Foreword
In recent years, the design difficulty of mobile communication antennas installed in mobile communication terminals has gradually increased. With the increase in the new communication method of LTE, the use of broadband is becoming more and more widespread. On the other hand, the usable space (antenna/area) is reduced due to factors such as the increase in size of the secondary battery. Therefore, the miniaturization of the antenna has become a top priority. However, if the antenna is miniaturized, it means that the impedance of the antenna will be lower than the input and output impedance of the RF circuit (50Ω system), which means that the RF circuit and the antenna impedance are integrated through the full communication bandwidth. is very difficult.
LC component issues
At present, in implementing impedance integration, LC components such as an inductor (L) and a Capacitor (C) are generally used. However, the LC component has a frequency characteristic, and the antenna Q value after the integrated impedance is degraded, and the frequency bandwidth is reduced.
Here, for materials that are difficult to exhibit in frequency characteristics during impedance conversion, the transformers used mainly in the low frequency field are exemplified. The transformer achieves the converted impedance by the ratio of the inductance (L value) of the two coils (transformers, coils) combined with the magnetic field, so the frequency characteristics in the ideal state cannot be guaranteed. So we consider using it into the integration of impedance.
Reasons not to use a transformer
There are three problems with using a transformer in an antenna for mobile communication.
1) In the microwave frequency band, it is difficult to achieve a high coupling coefficient due to "permeability of magnetic material ≒1";
2) The small input impedance of the antenna will cause a large loss of transformer loss;
3) The antenna input impedance value will vary due to the frequency band.
Because of these problems, transformers have not been used in the impedance integration of mobile communication antennas until now. And we solved this problem by a unique method.
The coupling coefficient refers to a change in the two transformers constituting the transformer, the distance between the coils, and the shape dependence of the magnetic flux caused by the coil. Since the transformation ratio is controlled while maintaining a high coupling coefficient, the shape of the transformer and the coil are in a uniform state, and a structure in which the L value of each coil can be freely controlled is developed.
This structure is constructed in LTCC (Low Temperature Co-fired Ceramic) and can be made with a distance between the transformer and the coil of several tens of μm. The coupling coefficient of the transformer can be controlled to 0.7 or more even in the microwave band.
Reversed connection port
Connecting a high-frequency transformer to an antenna with a 10Ω impedance, the insertion loss (insertion loss) due to the material properties of the transformer itself is relatively larger than that of a high-frequency device connected to 50Ω. Therefore, a transformer having a large L value and an impedance component which is generally used in a low frequency is difficult to use at a high frequency.
In order to reduce this impedance component and maintain the transformation ratio, we have adopted the configuration of the high-frequency transformer shown in Fig. 1. This configuration is contrary to the normal transformer configuration, which completely reverses the ground connection port and the antenna connection port. Therefore, the transformation ratio as shown in Fig. 1 can be achieved. With this configuration, the mutual inductance M value due to the combination is reflected to the transformer, and the value of the coil L used in the transformer is reduced, since the impedance component IL of the high-frequency transformer can be suppressed to be small.
Constant transformation ratio
The communication bandwidth used in the mobile communication antenna is divided into a low field "low band" and a high field "high band" by a limit of 1 GHz. In an open antenna, generally, the low band is the fundamental wave of the antenna and the high band is the high frequency wave of the antenna. When the anti-integration function of the short pin or the like is not installed inside the antenna, the impedance of the low band is about 10 Ω, and the impedance of the high band is about 19 Ω.
If a transformer with a certain transformer ratio is installed in such an antenna, it is impossible to integrate only the impedance of one band. Therefore, for antenna transformers, the necessary design requirement is to adapt the transformation ratio to the impedance frequency characteristics of the antenna. This adaptation method is shown in Fig. 2, which is an equivalent circuit that decomposes the ideal transformer part and the parasitic component part.
The parasitic components of the transformer structure developed this time are classified into two types: "series L" and "parallel L". Among them, the series parasitic component can be reduced by increasing the binding coefficient, and the parallel parasitic component may have a "binding coefficient = 1". In the high-frequency transformers with the necessary small L-value design, it is impossible to eliminate the influence of parallel parasitic components. However, the transformation ratio can be adapted to the impedance frequency characteristics of the antenna by controlling the value of such parallel parasitic components. The value of the parallel parasitic component can be controlled by switching the L value of the transformer coil. This time, we found a combination of the L value and the coupling coefficient K that can well control the parallel parasitic components.
Trial production of surface mount components
With the above-described structure, a high-frequency transformer was experimentally produced and evaluated for the purpose of simplifying the integration of the antenna group. The dimensions of the prototype are 2.0mm & TImes; 1.25mm & TImes; 0.6 mm surface mount components (SMD) (Figure 3). A 50 Ω measuring device is connected to the RF circuit side of the test piece, and the impedance and animation of the antenna connection side are as shown in FIG. The prototype transformer converts the low band (892MHz) into 12→50Ω, and the high band (1940MHz) is replaced by 19→50Ω.
Connecting an ordinary LC circuit to an antenna makes impedance integration difficult under wide bandwidth conditions depending on frequency characteristics. In response to this situation, the high-frequency transformer of this trial is connected to the antenna, and both the low band and the high band are converted into the most suitable impedance. That is to say, the impedance trajectory on the artisan map may change into a shape in which impedance integration is easy. The phase of the impedance trace is then fine-tuned by an external adjustment element, which may be very easily concentrated around 50 Ω. (Figure 5)
Dress evaluation
When the high-frequency transformer is installed in the communication terminal, what are the advantages, and the antenna characteristics are compared and evaluated in an environment where the installation is not performed.
Using the Sharp "ISW16SH" model on the market, the characteristics of the impedance integration in the LC circuit are compared and evaluated using the characteristics of the high-frequency transformer for impedance integration. (Figure 6)
The ISW16SH model has a USB interface directly under the antenna section, and the electric field of the antenna is evaluated by connecting with the interface under strict conditions.
The results show that LC and high-frequency transformers are unlikely to achieve "S11 < -6dB" (reflection loss 1.2dB) in the case of low band. In the case of the high band, the range of "S11 < -6 dB" is 300 MHz if the LC circuit is used, and 500 MHz when the high frequency transformer is used, achieving a 66% improvement. (Fig. 6(c)). In addition, the integrated characteristics of the antennas in the LC circuit and the high-frequency transformer show "Total Efficiency", and the high-frequency side of the low band also has an improvement of 0.6 dB. (Fig. 6(d)). This improvement is due to the wide bandwidth and improved IL.
Next, the effectiveness of antenna miniaturization will be tested. In general, antennas have the property of improving the grounding distance. However, if the antenna occupies a large space (excluding the GND area), the mounting space of components other than the antenna is pressed. Miniaturization of antennas and shrinking of antenna space are imminent. When a part of the antenna space is occupied by the GND, the change in the antenna characteristics at this time is evaluated. The width of the antenna space is 52 mm, of which 13 mm is occupied by GND, and the characteristics of the low band deteriorate, becoming Total Efficiency (Fig. 7(b)) equivalent to the LC circuit. This means that the area of ​​the antenna space can actually be reduced by 25%.
In summary, it can be proved that the high-frequency transformer of this trial can change the characteristics of the antenna compared with the ordinary LC circuit. The high-voltage transformer's transformation ratio adapts to the low-band and high-band impedance of the antenna. Even in the full-band range, it can be said that stable impedance integration can be obtained.
We will productize the equipment developed this time. This device will be considered as a "third impedance integrated device" after the inductor and capacitor. This device is a passive component that uses a transformer. Like inductors and capacitors, it is not as competitive as active devices such as transfer switches, but it has a high affinity. It corresponds to the multi-functionality of the portable terminal, and the antenna must evolve in the future. We firmly believe that the device will contribute to this evolution in the future.
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