How to Correctly Measure Wireless Communication Signals and EMC Analysis

With the rapid development of wireless communication technology and the continuous evolution of specifications, various wireless technologies, whether GSM, GPS, WLAN (such as Wi-Fi), Bluetooth, etc., have gradually appeared and become popular in daily life. Wireless communication technology itself is already extensive and profound, and when it is introduced into various electronic devices and application fields, it is necessary to consider the problems of electromagnetic interference (Electromagnetic Interference, commonly known as EMI) and electromagnetic compatibility (Electromagnetic Compatibility, EMC) to avoid interference with related functions and signal degradation, affecting their normal operation. However, although relevant electromagnetic specifications have been legislated around the world, focusing on the restrictions on electromagnetic radiation and RF (Radio Frequency), it is difficult to have a fixed set of standards to prevent or solve related problems when facing the situation where different communication modules may interfere with each other. This has therefore become the most important point that product developers need to overcome.

In addition, with the recent boom in portable devices and the diversification of communication functions, these related communication modules and antennas must be designed to be thinner and smaller to meet the needs of mobile applications. This makes it even more difficult to optimize product design. In order to build more different wireless modules and antennas in an extremely small and streamlined space, these components will inevitably be more likely to generate noise interference with each other, affecting their transmission performance, because it is often observed that the transmission distance is shortened, the transmission rate is reduced, and other conditions that are not conducive to the communication performance of the product. This article will introduce how to correctly measure wireless communication signals and perform electromagnetic compatibility analysis under wireless communication conditions.

Complex communication environment: Deterioration of reception sensitivity (De Sense) caused by platform noise (Platform Noise)

First, let's imagine a possible scenario for an average consumer using a new handheld device (whether it's a smartphone or a tablet): when it's time to dine and they want to find a nearby restaurant, they can take out their phone, click to open a pre-downloaded app, and then use voice control to say what type of food they want to choose. The app will then transmit the received voice information to the app's server on the Internet for interpretation, location locating and searching, and will display eligible options and even a map on the screen, so that users can follow the map to find the ideal restaurant.

In fact, in this seemingly simple operation process of just a few seconds, many components are involved, including the touch screen sensing, the combination of the product (hardware) and the user interface (software), the microphone receiving signals by eliminating background noise to transmit clean user voice, the activation of the 3G module, the ability to connect to the neighboring base station, the role of the GPS positioning system, the return of server search results, etc. Although for users, what they feel is the "good or not" usage experience; for developers, they must verify the mechanical structure, component selection, hardware and software integration, and communication module in detail to create a good user experience and fully realize the purpose of product use.

Therefore, understanding all components that may generate electromagnetic signals in the entire communication environment of the product can be said to be an important prerequisite when carrying out construction design. Through Figure 1, we can clearly see that there are four main types of components in general new devices that will generate electromagnetic signals. If the signals emitted by these components themselves cause mutual interference due to poor design, it can be called platform noise. These four types of components include system platforms (such as central processing units, memory, power supplies), internal and external connector coupling paths (such as various transmission interfaces such as USB, HDMI), outsourced platform modules (such as touch screens, camera lens modules, solid-state drives and other components purchased from manufacturers for assembly) and wireless chipsets/wireless modules (such as Wi-Fi 802.11 a/b/g/n, Bluetooth, GPS), etc. These four types of components require careful measurement and calculation to accurately find the best circuit design and properly carry out the overall product construction, avoid interference between each other, and minimize all possible problems. Risk.

What is the so-called platform noise interference? For example, the panel is the largest component of all control devices, and any signal emitted by the antenna in the device will hit the panel, and the noise emitted by the panel will also enter the antenna; similarly, the radio waves emitted by the antenna will also affect each interface; and the signals emitted by different modules will also become each other's noise, which is the so-called platform noise interference. When these modules and components are operating at the same time and the interference cannot be controlled under a certain limit, the phenomenon of "degradation of sensitivity" (De Sense) will occur, affecting the normal operation of the device's wireless performance.

For example, in the same frequency band, when mobile phone A can receive signals from 1,000 channels, while mobile phone B can only receive signals from 500 channels, users will actually think that mobile phone B has poor reception. Since the antenna, filter, and front-end circuit do not perform particularly poorly in any particular channel, in summary, this may be because mobile phone B has some shortcomings in its design, and is interfered by the carrier noise, resulting in the so-called deterioration of reception sensitivity.

It is not difficult to measure the interference of the carrier noise. You can choose a clean environment without external interference (such as an electromagnetic wave isolation box), and measure the signal throughput of a single wireless module connected to a circuit board (as shown in the yellow line in Figure 2), and measure the signal throughput of the module built in the product system platform (as shown in the blue line in Figure 2). By comparing the two, you will find that there is obvious signal degradation when it acts on the product platform. The difference in path loss between the two can be regarded as the interference of the carrier noise.

Here we must emphasize a concept, that is, the existence of carrier noise is inevitable. We cannot reduce the noise to zero because the module must be powered by the system, and the location of the module will also affect other adjacent modules and interfaces, which will inevitably generate noise. However, although the existence of carrier noise is inevitable, we can try to reduce its interference to a minimum without affecting the communication performance, which is why we need to measure the noise and find the interference source.

However, it is not difficult to measure the carrier noise interference, but to verify the sources of the carrier noise and the degree of interference caused by individual sources, very complex and detailed measurement methods are required, which is definitely a big challenge for developers. Just controlling the variables and cross-measuring the components that may cause interference can produce thousands of combinations. For example, different communication channels, Bluetooth and Wi-Fi, Wi-Fi and 3G, 3G and GPS, etc. may all cause signal loss due to signal coexistence (Co-existence), crosstalk, etc. The art lies in how to effectively determine the main noise source through the correct measurement sequence and techniques, and to automate the time-consuming cross-measurement.

The first priority in reducing noise: formulate a reasonable noise budget (Noise Budget) for adjustment

After understanding that the interference of the platform noise will cause the reception sensitivity to deteriorate, and knowing how to measure it, the next focus is to set the permissible value of the device noise, that is, to formulate a reasonable noise budget (Noise Budget), so as to make the most appropriate adjustments for the device. In other words, after knowing how the wireless communication technology can be demodulated (for example, it is known that the deterioration of the 3G module can be demodulated through the GPS module), knowing the noise level and Eb/No (system average signal-to-noise ratio), and setting a suitable noise tolerance value, the noise interference can be corrected (not eliminated).

However, such corrections are not the calibration of a single component, but require a series of interlocking verifications and modifications. For example, when the screen of a device interferes with the antenna reception, it is not just the panel itself that needs to be adjusted, but also the display card behind it, input and output power, circuit design, LVDS interface, etc., and even the surface current distribution method of the antenna. All of these need to be adjusted. As can be seen from the simple diagram in Figure 3, there are many variable factors that affect the signal reception ability of wireless devices, and they are all interdependent on each other. Therefore, according to the actual carrier noise conditions, setting a reasonable noise budget, and then adjusting it accordingly to reduce noise, is the key to effectively improving product quality.

Example: The biggest source of interference - touch panel

As mentioned above, the touch panel is the largest component in all types of new devices with touch as the core application, and the corresponding interference problems are more. Therefore, ensuring that the carrier noise caused by it can be controlled within the noise budget is naturally the first priority during verification. According to Allion's verification experience, currently in smart phones and tablet devices, about 60% of the interference problems come from the touch panel, of which 70% come from the IC control chip in the panel. Next, we will explain the key points of touch panel verification.

As the name implies, a touch panel is a panel with touch function. However, the first interference that a touch panel needs to overcome is not from other modules or interfaces in the same device, but from the panel itself. This includes the panel's pixel electrodes, pixel clock, storage capacitors, line-by-line address, backlight unit, etc., which can cause interference to the touch panel.

At this time, it is necessary to measure the voltage during touch, scan and observe the voltage changes at different times and using different touch points to understand the actual stage noise conditions, so as to make appropriate adjustments. Basically, the touch scan voltage is about 100~200k, and the screen update rate is five milliseconds (ms) to check all touch points. This low cycle frequency is very easy to cause interference to GPS and SIM cards. Therefore, the touch panel must increase the voltage to solve the interference of the panel, that is, by slightly reducing the sensitivity of the touch sensor in exchange for a reduction in the stage noise; and in actual measurement and observation, in addition to the need for precise fixtures and instruments, it is also necessary to measure the time domain (not the frequency) to obtain the true error rate (BER) data.

After measuring the noise of the touch panel itself and setting a reasonable noise budget value, we can start measuring the noise of the touch panel on various modules. The fishbone diagram of the touch panel noise budget in Figure 4 is the measurement and verification sequence we have summarized and studied based on experience. We must control the noise budget to observe the interference of the touch panel on different modules. In the actual measurement diagram in Figure 5, the red line is the noise budget value we set, and our goal is to reduce the noise value below the red line.

Below we will discuss several interference examples related to touch panels:

LVDS

Currently, many new devices such as tablets or Ultrabooks use the so-called LVDS for signal transmission when designing the panel display. LVDS is Low Voltage Differential Signaling, which is a technology that can meet the needs of high-performance and low-voltage data transmission applications. However, in actual applications, these signals may partially enter mobile communication bands such as 3G, which will generate a large ground capacitance imbalance current and cause interference. However, the traditional way of dealing with this is to alleviate this situation by sticking copper foil tape or conductive cloth, but the actual ground imbalance phenomenon is not solved, and the problem of LVDS cables is not really effectively solved. Only by measuring the noise difference of the LVDS signal itself in a closed environment and on the system platform can adjustments be made from the source of the problem.

●Line logic gate

In addition, the touch panel is connected to many circuits, and the logic gates of these circuits will generate frequency interference due to continuous switching. For example, when the logic gate generates about 45MHz interference, the difference between the transmit and receive frequencies of GSM 850 (869-896 MHz) and GSM 900 (925-960 MHz) is less than 45MHz, and external modulation will be generated to cause interference; another example is that Bluetooth is affected by the switching of the logic gate, which causes the current to change in size. Such external modulation causes the signal to enter the spectrum of GSM1800 and GSM1900 and cause interference.

Therefore, we must use the frequency domain simulation method to perform S-parameter analysis sampling to confirm that the error value between computer simulation and actual machine testing is within the allowable range in order to understand the noise conduction situation. This will not sacrifice the good touch experience of consumers, but also reduce the interference caused by the touch panel to other modules and components of the product.

● Solid-state drive

The emerging storage medium - solid-state drive (SSD) is still expensive due to the market price fluctuations of flash memory, but it has been widely used in tablet computers and other forms of mobile devices due to its thin and light size and low power consumption. However, the situation that traditional magnetic disk hard drives are easily affected by external communication conditions (for example, when a mobile phone is placed next to a computer hard drive to answer a call, it may interfere with the hard drive and cause data corruption) also occurs on SSDs.

In the case of SSD, the noise margin will decrease as the number of P/E cycles increases. As shown in Figure 7, after 10,000 P/E cycles, the noise margin has deteriorated significantly, and it is more susceptible to interference from the touch panel or other noise sources, affecting the actual function. In this situation, if the SSD can be erased evenly, it will be one of the effective ways to mitigate the rate of noise margin decline.

●Module multi-tasking operation

The power used by the touch panel comes from the system itself, and other modules such as communications or cameras are also powered by the system. Therefore, stable and sufficient voltage is the key to making these component modules work well. Among all the modules that need to use power, the 3G or Wi-Fi modules consume the most power when connecting to the Internet (data transmission). When all these communication modules are turned on, it is very likely to cause insufficient voltage, which will affect the stable power consumption of the touch panel. In addition, the electromagnetic waves of the communication module may also directly hit the panel at the same time, causing serious noise interference. At this time, we must go back to the previous fishbone diagram and perform verification of different module settings, location construction, and communication environment in sequence.

Precision measurement and verification can effectively improve communication quality and reduce noise interference.

At the end of this article, Allion also provides a complete verification step designed based on our experience as a reference for development verification. Through this verification sequence, we can reduce noise interference step by step and improve communication quality. As shown in Figure 8, a complete device with various communication modules and touch functions can be divided into the following three verification steps:

1. Conductive Test:

At the beginning of verification, the conduction test must be performed to accurately measure the device's own carrier noise, the degradation of receiving sensitivity, and the carrier noise during transmission and reception (Tx/Rx).

2. Electromagnetic compatibility (Near Field EMC):

After mastering the relevant information that can be obtained from the conducted test and setting the noise budget, it is possible to perform measurements including antenna surface current measurement, noise current distribution measurement and coupling path loss measurement, as well as camera and touch panel noise and RF coexistence external modulation.

3. OTA test (Over The Air Test):

After completing the conduction and EMC tests, we can perform independent and coexistence measurements on different communication modules, total radiated power (TRP) and total isotropic sensitivity (TIS), GPS carrier-to-noise ratio (C/N Ratio) measurements, and even DVB reception sensitivity tests.

Although the content discussed in this article is only one example of noise verification, we can already understand the profoundness of wireless communication signal technology and the depth of interference control technology. All related manufacturers and businesses need to conduct more in-depth research, invest more technical resources and energy during development to find the appropriate measurement methods and solutions to overcome the signal degradation and interference caused by the design of communication products.