Today’s communication systems require accurate measurements of receiver input power and setting of transmitter output power.
Transceivers are one of the basic building blocks for mobile phones, wireless LAN access points, and the cellular infrastructure. These devices handle both reception and transmission of multiple wireless formats, so testing can be complex, time-consuming, and costly.
Within a test station, a new class of PXIe vector signal generators (VSGs) can provide greater speed and throughput with capabilities such as quick frequency and amplitude switching, good signal linearity, and repeatability. The new VSGs also offer flexible synchronization of frequency and timing to support the required level of calibration.
Challenges in Transceiver Testing
The testing of any transceiver presents two major problems: breadth and synchronization. Breadth comes from the broad range of signal frequencies and power levels that must be tested. This becomes a problem when slow switching speeds in the VSG create a bottleneck in the testing process.
Synchronization is important in two areas: timing and frequency. During testing, the actions of the transceiver and the VSG must be precisely synchronized to ensure proper interaction. In addition, successful transmission and reception requires that the frequency reference in the VSG be synchronized with that of the transceiver, which may become an issue during testing. Some transceivers have an internal 13-, 26-, or 52-MHz reference; however, most test equipment can synchronize only with a 10-MHz reference.
Transceiver calibration is an important part of the test process. Typically, receiver calibration consists of providing an RF input over a wide range of frequencies and power levels, and then recording the value of the receiver signal strength indicator (RSSI) at each step. This calibration was managed by the test-system software through nested loops that set the DUT and signal generator to a series of frequency/power-level pairs, and then read back the associated RSSI levels.
As the need for speed grew, signal generators were designed to provide faster switching times. Along the way, the software overhead involved in controlling the DUT became the bottleneck that slowed the testing process.
To overcome this problem, many transceiver manufacturers developed special test modes that allowed the DUT to accept a single command that caused it to cycle through a large number of frequency and amplitude states and record the RSSI levels.
This method required the RF signal generator to cycle through a corresponding set of frequency and amplitude states in synchrony with the DUT. This is accomplished with a capability called list mode in many signal generators.
List mode lets the user predefine a large number of frequency and amplitude states that are output with either specified timing or in response to an external trigger signal. To cover the full range of requirements, the list mode must provide a large number of points, fast switching between those points, flexibility in timing and synchronization, and the ability to use a variety of modulated waveforms.
Synchronizing Actions & Frequency References
Two things drive the need for synchronization between a transceiver and a VSG: testing the DUT in either list mode or normal operational mode. Testing in list mode requires synchronization of the actions performed by the DUT and the VSG. When testing a device in its normal operating mode, it is necessary to synchronize the frequency reference in the VSG to that of the DUT.
Working with List Mode
Most list modes use two types of synchronization: unified triggering and point-by-point handshaking. In the first method, a single trigger event starts the lists in the DUT and the VSG, and the timing of both is initiated by the trigger. In this mode, it is important for every step in the list to be accurately scheduled relative to the initial trigger event. If the beginning of point N is timed from the beginning of point N-1, small errors in timing can accumulate, eventually causing the DUT and VSG to lose synchronization.
In point-by-point handshaking, the VSG will tune to a specific point when it receives an input trigger. When the VSG output settles at that value, it sends a trigger to the DUT or analyzer, which will then start a measurement at that point. When the measurement is complete, the DUT or analyzer will output a trigger that causes the VSG to advance to the next point. The back-and-forth handshaking repeats until the list has been completed.
The handshake method can be useful when the measurement time of the DUT or signal analyzer cannot be predetermined. On the upside, this approach can provide faster throughput because there is little or no need to include guard time in the lists. On the downside, this approach does require two dedicated trigger connections — one input and one output — on the DUT, signal analyzer and VSG.
Working with Operational Mode
Complete transceivers, such as mobile handsets or picocell or femtocell base stations, are often tested in an operational mode, which is typically done with either the operational software within the transceiver, or a special version of the software that has been modified to support testing.
The implications for the VSG depend on the type of device being tested. With mobile handsets, the DUT will synchronize to the timing and frequency of the VSG. In such cases, the waveform timing from the VSG remains constant through changes in frequency, amplitude, or waveform. If the VSG is unable to maintain this timing relationship, the DUT needs to be resynchronized with the VSG, and this can be a time-consuming operation.
When testing picocells or femtocells in operational mode, the synchronization requirements are more challenging because the base station is the frequency and timing master for the system. During testing, the VSG must synchronize to the frequency and timing references in the DUT.
For frequency sync, it is common to lock the VSG reference input to the oscillator within the DUT. As noted earlier, these transceivers use an internal frequency reference at 13, 26, or 52 MHz; however, most test equipment uses a 10-MHz frequency reference. To meet this need, the VSG must lock to a selectable input frequency higher or lower than 10 MHz.
For timing synchronization, the DUT will output a trigger pulse that is aligned with the frame boundary of the wireless format being tested (i.e., W-CDMA or LTE). To allow the DUT to demodulate the input signal, the VSG needs to start playback of the modulated waveform in alignment with the frame trigger. In most cases, the timing error must be within ±2 µs.
Test Throughput & Quality
The new class of PXIe VSG instruments includes features that provide the following capabilities:
- List mode with up to 3,200 steps.
- Fast switching of both frequency and amplitude.
- Flexible synchronization of timing and frequency.
Utilizing Lists & Fast Switching
Lists can be generated using the normal programming interface, allowing for easy transitions between programmed mode and list mode.
The new VSGs also offer a variety of modes for frequency and amplitude switching. For example, the traditional RF frequency and amplitude tuning mode can provide a minimum step time of less than 250 µs in list mode with ALC off (Figure 1) or < 365 µs for ALC on. These frequency changes can be over the entire frequency and amplitude range of the VSG.
Much faster switching times are possible with an innovative baseband offset tuning technology. This capability enables frequency changes across the entire modulator bandwidth, and amplitude changes of up to 20 dB, without degrading modulation performance. As a result, minimum step time in list mode can be as fast as 10 µs (Figure 2).
In addition to the frequency and amplitude values, each point in a list has an end event that defines how the list will proceed to the next point. That event can be either trigger- or time-based. Triggered end events support the handshake list mode.
In a time-based event, the user can select either the total step time or the dwell time, which is defined as the time after the VSG frequency and amplitude have settled. Total step time is useful in situations such as transceiver calibration.
To handle the typical reference frequencies used in many types of transceivers, the new multi-module VSGs include a flexible frequency reference module. As expected, the module includes a 10-MHz reference, but it can accept an external reference signal running at any value between one and 110 MHz.
Waveform playback can be initiated through an input for external trigger signals. The VSG external trigger uncertainty is less than 20 ns, and trigger delay resolution is equal to the reciprocal of the sample rate of the waveform data. For example, trigger delay resolution will be 65 ns for a typical W-CDMA signal. This allows synchronization with a picocell or femtocell to be better than 85 ns, which is well within the typical ±2 µs timing error allowed.
Core Benefits of Modular
Just as transceivers continue to pack more capability into less space, so do the new VSGs. The combination of capabilities described here provides the fastest throughput while also enabling complex testing of the transceivers used in mobile handsets, picocells, femtocells, and more. By providing these capabilities in a modular format, the PXIe-based VSGs support additional needs including smaller size, weight, and footprint for test stations; and a lower overall cost-of-test.
This article originally appeared in the January/February print issue. Click here to read the full issue.