This broadband RMS power detector features a wide linear dynamic range and the rapid response time needed for precise measurements in modern wireless communications systems.

Modern wireless communications systems often rely on signals with large differences between their peak and average power levels. For optimum performance, such high-crest-factor signals must be precisely measured and controlled. Unfortunately, typical power detectors can generally read the levels of continuous- wave (CW) signals at constant power levels but struggle with signals characterized by fluctuating power levels. The LT5570 root-mean-square (RMS) power detector from Linear Technology (Milpitas, CA), however, was developed just for these type of modern communications signals. It can perform a true RMS measurements on signals with even rapidly changing amplitude levels with the bandwidths of the most common wireless standards, from 40 MHz to 2.7 GHz.

The LT5570 is a monolithic RMS power detector (see figure) with excellent response time and wide dynamic range. Developed with commercial communications in mind, it can also be applied to many military power-measurement applications on pulsed signals through 2.7 GHz. It generates a DC output that is proportional to an input signal's RMS power level. The DC output is buffered with a low-output-impedance amplifier capable of driving a high-capacitance load. The RMS detector features exceptionally fast response time with full-scale risetime of 500 ns. It also features a fast fall time for speedy recovery of multiple pulses, at 800 ns. It is well suited for power measurements on signals with large peak-to-average ratios, as found in wideband-code-division-multiple-access (WCDMA) systems, CDMA2000 systems, and WiMAX systems, which use orthogonal frequency division multiplexing (OFDM) modulation.

The RMS power detector typically covers a 60-dB linear dynamic range at all frequencies. For example, at 500 MHz, the LT5570 exhibits 1 dB linearity error across a typical dynamic range of -52 to +13 dBm at temperatures from -40 to +85C. The output variation for that temperature range is a mere 0.5 dB. The deviation from continuouswave (CW) response for a signal with a 12-dB peak-to-average ratio is typically 0.3 dB. The detector provides a typical output slope of 36.9 mV/dB with a logarithmic intercept of -54.8 dBm at 500 MHz. At that frequency and a +10-dBm input signal, the secondorder harmonic distortion is typically -61 dBm and the third-order harmonic distortion is typically -66 dBc.

At the high end of the frequency range (2700 MHz), the LT5570 has 1 dB linearity error for a dynamic range of -35 to +13 dBm from -40 to +85C. The output variation versus temperature matches the 0.5 dB of the lower-frequency point. The deviation from a CW response for a signal with a 12-dB peak-to-average ratio is typically 0.3 dB. The typical output slope is 36.5 mV/dB with a logarithmic intercept of -40.6 dBm at 2700 MHz.

The LT5570 RMS detector is ideal for precision RF power measurements and level-control applications through 2.7 GHz, particularly in systems employing high-crest-factor signals, such as CDMA2000, WCDMA, and WiMAX. The RMS detector is supplied in a 10-lead DFN plastic package measuring 3 x 3 mm. It is designed for a typical supply voltage of +5 VDC and consumes only 26.5 mA current.

Linear Technology Corp.,
1630 McCarthy Blvd., Milpitas, CA 95035-7417;
(408) 432-1900, FAX: (408) 434-0507,