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F
ew years ago, an RF test engineer would have been very focused in his very unique field, performing tests on discrete RF devices such as mixers, power amplifiers, low noise amplifiers and RF switches. Today, the test industry faces increasing levels of integration such that many of these discrete device functions ar used as building blocks and are contained within one chip or module.

Furthermore, the integration levels are such that system-on-a-chip(SOC) devices contain baseband(analog) functionality as well as digital functionality. Earlier RF devices often contained three-wire serial communications for controlling things such as gain control, but current digital functionality of these devicesis becoming complex.

SOC devices are those that have more than one of the discrete RF devices (such as LNA, PA, Mixer, RF switch, VGA, Baseband/IF modulator/demodulator, Transmitter, Receiver and Transceiver) combined on a substrate to provide some function, for example, placing all of the said devices that make-up a mobile phone handset onto a single microchip. Over the past few years, there have been many attempts to place the complete wireless radio on a chip, but for practical reasons, what is termed SOC is often only a portion, such as that comprising the input/output at the antenna down to the analog baseband input/output on a wireless transceiver. Thus, an exception to the above statement is that a discrete transmitter, receiver, or transceiver may also be term as an SOC device.

The recent trends have been moving toward much higher levels of integration. This is primarily due to two reasons: reduced-cost at the consumer level and the desire for reduced power consumption (longer battery life). It is apparent that lower-frequency analog and lower-level digital functionality is core-siding on the SOC chip with RF front-end devices. This trend will continue as pressures to achieve the above two goals surmount. Below is an of RF SOC block diagram with higher level of integration.
SOC devices, as used in this discussion, have atleast one RF input(or output). Based on that, SOC devices for wireless communications can be broken down into the following types, based on input/output configuration: RF/RF; RF/IF; RF/baseband; and RF/Digital.

RF/RF and RF/IF are treated similarly with respect to testing procedures. The measurement techniques for IF frequencies still require attention to detail and an understanding of making measurements at high frequencies where traditional Ohm's Law-based calculations will not work. Examples of these types of SOC devices would include a chip consisting of a filter/LNA combination or filter/LNA/mixer combination to be used as the front-end-receiver. Additionally, they may have some digital signals for received signal strength indicator (RSSI) or automatic gain control(AGC).

RF/baseband SOC devices are used quite commonly today in WLAN modems. They may contain everything from the input filter/LNA all the way to the in-phase, quadrature (IQ) outputs. When testing these devices, the test engineer must have an understanding of RF measurement techniques, which are based on the frequency domain and time domain. RF/digital SOC devices are used quite commonly today in Bluetooth modems. The reason for this is that the bluetooth architecture is relatively simple to implement on a single chip. It has been explored quite exhausitively, and as a result the low cost pushes a minimum number of chips to be used in a bluetooth modem.

That's all for now. Stay tune on my next articles as I'm giving some few details on the RF discrete devices being mentioned.