Dual Detector Probes RF Power To 6 GHz

Sept. 21, 2010
This dual-channel power detector is bound to be a vital device in indoor and outdoor communications equipment requiring precise control of power levels from 40 MHz to 6 GHz.

Wide-dynamic-range communications systems need power control. Fortunately, a solution is available from Linear Technology, the new model LTC5583 dual-channel root-mean-square (RMS) power detector integrated circuit (IC). With a wide bandwidth of 40 MHz to 6 GHz, it provides coverage for most of the wireless standards, including those using wide-dynamic-range signals, such as wideband-code-division-multipleaccess (WCDMA), Long Term Evolution (LTE), and WiMAX systems.

The LTC5583 (see figure) is an extremely low-cost method of measuring power in any system through 6 GHz. The IC incorporates a pair of RMS detectors that are closely matched in performance (outputs within 1.25 dB) but well isolated to minimize leakage and errors. For each channel, the LTC5583 measures the logarithmic power level (in dBm) of AC input signals and provides a DC output voltage that is linear in proportion to the input power level, independent of the peak-to-average power (crest factor). The dual RMS detector can measure both peak and average power levels on the two channels over a better than 60-dB dynamic range, at power levels from -59 to +4 dBm, depending upon frequency. Its typical linearity is better than 1.0 dB for measurements through 6 GHz. The IC is simple to install: its single-ended input ports require no balanced-unbalanced (balun) transformers.

In order to manage wide-dynamicrange measurements for two channels on the same chip, the channels are isolated by more than 40 dB when measuring the same frequency at 2 GHz and by 50 dB or more when measuring the same frequency at 450 MHz. When the frequencies are separated by 10 MHz or more, the isolation improves further for most measurement frequencies. And by using differential inputs, the LTC5583 can improve isolation between channels to 55 dB or more.

The LTC5583 can be used to measure forward power, reverse power, or a combination of the two by applying its two channels at different test points in a circuit. The two matched detectors are useful for applications such as monitoring and controlling RF amplifier stage gain.

By using the two channels to measure forward and reverse power levels at the same frequency, for example, it can be used as a voltage-standing-wave-ratio (VSWR) monitor. A difference output pin on the IC provides the difference in power level between the two measurement channels. In addition, each channel includes a fast (140-ns rise time) envelope detector to track an input signal's envelope and provide a voltage that is directly proportional to a signal's instantaneous power level.

The LTC5583 delivers excellent performance over temperature, maintaining accuracy within 0.5 dB from -40 to +85C. The over-temperature performance makes it well suited for use in outdoor units and on cellular towers. The LTC5583 also includes provisions for first- and second-order temperature compensation to simplify calibration processes.

The 6-GHz dual RMS detector is supplied in a 24-pin 4 x 4 mm QFN package. It operates on a single +3.3-VDC supply with total current consumption of 80.5 mA. An enable pin makes it possible to power-down the detector. In shutdown mode, it draws maximum current of 10 μA. P&A: $9.50 (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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