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Impedance Matching and RF Voltage
One challenge that designers often have to overcome is the RF energy at the antenna. For example, the impedance matched to the antenna may generate a high RF voltage in the matching network.
When matching capacitors or inductors with higher impedance, a larger differential voltage may appear between the matching network and the antenna. When using components with lower voltage ratings, system performance may be degraded.
To eliminate this performance degradation, impedance matching devices with higher voltage ratings are used. These types of impedance matching devices are necessary to withstand higher RF voltages. Figure 1 shows how high the RF voltages are when transmitting at higher (GSM850/900) power levels.
Figure 1
Antenna impedance details
Many factors, including application and antenna design, affect the voltage level at the antenna. These include the following three factors:
1. Matching the antenna impedance of the device may result in higher voltage
2. Input power level in the application (i.e. Power Class 2 (PC2) or GSM)
3. Impedance of actual matching device
Let’s consider these three factors and take a closer look at how antenna patterns and antenna tuners can be used to optimize antenna design.
Learn more about impedance matching
Impedance matching components affect power levels and require the use of components rated at higher voltage levels to optimize antenna efficiency.
Figure 2 shows two antenna designs, Modes A and B. Next, we will describe how these design modes interact with impedance matching components of different voltage ratings. We will also show how to maximize the overall radiated efficiency by utilizing higher voltage rated components.
Figure 2
First, in Figure 3 we can see how antenna modes “A” and “B” are measured on the Smith Chart at low-band GSM frequencies. As shown, the antenna impedance is in the inductive region of the Smith Chart, so a series capacitor becomes the optimal matching solution. Therefore, our antenna matching solution will use a capacitor.
Figure 3
In our example, we measured and compared two similar devices shown on the left side of Figure 4 as antenna impedance matching components. One is 55VRF (DEVICE55) and the other is 65VRF (DEVICE65). Each device consists of a programmable capacitor with 32 different capacitance states and an independent switchable switch.
Figure 4
The state of each device is selected to achieve the maximum radiation efficiency in the low-band frequency range for antenna mode A. In addition, the selected device state should also meet the rated RF voltage requirements of each device: 55VRF for DEVICE55 and 65VRF for DEVICE65, as shown in the figure below.
The device was tested under GSM850/900 and LTE B12 (Band 12). The measurement graph (Figure 5 below) shows the antenna efficiency vs. frequency with the two devices connected.
Figure 5
The above output measurements were made using antenna pattern “A” for DEVICE 55 and DEVICE 65. As shown, the efficiency is significantly affected at GSM850 and GSM900 Tx frequencies if the lower voltage 55 V device is used. To achieve higher efficiency at GSM850, GSM900 and B12 while maintaining voltage levels, the voltage of DEVICE65 should be selected as its efficiency will exceed that of DEVICE55.
To improve the response performance of DEVICE55, we tried to use the mode “B” antenna design.
The output measurement graphs below show mode “A” using DEVICE65. For DEVICE55, we used mode “B”. Although using mode “B” antenna design at GSM frequencies improves the DEVICE55 solution, it is still not enough to meet the requirements of the DEVICE65 component. As shown in Figure 6, the efficiency of DEVICE65 once again exceeds that of DEVICE55.
Because DEVICE65 can meet higher RF voltage input impedance requirements.
Figure 6
Furthermore, the efficiency achieved using mode "B" and DEVICE55 is not as high as that achieved using mode "A" and DEVICE65, nor is the frequency band as wide, especially in the B12 frequency range.
Although DEVICE55 will show some improvement when using mode "B", the efficiency is not as high as using mode "A" and DEVICE65.
In summary, high voltage on the antenna does have an impact on efficiency and performance. Our measurements confirm that higher voltage rated devices achieve better performance in impedance matching applications at high RF voltages.
In our example, we use two configurable tuners from Qorvo, each containing a switch and a programmable capacitor array (PAC), one device rated at 55 V and the other at 65 V. The higher voltage rated components provide antenna designers with more margin.
This enables system designers to more effectively match circuits to multiple antenna patterns and RF voltage scenarios without modifying the design layout structure.
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提升卡
变色卡
千斤顶