Measurements of gain and phase are vital to the operation of many radio systems. At one time, this capability required complex and expensive instruments or circuitry. Fortunately, now this capability can be incorporated into wireless infrastructure subassemblies through a compact, single-chip integrated circuit (IC) that enables the direct measurement and comparison of two independent RF or intermediate-frequency (IF) signals.

Circuits used to measure power levels are generally referred to as detectors. The most common form of detector is the diode, which typically requires extensive calibration, linearization, and temperature compensation to accurately measure RF power over the normal range of temperature where a radio must meet its specifications. Discrete diode circuits can now be replaced with a single, highly integrated circuit which contains demodulating logarithmic amplifiers, enabling the measurement of alternating RF signals over a wide dynamic range, into a continuously varying low-frequency output voltage that corresponds to the ratio of the magnitudes of the envelopes of the RF input signals. More detailed information on demodulation logarithmic amplifiers is available in the AD8307 datasheet (available at www.analog.com/productSelection/pdf/AD8307_a.pdf).

The need to measure signal levels in wireless infrastructure equipment is critical to adjust transceiver automatic-gain-control (AGC) circuits, in the receiver (Rx) for maximum sensitivity to the varying inputs from the mobile users and in the transmitter (Tx) so that the output power is maintained at its optimum level for performance mask, power-amplifier (PA) efficiency and linearity, and government regulations.

As a result many different logarithmic amplifier circuits have been developed, and optimized for specific applications. Within an Rx, the received-signal-strength indication (RSSI) is used to adjust the gain of the Rx to extend dynamic range to 100 dB. For the Tx, accurately controlling the transmit signal power with a transmitted-signal-strength indication (TSSI) at RF frequencies at the higher power levels significantly eases the implementation of controls for PA operating level for maximum efficiency. As a sampling of available power-detector/logamp circuits, the models AD8309 and AD8310 from Analog Devices (Wilmington, MA) operate with maximum input frequencies of 500 and 440 MHz, respectively and dynamic ranges of 100 and 95 dB, respectively, while the company's models AD8313 and 8314 operate to 2.5 GHz, with dynamic ranges of 70 and 45 dB, respectively. The AD8309 and AD8310 log detectors are designed for RSSI applications, while the AD8313 and AD8314 are suitable for TSSI applications.

All of these detectors provide an output that is proportional to the logarithm of the amplitude of the incoming signal. In many applications, it is necessary to detect and compare power levels at different points within the circuit so that adjustments for optimal performance can be made. Temperature drift causes changes in PA gain and every decibel of power must be preserved for maximum efficiency and minimum power consumption. To measure the differences between two input signals, a new circuit was developed as embodied by the company's model AD8302 Gain and Phase Detector. This function allows users to effectively calibrate their PA gain and radio-transceiver AGC chains by computing the gain or attenuation between the input and output of a system or subsystem.

The AD8302 integrates two identical logarithmic detectors on a single chip, each having dynamic range of 60 dB, a digital phase detector, and circuits used for amplitude and output scaling (Fig. 1). With both logamps fabricated on the same die, their performance is matched very accurately as errors associated with each stage track each other, thereby effectively canceling each other. Two independent input signals, one of which might be a known reference signal, are applied to the Channel A and Channel B inputs. The outputs from the AD8302 are voltages proportional to the relative amplitude (i.e., gain or loss) and relative phase of the two input signals.

The AD8302 is the first integrated circuit (IC) to enable a direct ratio measurement between two independent RF input signals. The AD8302 enables designers to build accurate, low-cost system diagnostics and calibration into their final product.

A user can measure an amplitude difference range of up to 60 dB, which corresponds to input range from 0 to −60 dBm. Measurements at the center point at −30 dBm can be performed with exceptional accuracy. The phase measurement can be simultaneously measured over 180-deg. range. A full 360-deg. measurement range is possible when it is known a priori which channel leads or lags the other in phase.

The amplitude-signal output is scaled to 30 mV/dB and the phase output is scaled to 10 mV/deg. through on-chip output amplifier circuits. It is possible to adjust the scaling so that the user may, to a reasonable extent, customize these slopes. These output voltages may be fed to an analog-to-digital converter (ADC) or they can be used to drive analog circuits.