RMS Detector Reads 40-dB Range to 6 GHz

Nov. 12, 2008
This monolithic low-power and highly accurate RMS power detector scans 10 MHz to 6 GHz with a 40-dB dynamic measurement range for both CW and modulated signals.

Power control is vital to many modern wireless communications formats, including GSM/EDGE, CDMA, WiMAX, and Long Term Evolution (LTE) cellular systems. Of course, to control power, it is first necessary to detect it, and the LT5581 root-meansquare (RMS) power detector from Linear Technology (www.linear.com) is a fast-active integrated circuit (IC) with 40-dB dynamic range from 10 MHz to 6 GHz. It can even provide accurate power measurements on modulated signals with high crest factors and features excellent measurement accuracy over temperature.

The LT5581 is a monolithic broadband RMS power detector supplied in a miniature surface-mount package (see figure). It measures the logarithmic power level (in dBm) of input signals and provides a DC output voltage that is linear in proportion to the input power level. With such capability, it is well suited for a wide range of power monitor and control applications in portable and battery-powered wireless systems, cellular base stations, picocells and femtocells, fiber-optic transmitters, and instrumentation. The LT5581 can make measurements to 6 GHz, and offers a measurement range of -34 to +6 dBm at 2.14 GHz (it is rated for maximum input power of +10 dBm). Measurements can be made on modulated signals having crest factors (peakto- average power) as high as 12 dB. The LT5581 boasts a dynamic range of 40 dB at 880 MHz, 35 dB at 3.5 GHz, and 36 dB at 5.8 GHz, showing excellent performance for the frequencies of many of the latest wireless standards, including WLANs, LTE, and WiMAX.

Even though it covers a broad frequency range and wide power measurement range, the LT5581 maintains impressive linearity of 1 dB, regardless of whether making measurements for CW or modulated signals and across the full dynamic range. Depending upon the modulation format, deviations from CW response are as low as 0.1 dB and typically 0.5 dB for four-carrier CDMA signals. Output variations across an operating temperature range of -40 to +85C are within 1 dB. For filtering output modulation ripple, the LT5581 RMS power detector has an on-chip 300- ohm resistor. When combined with an off-chip capacitor of the proper value, low-cost filtering is provided for any number of modulation formats.

The LT5581 exhibits second-order harmonic distortion of typically -57 dBc and third-order harmonic distortion of typically -52 dBc. With its fast response timea rise time of 1 microsecond and fall time of 8 microsecondsthe LT5581 is suitable for time-division-duplexing (TDD) systems. The LT5581 operates at such low power levels, using only 1.4 mA of current from a +3.3-VDC supply, that it is ideal for portable instrumentation as well as measurement applications in wireless communications infrastructure systems. It is designed for supply voltages of +2.7 to +5.25 VDC, and also incorporates a shutdown feature. When its enable input pin is pulled low, the LT5581 draws typical shutdown current of a mere 0.2 microamps, with a maximum rating of 0.6 microamps. The RMS power detector is supplied in a compact 3 x 2 mm 8-pin DFN package and features single-ended RF input that does not require an additional single-endedto- balanced (differential) transformer. P&A: $2.29 (1000 qty.); stock. Linear Technology, 1630 McCarthy Blvd., Milpitas, CA 95035-7417; (408) 432- 1900, Fax: (408) 434-0507, Internet: www.linear.com.

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