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The basic purpose of a logarithmic amplifier (log-amp) is to convert an RF input signal to an output voltage proportional to the log of RF power. Modern log-amps accomplish this task with exceptional dynamic range, and their operation is simple. Inputs are single-ended or differential, and the detector outputs connect to the non-inverting terminal of an internal op-amp or transconductance amplifier. The integrated op-amp allows a user-adjustable slope and an easily implemented closed-loop operation for power-control applications. Output voltage is proportional to the log of the input power. Thus, the RF input power is calculated as follows:

where P_RFIN is the RF input power in dBm, P_INT is the zero-volt intercept point in dBm, and SLOPE is the detector slope in mV/dB.

In addition to basic power measurements, log-amps perform advanced RF measurements such as gain, return loss and VSWR. As shown in Figure 1, two log-amps are used for receiver applications in which you must monitor the receiver's conversion gain from RF to IF. Log-amp A monitors the RF power, and log-amp B monitors the IF power. Because the log-amps accommodate a wide frequency range, the measurement of conversion gain is relatively unaffected by the frequency difference in this superheterodyne architecture. The integrated difference amplifier provides a voltage output proportional to the log of the input powers, centered at 1 V. To calculate gain, use the DC output voltage V_OUTD:

P_RFINA and P_RFINB are the input powers in dBm, and their difference in dB is the gain. V_CENTER is the center voltage (1 V), and SLOPE is the output slope of the difference amplifier, which for the MAX2016, is 25 mV/dB. If V_OUTD equals 1.5 V, for example, the gain is 20 dB. V_OUTD = 0.5 V implies gain = -20 dB.

Log-amps can measure the voltage standing-wave ratio (VSWR) and return loss for a device under test (DUT). As shown in Figure 2, a full directional coupler couples the incident and reflected power to the log-amps, thereby producing two proportional output voltages at OUTA and OUTB. Because OUTD is a dc voltage proportional to the difference between OUTA and OUTB, you can easily obtain the return loss (RL) and VSWR by measuring V_OUTD as shown in Equations 4 and 5. Notice that Equation 4 is similar to Equation 3. As an option, you can use an ADC controlled by a microprocessor to obtain RL and VSWR as digital outputs.

Figure 3 shows a variation of the VSWR monitor used to verify the integrity of an antenna. Like the VSWR measurement, it uses a circulator instead of a coupler. Forward power is delivered to the antenna, but if the antenna is disconnected or damaged, power is reflected to the log-amp. A high level of reflected power indicates a poor antenna connection.

Steady advances in integrated circuit technology have enabled log-amps to expand their role beyond that of simple power detection and demodulation. They now perform advanced RF measurements such as VSWR and return-loss monitoring. Such RF measurements ensure that a system is operating with peak performance, while also providing a warning in the event of a system failure.

Ken Yang is a senior member of the technical staff at Maxim Integrated Products Inc., Sunnyvale, Calif.