Meeting the Challenges of mmWave Testing

For a long time, millimeter waves (frequency bands greater than 40GHz) were mainly used in the military field, including various radars, satellite communications, etc., and civilian applications were limited to microwave point-to-point applications. Due to the difficulty in designing and developing devices such as coaxial cables and connectors that work in the millimeter wave frequency band, many companies' products currently use waveguides as the main connection method. Anritsu has been deeply researching millimeter wave semiconductor devices, microwave devices , cables and connectors, and has been continuously investing for many years, and has always been in a leading position in the industry in this regard. At present, millimeter waves are increasingly used in industrial and consumer fields, and R&D engineers must be aware of the problems that coaxial cables used in test systems may bring to testing. For this purpose, Anritsu has developed a series of miniaturized instruments that can effectively reduce the number of coaxial cables and connectors used and effectively improve test accuracy.

Market Outlook

With the development of technology, more and more industries and applications are beginning to use millimeter wave frequencies.

5G — With the increase in smartphone users and the development of various mobile phone applications, the demand for wireless data transmission rate is increasing day by day. The existing spectrum resources are already very crowded and cannot meet these requirements. New spectrum resources are urgently needed to meet this demand. In view of this, in July 2016, the US FCC opened up nearly 11GHz of spectrum resources: 27.5 to 28.35GHz, 37 to 38.6GHz, 38.6 to 40GHz and 64 to 71GHz to meet this demand. Although 5G is still under development, at present, the fastest application will be home broadband millimeter wave access. After that, it will be widely used in mobile communications and base stations, and beamforming antenna technology will be used to compensate for the relatively large attenuation of signals during spatial transmission.

Automotive radar - The prerequisite for the realization of autonomous driving technology is that the car must be able to sense and avoid obstacles (see Figure 1). Therefore, the car needs a series of radars to detect and sense the environment around the car. In order to improve the resolution of radar, the main frequencies currently used are 24GHz, 77GHz and 79GHz millimeter wave frequencies.

Figure 1. Application of automotive radar

60GHz Wi-Fi (WiGig) — With the increasing demand for high-speed transmission rates, the 802.11ad standard was developed based on the original IEEE 802.11ac wireless local area network (LAN). The frequency range of 802.11ad is defined as 58 to 64 GHz, which is an unlicensed band. Recently, the frequency range of this band has been expanded to 71GHz (FCC Part 15). 802.11ad is mainly used for high-speed wireless multimedia transmission applications, including uncompressed high-definition television and real-time music and picture transmission.

Point-to-point microwave backhaul — In telecommunications data transmission applications, optical fiber and microwave are generally used. The advantage of optical fiber is high data transmission rate, but the disadvantage is that it is difficult to deploy. The advantage of microwave is that it is easy to deploy and is suitable for base station backhaul applications and is widely used. Especially with the large-scale deployment of various small base stations, such as picocells, microcells and metrocells, microwave backhaul is also being widely used. The traditional microwave backhaul frequency bands are 6, 11, 18, 23 and 38 GHz. The latest 60 GHz microwave backhaul frequency band is an unlicensed frequency band with the advantage of low cost of use, but the disadvantage is that the 60 GHz frequency band is affected by oxygen molecule absorption and has a relatively large attenuation. At present, some microwave backhaul uses the 80 GHz frequency band. The commonly used frequency band is the E-BAND frequency band, which covers the frequency range of 71 to 76 GHz, 81 to 86 GHz and 92 to 95 GHz.

Security and Defense — Radar and satellite communications are the main applications of millimeter waves in the military industry. Millimeter waves have recently begun to be used in the security field. Imaging technology developed using the characteristics of millimeter waves can detect metals and non-metals in a non-contact manner, which can be used to detect weapons or explosives. If you will be taking a flight in the United States in the near future, you may see and use these millimeter wave imaging devices at American airports.

Challenges of mmWave Applications

As mentioned above, based on the many advantages of millimeter waves, many applications can be developed. However, high-frequency signal transmission also inevitably brings problems such as high transmission loss, low test repeatability and difficulty in field testing. The relationship between RF and microwave signal propagation loss vs. frequency (f) and distance (d) is shown in the following formula:

At the frequency of millimeter waves, there will be a relatively large atmospheric propagation attenuation due to the influence of oxygen molecules in the atmosphere, especially. Figure 2 shows the relationship between atmospheric propagation attenuation and frequency. In the 60GHz frequency band, due to the increased absorption of electromagnetic waves by oxygen molecules, an attenuation peak will appear. Because the 60GHz transmission attenuation is relatively large, the transmission distance is relatively short, and the co-channel interference is relatively small, the government has designated the 60GHz frequency band as an unlicensed frequency band. At the same time, the large attenuation also brings challenges to testing. The test instrument needs a relatively large output power or a relatively high receiving sensitivity to ensure the accuracy of the test.

When the frequency reaches 70GHz, the diameter of the inner conductor of the coaxial connector is only 0.5mm, which is close to the limit of lathe machining capabilities. Any burrs or even dust on the connector will affect the matching performance of the connector in the millimeter wave band. Compared with low-frequency connectors, you must be careful when using high-frequency connectors to avoid damage. It is recommended to use a magnifying glass to check and clean before each use, and use a torque wrench for connection.

Figure 2. Atmospheric propagation attenuation vs. frequency

Meeting the Challenges of mmWave Testing

The spectrum analyzer is one of the key devices for millimeter wave testing. Together with the signal source and antenna, it can be used to test the fading characteristics of wireless channels. In the low frequency band, a desktop spectrum analyzer and an antenna are often used to form a test system. The antenna is usually placed on a turntable, and the desktop spectrum analyzer is placed on the test bench, and the two are connected by a coaxial cable. However, in the millimeter wave frequency band, the loss of the coaxial cable will increase dramatically due to the increase in frequency. For example, in the 70GHz frequency band, the loss of a 3m cable will be higher than 20dB. When such a cable is used for testing, the measurement range and accuracy will be greatly reduced. At the same time, the loss and phase characteristics of the cable will also change with temperature, which will increase the uncertainty of the test. In order to eliminate the many effects of the cable on the test, Anritsu has proposed a new solution, using an ultra-small spectrum analyzer to directly connect to the antenna, and the portable spectrum analyzer is connected and controlled by a PC via a USB cable (see Figures 3 and 4).

Figure 3. Using a benchtop instrument to test the millimeter wave frequency band will face the problem of excessive cable loss (b).
Using an ultra-small USB interface instrument, the instrument and the device under test can be directly connected (a)

Figure 4: 28 GHz wireless channel test, using a battery-powered portable signal source to transmit a 0 dBm signal through an antenna, and using a USB-based spectrum analyzer and antenna to receive the signal

Reducing the number of connections and cables in the test system will reduce test errors and the proportion of false detection. Reducing the use of cables will also reduce signal transmission mismatches, reduce test drift caused by cables, and improve test accuracy.

The power meter and spectrum analyzer test is a "scalar" test, which means that the phase of the signal is not included. The mismatch at the connection between the power meter and the spectrum analyzer will cause part of the signal to be reflected back to the signal source. After the signal is reflected to the signal source, the mismatch at the signal source port will reflect the reflected signal back to the power meter and spectrum analyzer. The phase of the reflected signal will change with frequency. The change in phase will cause the total signal strength to be added or subtracted when the reflected signal and the original incident signal vector are superimposed, resulting in an increase in the ripple of the total amplitude measurement result. In this way, the test result may be higher or lower than the actual situation.

The uncertainty of the mismatch can be calculated using the voltage reflection coefficient ρ at the connection. Assuming the reflection coefficients at the connection points of the cable are ρ1 and ρ2, the positive uncertainty u+ and negative uncertainty u- can be calculated using the following formula in dB.

A vector network analyzer can be used to measure ρ. Figure 5 shows the uncertainty curve obtained by the above formula. For example, a 70GHz signal source and a power meter or spectrum analyzer are connected by a cable. The standing wave ratio of the signal source and the power meter or spectrum analyzer port is 2:1 (ρ=1/3). The worst uncertainty of a 0dBm power test may be as high as +0.92dB to -1.02 dB. If a system has more cables or connections, the corresponding error will be greater.

Figure 5. Test uncertainty (±dB) due to reflections ρ1 and ρ2 at the connection

Using high-performance, low-loss cables can reduce test uncertainty, but it will bring problems such as rising costs. For example, a 2-foot-long precision test cable costs about $1,000. At the same time, the precision test cable cannot completely eliminate the test errors caused by the mismatch of the connection end face and the cable's own loss (see Figure 6). If multiple cables are used in a system, the problem will become more complicated. For example, suppose a cable has a loss of 5dB at 30GHz and a loss of 8dB at 70GHz. Another cable produced by the same manufacturer has a loss of 5dB at 30GHz and a loss of 10dB at 70GHz. In fact, this situation is very common. In this case, the loss calculation becomes complicated and a vector network analyzer may be required to test the actual loss at each frequency point, which will become cumbersome and prone to errors. If the use of cables can be reduced or even eliminated, and the DUT and test instrument are directly connected, the test process will be greatly simplified and the test accuracy will be improved. In the example of Figure 6, if the spectrum analyzer and the device under test are directly connected, the sensitivity will increase by 5dB and the test uncertainty will decrease by about 0.4dB due to the lack of influence of the cable.

Figure 6. Measurement uncertainty due to reflection and loss in a cable when connecting the test instrument to the DUT

Progress in mmWave Testing

Advances in millimeter wave testing technology have improved test accuracy. The 40 GHz K-type connector invented in 1983 (Anritsu patent), the 70 GHz V-type connector invented in 1989, and the 110 GHz W-type connector invented in 1997 are all examples of the gradual development of test interface technology.

Test instruments are also evolving to meet market demands: currently, vector network analyzers have a coaxial output that can support 70kHz to 145GHz, and there are very compact USB-connected spectrum analyzers that support a frequency range of 9kHz to 110GHz (Figure 7).

The external mixer of Anritsu's millimeter-wave vector network analyzer is very small. Due to the use of nonlinear transmission line (NLTL) technology, a single connection covers a maximum of 70kHz-110GHz/145GHz. And because of the use of coaxial output, it can be directly connected to the probe, which greatly improves the stability and ease of use of the test and is very suitable for wafer-level probe testing. The handheld spectrum analyzer, which is also developed using nonlinear transmission line (NLTL) technology, has a frequency range of 9kHz-110GHz and is only slightly larger than a smartphone. It can provide performance comparable to that of a desktop instrument, but at a relatively low price and small size. Due to its small size, the instrument can be directly connected to most of the devices under test without the need for coaxial cable conversion.

Figure 7. Current advanced millimeter wave test systems:
Anritsu’s VectorStar 70kHz-145GHz vector network analyzer (a)
Anritsu’s 9kHz-110GHz handheld spectrum analyzer MS2760A (b)

Summarize

In the past decade, with the development of semiconductors, microwave components, cables, connectors and test instruments, the difficulty and cost of millimeter wave applications have been greatly reduced, making millimeter wave technology widely used in price-sensitive commercial and consumer products and systems. By using advanced test instruments, the use of cables can be reduced, the test uncertainty caused by mismatch and cable loss can be reduced, the test accuracy of millimeter wave frequency bands can be improved, false measurements can be reduced, and product quality can be improved. The newly launched test instruments have greatly improved the measurement speed and accuracy, ensuring the smooth progress and cost reduction of R&D and testing.