InductorsMobile Phones -SAW Filter-

In high frequency circuits, it is important to perform impedance matching between circuits. Impedance matching is to match the output impedance of the sending side circuit, and the input impedance of the receiving side circuit in the signal transmission path.
By matching the impedance, the power on the sending side can be transmitted at the maximum to the receiving side.

Although a capacitor and inductor are used in the matching circuit, the actual capacitor and inductor are not ideal components, and a loss will be generated.
There is a Q to express this loss.
When the Q is higher, it shows that the capacitor and inductor have less loss.

The loss in high frequency circuits is affected by the level of inductor Q used for matching circuits.
In order to confirm this, Murata's SAW filter (800 MHz pass band) and RF inductor were used to replace the RF inductor with a different Q in the matching circuit, and the insertion loss of the SAW filter was measured and compared.

In the case of this circuit, only 1 RF inductor is used in the matching circuit.

Figure 2 shows the frequency characteristics of the replaced RF inductor Q, and Table 1 shows the structure, size and Q (Typ. value at 800 MHz) .

* Design support tool SimSurfing provided by Murata was used to display the graph in Figure 2.

Figure 3 shows the overall characteristics of the SAW filter when the RF inductor is replaced in the matching circuit, and Figure 4 shows the pass band characteristics.

From the results of this test, it is clear that when the level of the RF inductor Q is higher (low loss) the insertion loss of the SAW filter becomes less.
Accordingly, the level of the inductor loss is the level of the SAW filter loss including the matching circuit.

Keep in mind that the loss will change according to the high frequency circuit being used (SAW filter in this test) , matching circuit, frequency band and etc.

The actual inductance value of the inductor is not a continued value like 1.0 nH, 1.1 nH and 1.2nH. When performing matching, there are cases where fine constant steps are required to make fine adjustments. Since the tolerance of the inductance value becomes the variation in the matching, an inductor with less tolerance may be required to satisfy the necessity. In Murata's inductors, the film type LQP series is ideal for fine constant steps, and less tolerance.

For these reasons, the RF inductors to be used in the matching circuits of SAW filters must be selected after comparing and studying such factors as their Q characteristics, tolerance, size, and cost. If there is some leeway in the mounting space, the LQW15 series and LQW04 series of winding-type inductors with their high Q are suitable. Conversely, if the mounting space is limited, the LQP03HQ series and LQP03TN_02 series with their high Q yet compact 0603-package size are suitable.