An RFIC for the world

LOWELL, MA—When you use your cellphone indoors, do you lose the signal? When you need to travel overseas, do you need another phone because yours isn’t compatible with the network? Have you had to replace your phone because your carrier upgraded to a new network? If you answered “yes” to any of these questions, then you will appreciate the work being done at BitWave Semiconductor.

Systems engineer Mark D’Amato evaluates preproduction RFICs in simulated networks, often redefining the functions of a device. Photo by Mark Wilson.

The first prototypes of the device, called the “Softransceiver RFIC,” were available in early 2007, and the IC is now nearing commercial release, with engineers testing and characterizing the devices in production volumes. “Inside the Softransceiver” explains how the device works at frequencies from 700 MHz to 3.8 GHz.

System engineer Mark D’Amato evaluates preproduction devices in simulated networks. System engineer Rick Quintal characterizes the ICs for physical-layer compliance. And test engineer Sreekar Javvadi pulls the testing together by automating tests that would otherwise take years to perform.

Quintal, Javvadi, and D’Amato work in a lab equipped with several benches that contain identical equipment: spectrum analyzers, logic analyzers, vector signal generators, and digital serial interface modules (DSIMs) from Agilent Technologies. The engineers also use vector signal analyzer software and Baseband Studio baseband capture and playback software from Agilent and an in-house test tool called the “Softuner.” All benches are automated with LabView from National Instruments. Figure 1 shows how the equipment and software connect. Because all test benches contain the same equipment, engineers can run any test on any bench.

Figure 1.  RF and digital test equipment populate several test benches in BitWave’s engineering lab.

Before D’Amato can test a Softransceiver, Quintal must characterize the device’s programmable radio components, which include RF amplifiers, voltage-controlled oscillators (VCOs), mixers, filters, local oscillators (LOs), baseband amplifiers, and analog-to-digital converters (ADCs). He tests these components for parameters such as bandwidth, carrier suppression, sideband suppression, noise, and signal-to-noise ratio (SNR).

Figure 2.  An evaluation board provides access to the RF and digital sections of the BitWave RFIC. It also supplies power and control signals that configure the device. Courtesy of BitWave Semiconductor.

The Softuner provides Quintal with many pages that he uses to configure the RFIC’s programmable components. He also configures the RFIC using an application programming interface (API) when he needs to write directly to the device’s registers. Baseband Studio lets Quintal create and capture signals such as digital I/Q modulation signals and send them to the device in digital format through the DSIM.

Beginning with a receiver test, Quintal configures the RFIC for the most demanding channel bandwidths and frequencies. The goal: Determine the performance envelope of the device and compare it to the requirements for all wireless standards.

“You have to know that the chip works before you can characterize it,” said Quintal. He will spend up to two months characterizing initial parts before they’re ready for automated tests. He starts by configuring the RFIC’s receiver to get basic measurements.

Systems engineer Rick Quintal evaluates components such as RF amplifiers, mixers, and oscillators for compliance with physical-layer standards.  Photo by Mark Wilson.

The device’s transmitter section has an LO and a mixer. To test the transmitter, Quintal enters digital data from the DSIM and measures the gain and bandwidth from the transmitter’s analog output with a spectrum analyzer. He modulates the carrier with a sine-cosine tone from DC to 40 MHz and measures the baseband section’s response.

Once Quintal finds the frequency with the best response, he may need to tune the device based on customer requirements. For example, if the customer will use the RFIC for 2.1-GHz WCDMA, then Quintal will use the Softuner to optimize the device for that frequency. Using the Softuner’s tuning page, Quintal adjusts parameters such as filter bandwidth, which must change depending on the device’s configuration. The tuning page contains a virtual “knob” for bandwidth. Turning that knob actually causes the software to write to four or five registers that configure the device’s internal filters.

To characterize the receiver, Quintal must measure conversion gain, frequency response, and SNR. He injects a two-tone signal into the receiver from which he calculates third-order intercept, gain, and linearity. He will also use the vector signal analyzer software to measure S-parameters in order to characterize the device in the frequency domain.

Quintal also looks at VCO bias current, which can range from a few microamps to 20 mA. He then optimizes the device for minimum bias current. He also looks at performance versus power consumption.

Because the Softransceiver contains programmable function blocks, Quintal can change the device’s architecture, including the internal ADC. There isn’t one ADC architecture that works best for all wireless technologies. For example, the Softransceiver will configure itself for a 4-bit delta-sigma ADC when it needs to operate as a WCDMA interface, but it might use a 10-bit pipeline converter for other standards. Both architectures are already designed into the silicon and can be configured from the same shared building blocks; the Softransceiver configures whichever converter is better for a given application.

Test engineer Sreekar Javvadi automates tests based on test parameters stored in spreadsheets.  Photo by Mark Wilson.

The device uses four DC voltages (1.2 V, 1.5 V, 1.8 V, and 3.3 V) that the evaluation board regulates down from 5 V. Quintal tests the device by changing each voltage by ±10% from nominal.

Quintal looks at how electromagnetic interference (EMI) and noise affect the VCO’s phase noise. An increase in phase noise will decrease the eye opening of a received data stream, making it harder for the receiver to accurately detect bits. Quintal noted that an open board is just about the worst test condition possible with regard to EMI. Using a shield, Quintal gets a baseline phase-noise measurement. Then, he adds noise to a transmitter’s analog input, looking for phase-noise changes from the VCO output. He also tests the RFIC with noisy DC power, looking for phase-noise differences.

Quintal currently performs many tests manually using the Softuner tool but his colleague Sreekar Javvadi is working to automate the tests. Using LabView, Javvadi is developing a long list of tests that will automate Quintal’s work, reducing test time and providing more repeatable measurements.

BitWave hired test-services company Anagon to develop the overall architecture for automated testing. Javvadi supplements the architecture by developing and running specific tests.

Taking data from Quintal’s tests, Javvadi has developed Excel spreadsheets of test parameters that include the data needed for standards-compliance tests. To configure the device, the LabView code reads configuration data from an Excel sheet for a wireless standard such as GSM or WCDMA.

Each compliance test uses a set of LabView virtual instruments (VIs) that make API calls to the RFIC that configures its internal function blocks. The VI then makes API calls directly to the device’s registers. For example, an API call to the device will configure its RF section, such as an LO or a filter. Table 1 provides examples of file names used for some transmitter and clock tests. Figure 3 shows an example of LabView code used for measuring the transfer function of an RF filter used on the RFIC’s receiver block.

Figure 3.  LabView code writes data to the DUT’s registers to measure a filter’s transfer function. Courtesy of BitWave Semiconductor.

“At this stage, we pass real data to the device, modulated on an RF signal,” said Javvadi. “The modulation schemes depend on the compliance standard.”

Javvadi doesn’t just develop automation software, he runs tests, too. He’ll run automated tests to verify that his code is properly written by comparing his results to those measured by Quintal. Once satisfied with the software quality, Javvadi will test as many as 200 devices from which he can develop enough statistics to conclude that the design will pass compliance tests.

Because a test involves thousands of measurements on at least 16 wireless standards, an evaluation can generate more data than Excel can hold. To process the data, Javvadi will import the test results into Matlab for analysis. “We can call Matlab from LabView and analyze data on the fly,” he said.

While characterization and compliance tests are crucial, testing isn’t complete until systems engineer Mark D’Amato runs system-level tests. D’Amato combines the data from Quintal’s characterization tests and Javvadi’s test automation into tests that determine how well the Softransceiver will respond when integrated into a cellphone. Instead of treating the device’s receiver and transmitter separately, D’Amato treats the entire RFIC as if it were a “black box,” and he studies how it interacts with surrounding circuits.

D’Amato’s job is to fine-tune the RFIC until it works. He looks at its performance data and then investigates how to tune the RFIC’s functional blocks such as VCOs, filters, and mixers. The device has five RF stages and each needs tuning. By programming the device’s registers, D’Amato can adjust component values that change the device’s characteristics. He looks at the device amplifiers to determine their gain, bandwidth, and other parameters before reprogramming the device.

D’Amato uses the Softuner’s tuning pages to make adjustments. The tool provides access to the components that make up programmable amplifiers where D’Amato adjusts gain and frequency response. “When you change a setting on the tuning page,” said D’Amato, “it pushes that change down to the appropriate register. When you make a change to a register through an API call using LabView, the change pushes back up to the tuning tool.”

To perform the tests, D’Amato uses the vector signal generator to generate the cellular-modulation signals. Because the RFIC can also reconfigure itself to connect to WiFi networks to keep calls intact, D’Amato tests the devices using cells from RadioFrame Networks (see "Cellphones connect through Ethernet," to learn more about RadioFrame Networks).

“We send jamming signals to the device to verify how well it works in the presence of interference. We must maintain communications in the presence of noise. Thus, we measure signal-to-noise ratio on receivers to see how much noise they can tolerate and still receive properly.” SNR is just one of many tests required by wireless standards. The data for these tests resides in the spreadsheets that Quintal generates.

Given that the BitWave Softransceiver RFIC is the first reconfigurable SDR, D’Amato has run into limitations on the test equipment he uses. “Our I/Q modulation signal is digital as where most other RFICs use analog signals,” he said. D’Amato uses the DSIM to generate the 16-bit digital I/Q modulation signal. From that signal, the device’s embedded digital signal processor (DSP) generates a 12-bit I/Q signal that becomes the output of the RFIC’s receiver port. Using a logic analyzer, he captures the resulting 12-bit I/Q signal at 15.6 Msamples/s.

The logic analyzer lets D’Amato analyze the digital modulation for signal integrity with a vector signal analyzer. Looking at the RFIC from a system level, he uses the data to make adjustments to the device’s internal blocks and thus he can verify that the RFIC will work under all of the wireless standards it supports.

Because the Softransceiver supports so many wireless standards, BitWave engineers have perhaps the most expensive RF test equipment available. That’s because the company had to purchase a personality profile for each standard. Most RFICs support just one standard and thus need just one profile.

Table 1. Function calls and the tests they perform.