Accurately measure 802.11ac RF performance with a power analyzer

When measuring 802.11ac wideband signals up to 160MHz, engineers must use power measurement instruments with sufficient bandwidth to acquire, analyze, and accurately measure specific 802.11ac burst signal segments and ensure compliance with regulatory requirements during the design and verification phases, thereby accelerating the development of 802.11ac RF modules. When testing 802.11ac wireless local area network (WLAN) power amplifier (PA) modules during the design and verification phases, engineers must accurately obtain peak burst measurements when measuring 802.11ac wideband signals up to 160MHz. When measuring or debugging such wideband signals, it is important to select power measurement instruments with sufficient video bandwidth to ensure that specific 802.11ac burst signal segments can be acquired, analyzed, and accurately measured.

With the right power measurement instrument, the timing relationship between the control circuit and the RF burst signal can be analyzed. Therefore, selecting and configuring the right measurement instrument ensures that obstacles can be quickly eliminated, the required resources can be greatly reduced, and costs can be effectively reduced during the design and development process.

This article will provide an overview of the 802.11ac standard and explain how to use a peak power analyzer (PPA) to meet the unique design and verification test requirements of 802.11ac power amplifiers and transmitters. Test configuration diagrams and screen shots will be used to enhance the various test applications such as burst power vs. time, power on/off rise and fall times, complementary cumulative distribution function (CCDF), power added efficiency (PAE), and control trigger delay measurements.

A brief introduction to the 802.11ac standard

802.11ac is the next generation WLAN standard, which aims to enable mobile devices to operate at frequencies below 6GHz, three times faster than the existing 802.11n WLAN standard. Currently, the 802.11ac standard is still in the working draft stage and is expected to become a formal standard by the end of this year or next year. Table 1 lists all 802.11 WLAN standards, including 802.11ac.

The 802.11ac WLAN physical layer is basically an extension of the existing 802.11n standard and is compatible with the 802.11n standard. Therefore, electronic devices equipped with 802.11ac chips in the future will be able to operate on the current 802.11n WLAN system.

According to the 8012.11ac working draft, in the future, there will be channel bandwidths of 20, 40, 80 and 160MHz available for selection. However, currently, except for the 160MHz channel bandwidth, which is an optional bandwidth, all others are mandatory bandwidths. In other words, in the early stages of deploying real systems, all infrastructure, chips and terminal devices may use 20, 40 and 80MHz bandwidth channels. As for channel allocation, the aforementioned bandwidths may be continuous or non-continuous, especially the 80MHz channel. For example, two 80MHz channels spanning two frequencies can be used to construct a 160MHz bandwidth communication link (Figure 1).

Figure 1 802.11ac 160MHz bandwidth channel configuration

Peak Power Analyzer Helps Double 802.11ac Test Performance

Output power is an important 802.11ac transmitter performance test indicator.

Measuring the output power is necessary

During the design and development phase, engineers must measure and verify the output power of the transmitter to meet regulatory specifications. Figure 2 shows a power measurement configuration that can measure average, peak, and peak-to-average power ratios. To obtain accurate measurements, the stability of the acquired burst signal must be maintained. This can be achieved by using a peak power analyzer to select appropriate time trigger settings, such as trigger level, hysteresis, and delay time.

Figure 2 802.11ac transmitter power measurement configuration

Power vs. time

Although IEEE802.11ac does not specify test requirements for Power vs. Time (PvT) analysis, it is an extremely important measurement for all wireless standards. For example, the PvT burst measurement function is very useful when analyzing the preamble segment of 802.11ac. The preamble is an essential element for packet detection, automatic gain control, symbol timing, frequency estimation, and channel estimation.

802.11ac has ten symbols in the preamble segment, which is equivalent to a burst length of 40 (μs). Figure 3 shows an 80MHz 802.11ac burst signal measured using PPA. The PPA can be used to perform preamble burst measurements such as average, peak, and peak-to-average using the zoom function or by adjusting the time scale.

Figure 3 802.11ac 80MHz bandwidth preamble power measurement screen

Test power on, off and up/down

This test analyzes the time it takes for a transmitter to fully power on and then off, and is typically performed during the design and validation phase. The specifications for this transient time response vary, depending on factors such as the PA design (what amplifier is used) or other control circuits. 802.11ac transmitters must be designed to comply with the short guard interval of 400 nanoseconds (ns), in other words, the transmitter turn-on/off time must be well below 400ns.

If the transmitter is turned on too slowly, the initial data may be lost, but if it is turned off too quickly, it will increase the power spread to adjacent channels. This PPA can analyze the power turn-on/off or rise/fall times of the transmitter and receiver, as shown in Figure 4 and Figure 5.

Figure 4 Power on (rise time)

Figure 5 Power off (fall time)

Performing CCDF Measurements

Complementary Cumulative Distribution Function (CCDF) measurements define the characteristics and behavior of a PA. PAs are typically designed to operate at high crest factors, and CCDF is used to measure the percentage of time that the burst power reaches or exceeds a specific power level. As shown in Figure 6, the Y-axis of the CCDF trace plot represents the probability (percentage) that the signal power reaches or exceeds the power specified on the X-axis in dB. CCDF analysis of 802.11ac signals is typically performed on the preamble segment of a burst because the modulation scheme on the preamble segment is different from the data or payload segments. Therefore, the CCDF trace on the preamble is different from the trace on the data segment.

Figure 6 802.11ac CCDF plotted using a peak power analyzer

图6中后方拋物线是802.11ac前导码区段的CCDF图，前方拋物线是高斯线(Gaussian Line)，通常会开启高斯线以作为参考。同时，可绘制和分析两个射频(RF)通道的CCDF轨迹，在比较PA模块的802.11ac信号输入和输出时，这项功能非常有用。

Monitoring Power Added Efficiency

One of the challenges of transmitter design is to optimize the efficiency of the power amplifier. PAE measures the power conversion efficiency of the PA to determine how much DC power is converted into RF power (i.e., the efficiency percentage). The following formula 1 is the PAE formula. The PA operation category and the type of active components used will affect the PAE performance, and the specification can range from 20 to 60 seconds.

Figure 7 Power-added efficiency test configuration

Confirmation control or trigger delay measurement

When developing a transmitter module, engineers must identify and measure the delay time between the trigger or control signal and the actual RF burst output, that is, analyze the timing relationship between the actual RF burst and the voltage bias circuit. The voltage bias circuit includes DC power supply bias, switching, drive control, and voltage controlled oscillator (VCO) signals.

Figure 8 shows a typical transmitter functional block, where the design focus is usually on getting the shortest time delay result as quickly as possible. The PPA analyzes the timing information of the relevant control signals and the RF burst, and has a special function that automatically measures the time delay between the two channels and adds a marker on each signal.

Figure 8 Using a peak power analyzer

In summary, to support 80MHz and optional 160MHz channel bandwidths, a more powerful RF power measurement instrument is required, and the PPA is the best power meter that can meet the 802.11ac test requirements.

The PPA can be used for typical RF power measurements such as average, peak, peak-to-average, and CCDF analysis, as well as analysis of the power added efficiency of the power amplifier and delay timing information responsible for control signals within the transmitter module, since the PPA is equipped with two RF channels and two analog video channels in one liquid crystal display (LCD) instrument. In addition, the PPA provides an intuitive operating interface that makes it easier to design and verify 802.11ac power amplifier modules and transmitters.