Fast-response, precision RF power detector

EDN Europe, 04 Dec 2007

In basestation design, says Linear Technology’s James Wong – product marketing manager of HF products – power measurement is a critical function and one which becomes more difficult as digital modulation schemes become more complex. OFDM (orthogonal frequency division multiplexing) and QAM (quadrature amplitude modulation) yield high crest-factor signals (high PAR, peak-to-average ratio) which are difficult to measure accurately. His division has introduced the LT5570, a wide dynamic range mean-squared RF detector that provides accurate RMS power measurement of a 40 MHz to 2.7 GHz signal over 60 dB dynamic range, even with a modulation crest-factor of up to 12dB. Measurement accuracy is ±0.5dB over its full dynamic range and over a temperature range of -40°C to +85°C. Today’s basestation designs need to acquire power level data within 1 μsec, Wong says, offering as a solution the new chip’s full-scale rise time of 500 nsec, “this is ten times faster than has been possible with products already in the market.” Wong notes that previous techniques have included Schottky-diode peak detectors, plus average-power detection circuits: the 5570 uses a new proprietary scheme developed by Linear, Wong adds, which the company is not disclosing.
As well as LTE cellular and WiMAX transmitters, cable networks, microwave datalinks, satellite communications, and military radios have similar needs, Linear says: cable networks already use 256-QAM, moving ot 1024-QAM. Even if the input waveform has high crest-factor content, such as a 4-carrier W-CDMA modulated waveform, its RMS conformance accuracy is typically within 0.2 dB, compared to that of a CW (continuous waveform) power. The device has 61 dB dynamic range at 880 MHz, and 51 dB at 2.14 GHz. Its linear DC output is proportional to the input power in dBm with a scaling factor of 36.5mV/dB, typical. Minimum sensitivity is -53dBm at 880MHz, and -43dBm at 2.14GHz. It operates from a single 5V supply, and uses 26.5mA, with shutdown to 0.1uA. In a 10-lead 3 x 3 mm DFN package it costs $5.75 (1000). Next, Wong anticipates a move to a modular solution to ease the signal integrity issues of handling 150 Msample/sec digital RF waveforms; all of the amplification and buffering associated with driving the key analogue-to-digital converter function will be combined.