Essential testing on Bluetooth transceiver ICs

Designing these type of radios is indeed tough, but manufacturing and testing are no less challenging. All radio appliances with a Bluetooth application must comply with a series of rigorous tests. Table 1 shows some outline test requirements.

Figure 1 shows a typical ATE arrangement for testing a Bluetooth transceiver IC—in transmit mode—configured for testing the –20 db bandwidth parameter. The ATE powers the IC and programs its PLL/VCO to a predetermined Bluetooth channel (2.4 ~ 2.5 GHz) using the corresponding digital pattern. The ATE then introduces a predefined wait time for the VCO to settle to the programmed carrier frequency. (See "What's in a Bluetooth transceiver IC?")

Next, the ATE sets the IC’s PLL in open loop to allow modulation to take place using a pseudo-random bit sequence (PRBS) via the IC’s TX data pin. The ATE’s microwave receiver connects to the antenna of the Bluetooth IC’s transmitter using a microwave port. After further predefined delay following setting of the digital pattern, the ATE triggers the microwave receiver to capture the transmit signal. The ATE performs a fast-fourier transform (FFT) on the digitised samples collected by the microwave receiver.

From the captured signal (see Figure 2), the ATE scans and measures first the power of the carrier frequency, second the frequency component which is –20 dB from its programmed carrier frequency on both sides of the GFSK signal, and third the bandwidth.

The ATE arrangement shown in Figure 1 is equally applicable for testing receiver sensitivity parameters of a transceiver IC when set to receive mode.

Testing can be a very expensive part of the manufacturing process in ramping-up to production volumes of Bluetooth ICs. Automatic test equipment (ATE) is your best bet to provide a cost-effective test solution, providing you use a comprehensive test list with very short test times. The ATE must have a high-quality front end with the versatility to meet all testing needs. In particular, the ATE’s RF signal sources must have much faster settling times than the device under test. This invariably means that the ATE’s digital subsystem must be state of the art in order to meet the stringent digital requirements of these mixed-signal RF devices. This, in turn, means your ATE must have fast, high-resolution DSP instruments to acquire signals from the device as well as powerful DSPs to do the processing needed for each test.

A Bluetooth radio consists of two major modules, a RF transceiver and a baseband controller. Generally, there are no major problems in testing the baseband controller device because this is a mainly digital circuit. However, the testing of the integrated RF transceiver of the Bluetooth needs to comply with the Bluetooth specification, which requires the ATE tester to have the required supporting architecture.

 Test challenges

One of the biggest challenges in designing Bluetooth radios is in designing local oscillators that have very fast settling times. In practice, this means that frequency settling times of less than 200 msec. A further challenge is in controlling the deviation produced by data modulation of a carrier to prevent interference to users on adjacent channels. At the same time, deviation must be sufficient to assure a good bit error rate.

To generate a modulated carrier for the sensitivity test the ATE creates a PRBS data stream, which it stores in an array in the test program. A math-based GFSK modulator produces the random bit stream to modulate the IF carrier. A VHF arbitrary waveform generator sources the IF modulated signal, which becomes up-converted to the IC receiver’s test frequency by the RF modulation source. The test signal reaches the IC under test via the ATE’s microwave port module.

Figure 1. This typical setup for testing Bluetooth RF transceiver ICs uses Teradyne’s MicroWave-6000 option.

Figure 2. The ATE uses this typical Bluetooth transceiver RF output signal to measure carrier power, -20 dB frequency, and bandwidth.

The transceiver IC demodulates the RF signal and sends the bit stream from its RX data pin. The ATE’s VHF digital capture instrument captures the output signal. The ATE then computes the IC’s BER by comparing this captured data with the original PRBS data, which was used during the modulation. In practice, you need to measure BER at different input power levels to fully test that receiver sensitivity meets specification. TME

For Further Information

Robinson, Angus, Anritsu, “On Your Marks for Testing Bluetooth”, Test & Measurement Europe, Jun/Jul 2000, pgs. 17-24. 

Bluetooth Special Interest Group Web site www.bluetooth.com. 

 ST Assembly & Test Services Web site, www.stats.com.sg

Teradyne Web site, www.Teradyne.com.

Nelson Lee T K is the Director of Test Technical Support  with ST Assembly & Test Services (STATS). He has 20 years of experience in semiconductor test and specialises in RF device testing for next generation communication systems such as 3G wireless, Gigabit Ethernet, SONET/SDH, and Bluetooth. Yogan Senthilkumar is a Senior Mixed Signal Applications Engineer with Teradyne’s South Asia Test Assistance Group. He is currently supporting Teradyne’s A5/Catalyst tester platform.

What’s in a Bluetooth transceiver IC?                         Figure A. The essential blocks of a Bluetooth transceiver IC. Figure A shows the essential blocks within a typical Bluetooth RF IC. The transmitter path starts with baseband Gaussian Frequency Shift Keying (GFSK) modulation data, which passes through a Gaussian filter. The output of the filter modulates the VCO frequency, which then deviates from its centre frequency, either up or down, depending upon the logic of the serial input data stream. The amount of deviation determines the modulation index of the transmitter. The modulated signal passes via a preamp and power amp to the antenna. The Bluetooth radio operates as a half-duplex radio. An RF multiplex switch connects the antenna either to the transmitter or receiver section of the IC. On the receiver side, a low noise amplifier (LNA) boosts the return GFSK signal transmitted by another Bluetooth device. The mixer down-converts the received signal to an IF frequency typically at a few MHz. For this to happen, the same PLL/VCO in the transmit portion acts as the local oscillator for down conversion in the receiver. The IC then demodulates the IF signal and outputs RX data. GFSK is a modulation technique where the data change the frequency of a carrier linearly for some amount of a carrier cycle during the duration of a bit. The rate of frequency change is a function of the data rate. The amount of frequency change is a function of the amplitude of the data. The frequency deviation of the carrier—in transmit mode—depends on the amplitude of the input data stream. The converse is also true: the amplitude of data from a demodulated carrier is a function of the carrier deviation—in receive mode. This aspect has an important influence on the bit error rate (BER) of the system. BER is a function of the energy of each transmitted bit relative to the noise contained in each bit. There is a minimum BER for effective communication. The Bluetooth specification is 0.1% BER at 72 dBm, which means 1 error per 1000 bits over a stream of data. The conformance specification requires sensitivity to be measured as a BER of more than 1,600,000 bits at three frequencies. This test alone would take at least 25 seconds using standard single-slot (DH1) packets and so, in practice, the test measures fewer bits even at a reduced number of frequencies.