Fujitsu and Fujitsu Research Institute announced on October 17, 2013 that they have developed technologies that can increase the sensitivity of signal receiving IC chips for large-capacity wireless communication devices using the millimeter wave band (240 GHz band). This technology is related to the amplifiers that make up the receiver, which can prevent the oscillation caused by the leakage signal and increase the amplification.
With the increase in data communication demands of smart phones and other devices, the millimeter wave band, which has a bandwidth of more than 100 times that of existing mobile phones, is expected. However, in the millimeter wave band (30G to 300GHz), when the extremely high frequency of 240 GHz is reached, the radio waves propagating in the space are greatly attenuated. Therefore, it is necessary to use a high-sensitivity receiver (composed of an antenna, an amplifier, and a detector) capable of receiving a weak signal. In order to effectively improve the receiving sensitivity, it is necessary to increase the amplification of the amplifier.
Fig. 1 Wavelength comparison of millimeter wave band and 2GHz band
Figure 2 Signal leakage via ground plane
In the 240 GHz band, the wavelength of the signal is very short, less than 1 mm, which is smaller than the chip size of the amplifier (Figure 1). The following new problems will arise: Part of the output signal of the amplifier will leak to the ground plane (electrical reference plane) formed on the surface of the amplifier chip. These leakage signals will be returned to the input terminal of the amplifier and input to the amplifier again ( In Fig. 2), when the signal input again is amplified by the amplifier, it becomes a larger leakage signal and returns to the input terminal again. This is the oscillation phenomenon, which will cause the receiver to receive the signal normally. To achieve higher magnification in the millimeter wave, it is necessary to use a technique that can suppress this oscillation without losing the magnification. This time Fujitsu and Fujitsu researched the Fujitsu Institute's InP HEMT technology and developed the following two technologies.
Techniques that suppress oscillations and enable multistage amplifier connections
The leakage signal of the amplifier has a maximum amplitude where the amplitude increases at a specific position and an amplitude zero where no vibration occurs at all. When the position of the amplifier input terminal coincides with the maximum point of the amplitude of the leakage signal, a larger leakage signal is input to the amplifier, causing oscillation (Figure 3). When the input terminal is in the zero position of the amplitude, the leaked signal will not vibrate at all and the amplifier will not amplify the leaked signal. Therefore, the position of the input terminal and the output terminal of the amplifier can be matched with the amplitude zero point of the leakage signal (lower in Fig. 3). Connecting multiple amplifiers designed in this way increases the amplification without causing oscillations.
Efficient transmission of amplifier output signals to the next level of technology
In order to efficiently transmit the output signal of the amplifier to the next-stage amplifier, it is necessary to perform impedance matching on the line connecting the amplifiers. For this reason, a certain line length must be guaranteed. However, when the amplifier's input and output terminals are aligned with the amplitude zero position, the size of the amplifier is limited and the line is also limited to a specific length, so impedance matching is very difficult. The R&D personnel achieved the results of impedance matching even if the amplifier is limited in size by adopting U-shaped lines and adjusting the U-shaped longitudinal and lateral lengths.
Figure 3 shows the previous amplifier, and the figure below shows the amplifier developed this time.
Figure 4 Terminal usage
According to reports, by adopting these technologies, the sensitivity of receiving ICs can be increased by about 10 times than before. If this kind of IC is equipped on a smart phone or other device using a small antenna, the antenna with a wider directivity can be used, so it is no longer necessary to precisely align the angle of the terminal with the transmitter (Fig. 4). The user's convenience. Future developments such as Fujitsu will continue to develop small antenna-integrated packages that can package such receiver ICs. It is planned that transmission experiments will be conducted before 2015, and will be practical before and after 2020. (Reporter: Chi Chi, Real Tech-On!)
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