Application Note

802.11ad Testing with the Spectrum Master™ MS2760A Ultraportable Spectrum Analyzer

802.11ad: A Growing Millimeter-Wave Technology 802.11ad is a newer WLAN standard that offers throughput rates up to 7 Gbps. To achieve these speeds, the technology uses newly allocated millimeter-wave (mmWave) ISM bands, specifically 57 to 71 GHz. With applications including docking stations, virtual reality, media downloads, and wireless HD video, 802.11ad (or WiGig) is becoming a standard technology for most major hardware manufacturers. As these technologies become established and grow in the marketplace, the need for affordable, reliable test will be ever- present. At mmWave frequencies, however, test equipment has historically been extremely expensive and difficult to characterize. As a longtime leader in mmWave technology, Anritsu has leveraged decades worth of experience and knowledge to bring forward new equipment to help companies overcome these challenges. In particular, the Spectrum Master MS2760A Ultraportable Spectrum Analyzer — an innovative new mmWave spectrum analyzer — is available today for use in 802.11ad testing.

Advantages of the Spectrum Master MS2760A for 802.11ad WiGig TestNo High Frequency MixersUntil recently, one of the only options available to engineers for making spectrum analysis measurements at mmWave frequencies required using external harmonic mixers to downconvert signals into frequency ranges of available spectrum analyzers. These mixers can produce artifacts, making it difficult to distinguish between the behavior of the device under test (DUT) and the mixer. These artifacts can also add erroneous power to the signal, impacting the measurement. This is in addition to the already reduced amplitude sensitivity, frequency accuracy/stability, and the increased cost of adding the mixer.

The Spectrum Master MS2760A does not use harmonic mixers in order to reach the required frequencies. By utilizing Anritsu’s patented non-linear transmission line (NLTL) technology, it is capable of performing single sweep coverage from 9 kHz up to 110 GHz (model dependent). As a result, the Spectrum Master MS2760A provides a cleaner response in the mmWave bands.

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Dynamic RangeDue to the inherent noise floor limitations of frequency filtered receivers in spectrum analyzers, many will not have enough dynamic range to measure the full 802.11ad IEEE spectral mask. As the modulation bandwidth of signals increases, the noise floor of the spectrum on the analyzers will get higher and higher, eventually hiding the skirt of the signal. IEEE requires that adjacent and auxiliary channels be suppressed by up to 30 dB from the main channel, but if the analyzer noise floor is higher than the adjacent and auxiliary channels, the measurement is impossible. Many spectrum analyzers require users to purchase and integrate external preamplifiers to raise the signal high enough to make the measurement, adding cost and complexity to the test system. Thanks to Anritsu’s NLTL receiver, the Spectrum Master MS2760A is specified with over 100 dB of CW dynamic range across the entire frequency band, which means that, even with wider signals like 802.11ad, the analyzer has enough range to show the signal channels.

A New ParadigmmmWave testing can be very difficult. Standalone mmWave spectrum analyzers (i.e. those that do not incorporate harmonic mixers) are extremely complex. As a result, many companies are using more affordable, lower frequency spectrum analyzers with harmonic mixers. Unfortunately, by the time all of the options, licenses, and accessories have been added up, it can become unsupportable. Another advantage to utilizing Anritsu’s NLTL receiver is that it provides unbanded frequency coverage without external hardware in a simple, portable package.

This can be especially helpful in sites with many labs trying to make the same or similar measurements. It is usually unrealistic to have budget sufficient to equip every lab with expensive mmWave equipment. Consequently, companies often buy one or two sets of the more expensive equipment and expect it to be shared among the labs. By forcing engineers to timeshare limited equipment, development is sequential and projects can be delayed, which in turn delays profits and opens doors for competition to establish an earlier position in the market. The affordable price of the Spectrum Master MS2760A allows companies to place mmWave spectrum analyzers with more engineers, enabling them to work in parallel rather than sequentially while avoiding project delays and unrealistic capital expenses.

Figure 1. The MS2760A provides enough dynamic range to easily measure the IEEE 802.11ad spectral mask.

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Furthermore, for any long-term tests such as environmental or EMI tests that can run continuously for several days, the MS2760A can provide constant coverage without tying up unnecessary capital expense. In a situation where equipment is limited, it is going to be hard to justify monopolizing an extremely expensive piece of equipment for that long. The Spectrum Master MS2760A is a perfect stand-in to monitor any changes in signal power or frequency. With an operating temperature of 0 to 50˚C and built-in temperature calibration factors, under some conditions, it can even be placed in the chamber with the DUT.

Figure 2. The MS2760A puts mmWave measurement capability in the hands of more engineers/technicians to drive faster product development.

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Direct Connect to Devices Under TestBefore 802.11ad modules are integrated into consumer or commercial goods, the wafers, chips, and circuit boards go through rigorous testing and verification. The propagation loss of signals through cables gets worse at higher frequencies, as shown in Figure 3. Added cables means more connections, which will inherently add more mismatch uncertainty to the measurement.

The ultraportable form factor of the Spectrum Master MS2760A is small enough to be connected directly to most devices, mitigating the need for cables and improving the sensitivity and accuracy of the test. It is even designed for easy connection to wafer probes for direct on-wafer spectrum measurements.

Figure 3. Theoretical example of how propagation loss of RF signals is much higher at mmWave frequencies.

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Over-the-air (OTA) test is another common solution for mmWave frequencies. This is because OTA is often representative of the final product use case (such as with a wireless docking station or tablet, for example) or because access to the device under test is restricted. OTA, however, can pose similar problems to cables. Not only is propagation loss more significant at higher frequencies, but as the wavelength of the signal begins to match the wavelength of common particles in the air (i.e. water), the loss can be worse. This causes a non-linear loss across frequencies that can be difficult to characterize. As shown in Figure 4, the problem is particularly bad at 60 GHz, right in the middle of the 802.11ad band. The size of the Spectrum Master MS2760A enables users to place the spectrum analyzer closer to the DUT. For example, in a small antenna chamber, traditionally large spectrum analyzers are forced to be outside of the chamber and require long cables to the receiving antenna within the chamber. The loss due to the cabling significantly reduces the available measurement range of the spectrum analyzer.

Figure 4. Propagation loss of RF signals over the air is not linear across frequencies and is particularly high around 60 GHz.

MeasurementsIn the lab, during certification, and on the manufacturing line, it is critical to demonstrate that the 802.11ad signal does not exceed the predefined regulatory limits of power and frequency and to ensure consistency from device to device. The Spectrum Master MS2760A can help ensure product performance by supporting the following tests:

• Channel Power

• Adjacent Channel Power

• Spectral Mask using limit lines

• Occupied Bandwidth

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Channel PowerChannel Power measurements with the Spectrum Master MS2760A are simple. To set the analyzer for a Channel Power measurement, follow these buttons:

From the home menu screen, follow steps below:

1. Select MODE MEAS

2. Select Measurement

3. Select Channel Power

Figure 5. Mode Meas menu.

Once in the Channel Power measurement, the user can enter the desired channel bandwidth for making channel power and/or spectral density measurements. The settings are located either below the spectrum display in the Channel Power menu/display or in the MEAS SETUP menu (Figure 6, below).

Figure 6. Channel power reading of an 802.11ad signal.

802.11ad is typically modulated with a 2.16 GHz bandwidth. To set the analyzer’s integration bandwidth, click or touch the Integration BW parameter in the menu or directly on the value in the CHANNEL POWER MEASUREMENT table below the spectrum display.

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Figure 6 shows that this particular 802.11ad signal has a 3.79 dBm integrated power in the main 2.16 GHz channel. The vertical green lines indicate the integration bandwidth used for the measurement. This process can also be automated via a remote interface by using basic spectrum analyzer SCPI commands.

Figure 7. Setting the Integration Bandwidth.

Figure 8. Channel Power Measurement of an 802.11ad signal.

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Adjacent Channel PowerThe Spectrum Master MS2760A can also be set to measure the integrated power of adjacent channels. This is done by selecting the Adjacent Channel Power (ACP) measurement from the MODE MEAS menu. Once the measurement is selected, the widths of the channels can be set from the ACP MEASUREMENT table below the spectrum plot, or from the MEAS SETUP menu.

Figure 9. ACP Measurement of an 802.11ad signal.

Spectral MaskAnother basic test run in the lab and in manufacturing is verification that the signal meets IEEE adjacent and alternate channel suppression requirements through a Spectral Mask test. This is important because large signals outside the allotted primary channel can cause interference with other nearby signals.

Testing this can be done by setting limit lines equivalent to the IEEE specifications with a pass/fail condition. The IEEE standard section 21.3.2 (titled “Transmit mask”) states:

The transmit spectrum shall have a 0 dBr (dB relative to the maximum spectral density of the signal) bandwidth not exceeding 1.88 GHz, –17 dBr at a 1.2 GHz offset, –22 dBr at a 2.7 GHz offset and –30 dBr at a 3.06 GHz offset and above, inside the channels allowed for the regulatory domain in which the device is transmitting. The resolution bandwidth shall be set to 1 MHz.

This mask definition is illustrated in Figure 10.

Figure 10. IEEE 802.11ad spectral mask.

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Setting Up the Spectral MaskTo set limit lines on the Spectrum Master MS2760A to match the 802.11ad spectral mask, go to the Limit Setup menu from the MEAS SETUP menu (the analyzer should be set to “Spectrum” measurement mode, otherwise the Limit Setup option will not be available). The limit menu will open below the spectrum window.

Figure 11. Limit Setup in the MEAS SETUP menu.

Figure 12. Limit Setup menu.

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There are several ways to create and edit a limit lines menu. Since the frequencies, amplitudes, and number of points are known for this application, the Create Envelope option will make the setup simple and straightforward with these steps (shown in Figure 13).

Figure 13. Limit menu.

STEP 1 (Figure 13, 1): Select the ENVELOPE section of the Limit Setup menu:

STEP 2 (Figure 13, 2): Enter the number of points needed for the envelope. To recreate the IEEE mask from Figure 10, the correct number of points will be 10. Values for Select Limit, Amplitude Offset, and Shape can stay at their default values of Upper, 3, and Square, respectively.

STEP 3 (Figure 13, 3): Click or press the Create Envelope button. The Spectrum Master MS2760A software will automatically create an envelope with the correct number of points. If there is a live 802.11ad signal in the spectrum window, the software will automatically create an envelope that approximately fits the signal.

STEP 4 (Figure 13, 4): Select EDIT on the top row of the table and edit the points to create the desired spectral mask shape. To do this, go to the EDIT section of the Limit Setup menu. A table of limit line points will automatically open on the right side of the spectrum window. The frequency and amplitude values of each point can be edited by clicking or touching the current value. If the center frequency of the signal is X and the maximum power limit is Y, the point values should be set as follows, with each point number corresponding to a numbered point on the mask in Figure 10:

Point Frequency Amplitude

1 X - SPAN/2 Y - 30 dB

2 X - 3.06 GHz Y - 30 dB

3 X - 2.7 GHz Y - 22 dB

4 X - 1.2 GHz Y - 17 dB

5 X - 0.94 GHz Y

6 X + 0.94 GHz Y

7 X + 1.2 GHz Y - 17 dB

8 X + 2.7 GHz Y - 22 dB

9 X + 3.06 GHz Y - 30 dB

10 X + SPAN/2 Y - 30 dB

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It follows then that, if the center frequency is at 60.48 GHz, the span is 10 GHz, and the max amplitude allowed is at –20 dBm, the points should be set to:

Point Frequency Amplitude

1 55.48 GHz –50 dBm

2 57.42 GHz –50 dBm

3 57.78 GHz –42 dBm

4 59.28 GHz –37 dBm

5 59.54 GHz –20 dBm

6 61.42 GHz –20 dBm

7 61.58 GHz –37 dBm

8 63.18 GHz –42 dBm

9 63.54 GHz –50 dBm

10 65.48 GHz –50 dBm

Figure 14. 802.11ad signal with IEEE spectral mask.

With the limit lines now set to match the regulatory limits for 802.11ad, the user can set the analyzer to provide a pass/fail condition that will fail any time the signal crosses the defined limit line.

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Occupied BandwidthFinally, the Spectrum Master MS2760A can be used to measure the occupied bandwidth of the WiGig signal to ensure that: 1) the signal is transmitting with the expected bandwidth and 2) that the correct percentage of the power is contained with the main channel of the signal.

Occupied Bandwidth (OBW) is a common measurement performed on radio transmitters. This measurement calculates the bandwidth containing the total integrated power occupied in a given signal bandwidth. There are two different methods of calculation:

• % Integrated Power Method: The occupied frequency bandwidth is calculated as the bandwidth containing the specified percentage of the transmitted power.

• >dBc Method: The occupied frequency bandwidth is defined as the bandwidth between the upper and lower frequency points at which the signal level is a desired number of dB below the peak carrier level.

For 802.11ad, the % Integrated Power Method will be the most common measurement. To set the analyzer to measure occupied bandwidth, simply go back to the MODE MEAS menu and set the Measurement drop down menu to Occupied BW. A menu will pop up below the spectrum window and green indicator lines will appear on the screen showing the occupied bandwidth. The analyzer will typically default in the % Integrated Power Method at 99% power. This can then be adjusted for specific measurement needs. Note that for the green lines to be correctly set to the start and stop frequencies of the occupied bandwidth, the signal must be centered within the span.

ConclusionThe new world of technology is pushing products to higher frequencies all the time. As a developer of these technologies, it may seem like some projects are out of reach due to the prohibitive cost of mmWave test and measurement instruments. The Spectrum Master MS2760A is key to unlocking mmWave measurements across all facets of the product lifecycle with key benefits:

Spectrum Master MS2760A Advantage Benefit

Patented NLTL sampler-based down conversion (no harmonic mixers)

Provides a wider image and/or spur free frequency range within the defined 802.11ad band

Wider dynamic range Allows full test of the IEEE 802.11ad spectral mask without the need for external amplifiers that will add cost and complexity to the test system

Affordability More affordable mmWave test devices means equipping more engineers with the necessary tools to bring products to market faster

Closer measurements to the device under test Avoid high propagation loss and/or expensive precision cables at mmWave frequencies, which saves money and improves the accuracy and repeatability of measurements

11410-01049, Rev. A Printed in United States 2017-10©2017 Anritsu Company. All Rights Reserved.

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