Current Issue An Independent Journal Dedicated to the Advancement of Chip - Scale Electronics April 2001 Email the editor

By Nelson Lee, ST Assembly Test Services Ltd., Singapore

Advances in CMOS semiconductor technology drive the market toward higher integration of discrete RF components, such as those used in LNAs, mixers, up/down converters, IF transceivers and digital modulation, into a single silicon chip solution.

This trend is evident in the recent emergence of Bluetooth* devices in the semiconductor market.

This article addresses the RF test challenges faced by semiconductor test subcontractors to support production testing of integrated wireless communications devices.

With the eroding average selling price (ASP) of the product, there is a continuous "squeeze" on the manufacturer to decrease testing costs through test-time reduction and by increasing UPH throughput.

The testing of today's highly integrated RF devices requires test equipment that can respond to higher analog signals (above 2.4 GHz), while retaining the traditional capability of a mixed-signal analog tester (with formatted digital drive/compare pins and a DSP processor for evaluating the mathematics of the digitized signals).

Figure 1. RF frequency spectrum

What Is RF?

RF refers to radio signals used in radar, wireless communications, electronic navigation, satellite television and a growing list of electronic applications. RF covers a large spectrum of frequencies from 10 KHz to 300 GHz, as shown in Figure 1.

In most IC testing environments, the range of frequencies from 10 KHz to 300 MHz is referred to as the RF baseband frequency, while the frequency range above 800 MHz (and up to 300 GHz) is identified as RF front-end frequency, microwave or terrestrial frequency. Such signals possess short wavelengths measured in centimeters (roughly 30 cm to 1 mm).

The baseband frequency contains the "signal ingredient" for video, audio or data appliances. This frequency is usually modulated by another higher frequency carrier signal for wireless transmission.

Traditionally, modulation is achieved using an analog method, such as AM, FM, PAM or PWM. Due to the limitations of the analog modulation bandwidth which restricts the number of users within the allocated bands, and because of the need for better clarity in recovering transmitted signals, the digital modulation method was developed.

Figure 2. Generic wireless RF building blocks

Testing Parameters

Figure 2 illustrates the typical building blocks of all wireless RF appliances and normally consists of the following module groups:

• Input/Output Refers to appliances such as mobile phones, television sets, links to computers, home equipment, etc.

• RF Baseband Used for efficient encoding and decoding of signals into a smaller number of digital information bits. This enhances the bandwidth or number of users allowed within the allocated frequency bandwidth. DSP (digital signal processing) chips are typically used for these kinds of complicated modulations.

• RF Front-End Used for wireless communication with another appliance (such as communications between mobile phones, receiving television signals, etc.).

In most RF front-end testing, the test coverage consists of the following checks on the device-under-test:

• Detects small signals in the presence of noise

• Does not add excessive noise or distortion to the electronic systems

• Transmits efficiently, resulting in lower battery consumption

• Transmits clearly, eliminating pollution of the RF spectrum environment

Some common RF testing parameters include S-Parameters, VSWR, RF-IF-LO Isolation, phase noise, carrier suppression, gain compression, I/O balance, conversion gain, IP3 (TOI), noise figure, and RF power.

Minimal Testing on Discretes

Some IC manufacturers conduct only minimal testing on their discrete RF devices, based on the belief that RF devices can be fully verified for gross functionality by the S-parameters and power measurement testing.

Other RF test parameters are usually "guaranteed" through sufficient design-wafer process marginality, which normally causes minimal impact on test yield. Moreover, these discrete RF devices are subjected to further testing at the application board level.

Tester Types

The RF front-end consists of discrete RF components with some level of integration, such as that which occurs between LNA and the RF mixer (up/down converters employed in satellite transmission). The major wafer processing technologies used to implement RF semiconductor devices are gallium arsenide (GaAs), silicon germanium (SiGe) and silicon (Si).

Figure 3. RF semiconductor technologies

Figure 3 shows the type of semiconductor technology used by RF designers in the fabrication of RF components. The testers used to support these semiconductor chips and common packages are shown in Figure 4.

The tester type is mainly focused on testing specific modules. One reason for this level of specificity is that discrete assembly packages may contain various types of RF modules, which may even be manufactured by different companies.

Figure 4. Spectrum of RF devices

Integrating Digital I/O

Vendors for low-cost RF testers for discrete RF components are now integrating digital I/O capability and adding DSP processors to evaluate the mathematics of the mixed-signal testing.

Next