Inductance matching analysis based on high frequency circuit

For high frequency circuits, inductive matching between circuits is important. Inductance matching means that the output impedance of the transmitting circuit is consistent with the input impedance of the receiving circuit on the transmission line of the signal. After matching, the power of the transmitting end can be transmitted to the receiving end to the utmost.

The matching circuit uses capacitors and inductors, but the actual capacitors and inductors are different from the ideal components and have losses. Indicates that there is a Q value for this loss. The larger the Q value, the smaller the loss of the capacitor and inductor.

The Q value of the inductor and the loss of the high frequency circuit

The magnitude of the Q value of the inductor used in the matching circuit also affects the loss of the high frequency circuit.

In order to confirm this, we used Murata's SAW filter (passband 800MHz band) and RF inductors, and replaced the RF inductors with different Q values ​​in the matching circuit to measure and compare the insertion loss of the SAW filter.

Figure 1 shows a circuit diagram. This circuit, although a matching circuit, has only one RF inductor.

Figure 1: SAW filter and matching circuit

Fig. 2 shows the frequency characteristics of the Q value of the RF inductor that has been replaced this time. Table 1 shows the structure, size, and Q value (Typ. value at 800 MHz).

Figure 2: Comparison of Q values ​​of RF inductors (both 7.5nH)

Table 1 Comparison of RF Inductance

* The chart in Figure 2 is represented by SimSurfing, a design aid provided by Murata.

The overall characteristics of the SAW filter when the RF inductor of the matching circuit is replaced are shown in Fig. 3. The passband characteristics are shown in Fig. 4.

From the passband characteristics of Fig. 4, it can be confirmed that the insertion loss of the SAW filter differs depending on the RF inductance used. This level of loss in high frequency circuits is becoming more and more important.

From the experimental results, the larger the Q value of the RF inductor (the smaller the loss), the smaller the insertion loss of the SAW filter. That is to say, the size of the inductor loss is the size of the SAW filter loss including the matching circuit.

Please note that the used high frequency components (this time is a SAW filter), matching circuit, frequency band, etc., the loss will also vary.

Inductance deviation and influence on matching circuit

In addition, the actual inductor has an impedance value of 1.0 nH, 1.1 nH, and 1.2 nH. When making a match, it is sometimes necessary to fine-tune it with a detailed constant step. At the same time, the deviation of the impedance value (standard deviation) becomes a matching standard deviation, and in order to satisfy the necessary characteristics, an inductor with a small deviation is sometimes required. Among Murata's inductors, the thin film type LQP series is the most consistent with the detailed constant steps and small deviations.

Therefore, customers are advised to select the RF inductance of the matching circuit based on comparison and study of Q characteristics, deviation, size, and cost.