7.2.2 Impedance Matching TX_Out (Pin 5) to 50 Ω

The output impedance of the TRF7963A when operated at full power out setting is nominally 4 + j0 Ω (4 Ω real). This impedance must be matched to a resonant circuit, and TI recommends a matching circuit from 4 Ω to 50 Ω, as commercially available test equipment (for example, spectrum analyzers, power meters, and network analyzers) are 50-Ω systems. See Figure 7-2 and Figure 7-3 for an impedance match reference circuit. This section explains how the values were calculated.

Starting with the 4-Ω source, Figure 7-2 and Figure 7-3 shows the process of going from 4 Ω to 50 Ω by showing it represented on a Smith Chart simulator (available from http://www.fritz.dellsperger.net/). The elements are grouped together where appropriate.

Figure 7-2 Impedance Matching Circuit

This yields the following Smith Chart simulation.

Figure 7-3 Impedance Matching Smith Chart

Resulting power out can be measured with a power meter, spectrum analyzer with power meter function, or other equipment capable of making a "hot" measurement. Take care to observe maximum power input levels on test equipment and use attenuators whenever available to avoid any possibility of damage to expensive equipment. Table 6-3 lists the expected output power levels under various operating conditions.