Figure 7-4 shows a practical RF power control loop realized by using TI’s LMV242x with integrated RF detector. The RF signal from the PA passes through a directional coupler on the way to the antenna. Directional couplers are characterized by the coupling factor, which is in the 10dB to 30dB range, typically 20dB. Because the coupled output has to deliver some power (in this case, to the detector), the coupling process takes some power from the main output. This coupling process manifests as insertion loss, with the insertion loss being higher for lower coupling factors.

Choose the correct attenuation between PA output and detector input to achieve power control over the full output power range of the PA. A typical value for the output power of the PA is +35.5dBm for GSM and +30dBm for PCS/DCS. To accommodate these levels into the LMV242x detection range, the minimum required total attenuation is approximately 35dB (see also Section 5.6 and Figure 7-2). A typical coupler factor is 20dB. Insert an extra attenuation of approximately 15dB.

To achieve extra attenuation Z between the coupler and the RF input of the LMV242x, use two resistors. R_X and R_Y. according to Figure 7-3

where

or

Equation 2.

For example, R_Y = 300Ω results in an attenuation of 16.9dB.

To prevent reflection back to the coupler, ensure that the impedance seen by the coupler is 50Ω (R_O). The impedance consists of R_X in parallel with R_Y + R_IN. Calculate R_X with the following formula:

For example, with R_Y = 300Ω, R_IN = 50Ω → R_X = 58Ω.

Figure 7-4 Simplified PA Control Loop With Extra Attenuation