less power. They perform a direct conversion from baseband signals in the analog domain to RF or vise versa. It is this exposure to the analog domain and less control of the image frequency that can introduce IQ impairments.

No matter what transceiver architecture is used, it can be difficult to produce a mixed-signal IC with no IQ impairments. For transceivers with less than perfect native performance attributes, it may be necessary to adjust baseband signals to overcome impairments in the transceiver, in the baseband to transceiver link or in the system itself. This has the effect of improving the modulation accuracy along with improving the spectral properties of the modulated signal.

Impairments that may be acceptable for low bandwidth standards could be unacceptable for latest generation higher bandwidth standards. Take, for example, the evolution of the cellular standards GSM, WCDMA, and LTE. 3G GSM and WCDMA employ GMSK and QPSK modulation schemes respectively, providing two bits per symbol each. 4G LTE, however, employs a 16QAM or 64QAM scheme with the capability of tripling the bit density of each symbol relative to its 3G counterpart. In wireless LAN systems, the demand for higher data rates drives 256QAM. This requires the modulator to produce higher quality modulated signals in order to maintain higher data rates at the furthest extent of the required coverage area in the presence of broadband noise. An example of this higher quality modulation requirement is the 802.11ac standard [1], where the error vector magnitude (EVM) system requirement for QPSK is 22.4% and for 256QAM is 3.1%. As future standards drive increased data rates over increased distances, the importance of compensation for IQ impairments will become even more important.

Correct for impairments
There are two fundamental ways to design systems with improved modulation accuracy. One is to improve the architecture of the devices to remove the sources of I and Q impairments. This approach comes at cost of complexity and price. Another approach is to measure the inaccuracies and to perform a one-time or real-time correction to the I and Q signals to correct for these impairments that result in improved modulation accuracy. Figure 1 shows an example of such a device. A feedback path is used to evaluate the IQ impairments. The detection of IQ impairments is typically done by using either side band suppression measurements or digital demodulation to evaluate these impairments. Once impairments have been determined, depending on the system architecture, corrections can be made by affecting the analog circuits or pre-distorting either the I or Q waveforms digitally to achieve the I and Q balance required.

Figure 1: Mission Mode: Quadrature modulator showing impairment correction in end application.


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