Transmitters require circuitry to measure and control RF power for proper operation. Since system requirements vary widely, a power-control circuit may be as simple as a low-dynamic-range diode detector. The whole purpose for this device is to detect catastrophic events such as the sudden increase in VSWR that might occur when an antenna breaks. But for more complex power detection control, as in a GSM base-station transmitter, an RF power detector may need measurement uncertainty of less than ±1 dB over an input power range of 60 dB or more. There are many ways to control RF power, but ever-improving logarithmic-amplifier (logamp) technology offers many solutions.

The GSM transmitter is an example of a demanding application. A +47-dBm (50-W) transmitter operating at full power must transmit a power level between +45 and +49 dBm¹ (+44.5 to +49.5 dBm under extreme conditions). To stabilize the output level, the amplitude of the transmitter's input signals must be precisely known, often within ±1 dB over a wide dynamic range.

There are a number of ways to measure this power, including the use of a closed-loop architecture (Fig. 1a). In this approach, a directional coupler (with typically 10 to 30 dB coupling) is used to sample power from the power amplifier (PA) to the antenna. Some additional attenuation is generally necessary to reduce the power within the safe measurement range of the detector. This measured result is compared to a set-point voltage; the difference drives an integrator (also commonly referred to as an error amplifier).

The error amplifier's output will rise or fall until the output power of the PA corresponds to the set-point voltage. The error amplifier will not necessarily drive the bias control of the PA; the system will be just as effective if the PA has fixed gain and the error amplifier is used to control the gain of an intermediate-frequency (IF) variable-gain amplifier (VGA).

This type of power control (known as controller mode from the perspective of the detector) is useful in systems requiring fast control of power, such as time-division-multiple-access (TDMA) systems where power is transmitted in precisely timed bursts. The fast "local" control allows the power to be ramped up and down in a controlled fashion. If a logarithmic detector is used, the power can be controlled over a large dynamic range (typically 40 to 60 dB).

Figure 1b shows a power control loop where the detector output is digitized. Software in the digital signal processor (DSP) or microcontroller makes a decision based on the measured result and then adjusts output power using a digital-to-analog converter (DAC). Since this arrangement does not allow for fast control, it is more useful in systems where power is transmitted continuously, such as CDMA, WCDMA, and TD-SCDMA systems. With digital control, extra calibration can be added to the measurement loop. For example, if the power detector drifts (but with good repeatability) with temperature, a compensation algorithm can be implemented if the system contains a temperature sensor.

Figure 1c shows a wireless transmitter with an auxiliary receiver in which the signal being transmitted is sampled and mixed back down to baseband. The use of an auxiliary receiver is common in HPA linearization schemes such as feedforward and digital pre-distortion, where it provides feedback to the algorithm about the quality of the transmitted spectrum. In this implementation, measurement of transmitted power comes for free. The measurement will be accurate as long as the gain of the receiver does not vary significantly with temperature or frequency.

Figure 1d shows alternative power control architecture used in some handsets. The architecture assumes that the transmitted power should be determined based on the received power. For example, if the received power is decreasing, the transmitted power should be increased. This is a slow and somewhat imprecise system. However, it is a useful way to set power during the initiation of a link.

In general, power-measurement accuracy is most critical when the PA is at, or close to, full power. For example, in a +50-dBm (100-W) transmitter, a ­1-dB error in the voltage from the power measurement circuit will result in a transmitted power of +51 dBm (126 W). This forces the PA to be over-dimensioned by 25 percent (making it physically larger and more expensive) to guarantee safe operation. However, at low power levels, the tolerance of the output power is only required to be within the limits of the wireless standard.

The temperature stability of the detector in these applications is critical. Traditionally, diodes have been used to perform this function. While diode detectors have good temperature stability when driven hard (good performance is generally achieved at input powers in the +15 dBm range), they have limited dynamic range (20 to 30 dB) and drift severely at low input power levels.

Demodulating logamps are becoming an increasingly popular choice when systems call for measurement and control of RF power. Figure 2 shows the transfer function of a logamp at 2.2 GHz. The AD8318, specified from 0.001 to 8 GHz, was used to generate this plot. The figure shows output voltage and calculated error, both versus input power.

As the input power varies from ­65 to 0 dBm, the output voltage varies from 2 V to about 0.5 V.

Calibration is required to achieve the rated accuracy of a log detector. This is true even if the detector is factory trimmed. Looking again at Fig. 1, we can see that there can be uncertainty about the signal level reaching the logamp. Signal trace losses, and part-to-part variability in the coupling factor of the directional coupler and attenuator can easily produce 1 dB or more of uncertainty.

The recommended method of calibration is to set the PA output to two or more approximate levels and measure the detector's output voltage.

Within its linear operating range, a logamp with approximately follow the equation:

Slope is the incremental change in output voltage for a corresponding change in input power (unit is mV/dB). The intercept is the point (in dBm or dBV) at which the extrapolated linear transfer function touches the x-axis.