Pre-compliance testing for WLAN transmitters - Embedded.com

Pre-compliance testing for WLAN transmitters

It’s no longer enough to have smart gadgets. Any gadget that wants to grab consumer attention has to be both smart and connected. The connected piece means that wireless must be part of the design as well. Wireless also opens up many new design possibilities and is starting to show up in some unusual places. Here are a couple of the more interesting examples:

Start-up Velo Labs is developing a solar-powered bike lock that can hook up to Wi-Fi networks to send alerts to the bicycle owner’s smartphone if the lock’s motion detector senses that the bicycle is potentially being stolen.

A new intelligent electric toothbrush from Oral-B incorporates wireless to capture information about the user’s brushing habits where it presumably could become part of dental records. The question of whether or not this is a good thing is up to the consumer to decide.

The point here, of course, is that there are thousands of products on the shelves, in the works, or yet to be imagined that will incorporate low-power wireless capability to meet consumer demand or to become part of the so-called Internet of Things.

One challenging part of this demand for connectedness is that product manufacturers – many of whom have little to no RF experience – need to learn how to add wireless capability to their products. The most common and practical approach is to incorporate a prepackaged WLAN module into your design. Not surprisingly, the market for such modules is growing at a double digit pace, with continued growth forecast.

While the use of wireless modules eliminates many technical issues, there are still many decisions to be made (Figure 1 ). The most critical – and daunting – task is to ensure that the end product meets complex FCC and international regulatory requirements. Compliance testing is exhaustive and time consuming, and a failure at this stage of product development can cause expensive re-design and delay product introduction.

Regulatory pre-compliance checks, as highlighted in step 6 of the flowchart, are vital to avoiding such worse-case scenarios. Fortunately, cost-effective and familiar test equipment can be used in-house to perform pre-compliance testing and ensure that your wireless-enabled products have a high probability of passing compliance tests on the first go round. The goal is to uncover potential problems early on and reduce the risk of costly failure at the compliance test stage.

Figure 1: Using a wireless module lowers design complexity, but still involves a number of important steps, the most critical being ensuring that the design passes regulatory compliance tests.

From the very first wireless transmissions, spectrum emission has been a concern for design engineers. Regulatory agencies around the world have placed limits on the emission levels, and have defined measurement methods for compliance testing. Formal certification, which must be completed before a product can be sold, must be done at an independent lab and can cost between $5,000 to $10,000 per day not including travel and other expenses.

The use of off-the-shelf modules, even ones that have been certified on their own, doesn’t necessarily make the job of obtaining certification much easier. This is because the complete assembly of the final product must be tested and qualified as well. Design issues like PC board layout, antenna design and placement or system interactions can lead to a product failing to meet certification requirements.
Sorting out these issues during the design phase is made much easier with instrumentation that not only lets designers perform a full range of pre-compliance testing on their own test bench, but also helps designers identify the root cause of problems that could cause a product to fail formal tests.

Pre-compliance testing vs. full compliance
Pre-compliance testing is done after system integration of the wireless module to determine any problem areas in the design. Pre-compliance testing doesn’t necessarily need to map to every single international standard since the goal is to simply uncover potential problems and reduce risk of failure at the expensive compliance test stage. The equipment used also does not have to include every feature and specification required by the standard, and can have lower accuracy and dynamic range than compliant receivers if sufficient margin is applied to the test results.

General-purpose spectrum analyzers with general purpose filters and detectors are a good starting point for pre-certification and EMI radiation testing. However, for more comprehensive analysis is it useful to have a WLAN-specific test and certification solution that supports the full range of test and analysis required for IEEE 802.11 standards.

Such a solution, particularly when based on a mixed domain oscilloscope and coupled with vector signal analysis software allows designers to correlate events in the time domain with frequency domain analysis for fast identification of problems that could cause a product to fail certification. For instance, a glitch that only happens during wake-up in the time domain could be causing an out-of-band emission in the frequency domain.

A common question often has to do with how quasi-peak (QP) detectors, used for full compliance tests, compare to the simpler peak detectors typically available for pre-compliance testing. In actual fact, external labs typically begin their testing by performing a quick scan using peak detectors to find problem areas that exceed or are close to the specified limits. For signals that approach or exceed the limits, they then perform a QP measurement.

The QP detector is a special detection method defined by EMI measurement standards that serves to detect the weighted peak value (quasi-peak) of the envelope of a signal. It weights signals depending upon their duration and repetition rate. As shown in Figure 2 , signals that occur more frequently will result in a higher QP measurement than infrequent impulses.

Figure 2: This example shows the effect of peak and quasi-peak detection on a signal with an 8 µs pulse width and 10 ms repetition rate. The quasi-peak value is 10.1 dB lower than the peak value.

A good rule to remember is QP will always be less than or equal to peak detect, never larger. This means peak detectors give you margin so you can confidently use peak detection to do your spectrum emission troubleshooting and diagnostics. You don’t need to be accurate to a full compliance department or lab scan, since it is all relative. If your lab report used the QP detector and shows the design was 3 dB over and your peak detect is 6 dB over, then you need to implement fixes that reduce the signal by -3 dB or more.

Pre-compliance probing technology
In a full compliance lab, EMI receivers and well-calibrated antennas are used to test the electronic devices over a distance of 3 or 10 meters. In other words, the measurements might be done in the far field. In essence, the far field test can accurately tell whether the product passes or fails as a whole but cannot point the source of a problem.

Using only the far-field test, you cannot isolate problems down to specific components or locations, like too much RF energy “leaking“ from an opening in a metal enclosure or help identify a cable radiating too much RF energy. A near-field test is the only way to locate such emission sources and is typically performed using a spectrum analyzer and near-field probe.

Near-field probes such as that shown in Figure 3 for EMI are electromagnetic pickups used to capture either the electric (E) or magnetic (H) field at the area of interest and are used with the spectrum analyzer. Manufacturers provide kits of probes that offer the best compromise between size, sensitivity and frequency range, and you may need all the sizes in your toolkit to solve your problem.

Selection between an H-field or E-field probe may be driven by location of a signal in your design, or by the nature of its source (voltage or current). For example, the presence of a metal shield may suppress the E-field, making it necessary to use an H-field probe for the application. Near-field probes must be used to either pick up the signal near the device under test.

Figure 3: A near-field probe is used to discover the location of unintended RF emissions

Voltage probes are used with oscilloscopes and spectrum analyzers to attach directly to the circuit of interest. Conventional oscilloscope probes can be used with spectrum analyzers with loss in sensitivity depending upon the impedance of the probe. For example a 500 Ohm Z0 oscilloscope probe connected to a 50 Ohm spectrum analyzer will result in a 10:1 divider and a reduction in signal to the spectrum analyzer input of 20 dB. However, when connecting directly to a circuit, the signals are generally large, and can be seen by the spectrum analyzer even with the reduced signal level. Furthermore, the noise floor and sensitivity of a spectrum analyzer is typically orders of magnitude better than an oscilloscope, so loss from a probe is rarely a limiting factor. Voltage probes must be attached directly to the circuit to pick up the signal.

Three basic steps for pre-compliance testing
As shown in Figure 4 , for pre-compliance testing, the frequency domain is divided to three sub-domains or zones. Table 1 shows 2.4 GHz band WLAN compliance requirements for North America and Europe.. Each zone has its individual regulations, and wireless device implementers need to be successful in the three-step spectrum pre-compliance test before they can bring their products to market. These steps are:

  • In-band (channel) domain: Check the transmit power output, the transmit bandwidth, and power spectrum density, etc.
  • Out-of-band domain: Check the spectrum emission or the adjacent channel power ratio (ACPR). The mask is usually defined by communication standards like IEEE.
  • Spurious domain: Check spurious emission levels.

Figure 4: For pre-compliance testing, the frequency domain is divided into three sub domains.

Regions

US/Canada

Europe

Rules

FCC 15.247/IC RSS 210

ETSI EN 300 328 (V1.8.1)

Frequency Range

2400-2483.5 MHz

2400-2483.5 MHz

Bandwidth

>500kHz @ 6dB BW

<20 MHz@ 99 % BW

Maximum Output Power

1 W (Antenna Gain<6dBi)

100 mW (20 dBm)

Reduced by 1 dB for every 3 dB (Antenna Gain=6dBi)

Power Spectrum Density

The peak < 8 dBm/3 kHz

The peak < 10 dBm/MHz

Spectral Emissions

No additional requirements.
Please refer to the IEEE 802.11 standard.

 -10 dBm/MHz
(2400 MHz-BW to 2400 MHz and 2483.5 MHz to 2483.5 MHz+BW)

-20 dBm/MHz
(2400 MHz-2BW to 2400 MHz-BW and 2483.5 MHz+BW to 2483.5 MHz+2BW)

Spurious Emissions

In any 100 kHz bandwidth outside the frequency band of operation the power shall be at least 20 dB below that in the 100 kHz bandwidth within the band that contains the highest level of the desired power.

Radiated harmonic and spurious emissions which fall within the restricted bands, as defined in FCC Part 15.205, must comply with the radiated emission limits specified in FCC Part 15.209.

Outside±2.5 time BW:
-36 dBm/100 kHz
(30 – 47 MHz);
-54 dBm/100 kHz
(47 – 74 MHz);
-36 dBm/100 kHz
(74 – 87.5 MHz)
-54 dBm/100 kHz
(87.5 – 118 MHz)
-36 dBm/100 kHz
(118 – 174 MHz)
-54 dBm/100 kHz
(174 – 230 MHz)
-36 dBm/100 kHz
(230 – 470 MHz)
-54 dBm/100 kHz
(470 – 862 MHz)
-36 dBm/100 kHz
(862 MHz – 1 GHz)
-30 dBm/1 MHz
(1 GHz – 12.75 GHz)

Table 1: FCC and ETSI WLAN 2.4 GHz band compliance requirements.

Transmit power measurement
When planning or updating awireless device installation, it's often necessary to determine if yourwireless equipment can achieve a certain transmission distance. However,this information is not printed in the specs for wireless devices andantennas. Maximum allowable output power is measured in accordance withpractices specified by the regional regulatory bodies. Therefore, youneed to check the standards and regulations to ensure that your deviceis able to pass the compliance test. The steps for performing this testusing vector signal analysis software along with a spectrum analyzer areshown in Figure 5 .

It is important to note that some WLANsignals exceed the bandwidth of an analyzer to perform the transmittedpower measurement. For example, an 802.11ac signal would require abandwidth of at least 160 MHz to perform the burst power test.

Figure 5: This examples shows the steps for performing IEEE 802.11g transmit power measurement.

Power Spectrum Density measurement
Power Spectral Density (PSD) is the power within each unit of frequency (Figure 6 ).For example, the FCC requires that the power spectral density conductedfrom the intentional radiator to the antenna shall not be greater than 8dBm in any 3 kHz band during any time interval of continuoustransmission, then you need to set the spectrum analyzer centerfrequency to the channel center frequency, set the RBW to 3 kHz, and usepeak detector and marker to determine if the maximum amplitude level isgreater than 8 dBm.

Figure 6: IEEE 802.11g Power Spectrum Density measurement

Occupied bandwidth measurement
Occupied bandwidth (Figure 7 )is a measurement of the frequency band bandwidth that contains aspecified percentage of the total power of the signal. OccupiedBandwidth (OBW) is a measurement of how much bandwidth a signal consumeswithin an allocated channel. Typically, the OBW is specified as apercentage of the total power within the allocated channel bandwidth,such as 99 percent or the X dB down bandwidth. In the example in Figure6, the FCC requires that the minimum 6 dB bandwidth be at least 500 kHz,so you need to make the 6 dB down occupied bandwidth measurement.

Figure 7: IEEE 802.11g Occupied Bandwidth Measurement

Spectrum Emission Mask measurement
Oncethe transmit power output of your device meets the in-band compliancerequirement, you can move on to test the out-of-band emissions. Aspectral mask is a mathematically-defined set of lines applied to thelevels of radio transmissions. This mask provides the limit under whichthe signal power is allowed to distribute over the channel. The TransmitSpectrum Mask is defined for each variant of the standard.

Generally,the Spectrum Emission Mask (or out-of-band) domain starts at afrequency offset of 0.5 times the necessary bandwidth (allocated channelbandwidth) and extends up to 2.5 times the necessary bandwidth. Forexample, the IEEE emission mask domain of a 20 MHz bandwidth 802.11gsignal is from ±10 MHz to ±50 MHz frequency offset from its centerfrequency. The process for this measurement is shown in Figure 8 .

Figure 8: IEEE 802.11g spectrum emission mask measurement process.

Spurious emission measurement
Aspurious emission is any radio frequency not deliberately created ortransmitted, especially in a device which normally does create otherfrequencies. A harmonic or other signal outside a transmitter's assignedchannel would be considered a spurious emission. The local regulatorystandards such as FCC in US provides the limit (permissible value) ofspurious emission power of a given unwanted emission domain. In theexample in Figure 9 , the limit lines are loaded from a savedsetup file with the appropriate settings for each of the frequency zonesestablished in the standard.

Figure 9: IEEE 802.11g spurious measurement process

Overcoming RF integration complexity
Manymanufacturers assume that they can just buy a wireless module and havetheir product certified and ready for market with very little effort.Yet for even fairly simple integration efforts, there are many potentialareas for problems and complex regulatory requirements that must bemet. Given the cost of time of going to a compliance certificationfacility, pre-compliance testing is a must.

However, checking allthe standards and regulations is difficult and time consuming. Evenwhen all the regulatory information has been collected, you still needto get familiar with test equipment and make sure all your measurementsare setup correctly. In fact, pre-compliance procedures can add up tohundreds clicks on spectrum analyzers.

To simplify this process,test and measurement suppliers have started offering step-by-stepguidance or wizards for using mixed domain oscilloscope or spectrumanalyzers to perform pre-compliance measurements for WLAN devices.Further, diagnostics are not limited to pre-compliance testing.

Thesetools also support extensive diagnosis and troubleshooting to ensurethat the RF subsystem performs up to specified levels without beingdegraded by other parts of the integrated system.

Xiao Li is product line application engineer for Tektronix with a focus on applications of commercial wireless, spectrummanagement, and RF education. He joined Tektronix in 2012. He received aB.S. from Shenyang Ligong University (China), and M.S. and Ph.D.degrees from Portland State University (Oregon, USA). He was also anadjunct faculty (part-time) at Portland State University. His researchinterests include wireless communications and signal processing.

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