|
 |
wirelessZONE Products for the week of October 6, 2008
Linear Technology Says…
LT5581: 10MHz to 6GHz Low Power Rms Detector with 40 dB Dynamic Range
Provides Accurate RF Power Measurement
Linear Technology introduced the LT5581, a broadband 6GHz RMS detector, featuring 40dB dynamic range and a low operating supply current of 1.4mA. The device is well suited for a wide range of power monitor and control applications in portable and battery- powered wireless systems, cellular basestations, picocells and femtocells, fiber optic transmitters and instrumentation. The LT5581 outputs a DC voltage that is linearly proportional to the log input power, providing an easy-to-use, mV/dB scaling with exceptional linearity of better than +/- 1dB across 40dB range. The LT5581’s RMS measurement capability provides accurate RF power readings to within +/-0.2dB regardless of waveforms that have high crest-factor modulated content, multi-carrier or multitone. Moreover, the LT5581 offers exceptional accuracy of +/-1dB over its operating temperature range of -40°C to 85°C.
EN-Genius Says…
The detection of RF signals has got to have become one of the fastest growth applications in the history of electronics. It was an everyday thing for me, spending so many years with broadcast transmitters, but this is a new arena for many. In these days of modulations schemes with crazy crest factors – which the analog guys then have to find solutions for – the measurement of the true rms value of RF power becomes critical for best, and most efficient, operation. And that is true for both basestation and handset, with the former consuming wads of power and the latter the desire to extend battery life to the maximum possible.
The days of having a basestation in a small hut at the bottom of a tower have gone, and the racks of analog equipment have either been carted away or left to rot. Basestations now are at the top of the tower with the feeder losses reduced to fractions of a dB. (In my neck of the woods they are even put above the main power grid lines!) As such they are exposed to all sorts of temperature variations depending on the country and location within that country. The LT5581’s best specification to my mind, because of that, is the accuracy of ±1 dB over the temperature range of -40 ºC to +185 ºC. This number is conditional on a certain power input range (depending on frequency band) but those ranges are at least -27 dBm to -10 dBm up to 3.5 GHz: a remarkable achievement.
The part also achieves at least 40 dB of CW dynamic range (for ±1 dB linearity error) up to 2.6 GHz, falling to 36 dB at 3.5 GHz, and 31 dB at 5.8 GHz. Measurement accuracy is claimed to be within 0.2 dB accuracy with crest factors up to 12 dB, including LTE, GSM/EDGE, W-CDMA, CDMA2000, and WiMAX.
Operation is with a nominal 3.3 V rail (2.7 V to 5.25 V permitted, 5.5 V absolute maximum) taking a typical 1.4 mA. An enable pin is included on the package with a typical shutdown current of 200 nA.
The input to the LT5581 is single-ended so no transformer is necessary – unless you want to use one to match a 50 Ω directional coupler to the input. Otherwise a single shunt resistor can be used for the upper and lower frequencies, adding a L/C matching network for the middle frequency bands. A full S11 table is provided in the data sheet from 10 MHz to 6 GHz. An experienced RF designer will probably just design a simple strip “sniffer” on the circuit board and provide a half-way decent match “by design.” The RF signal must be ac-coupled into the part.
Apart from that coupling capacitor, a rail decoupling capacitor and an internal bias must be decoupled externally. The output has a push-pull stage with a series 300 Ω resistor. An external shunt capacitor can be used with this to form some additional filtering for any residual modulation ripple. The output can source or sink 5 mA. With a response time of typically 1 µs – both turn-on and settling – the part is suitable for TDM modulation schemes.
The over-temperature accuracy and the dynamic range make this part ideal for modern basestations. Add the low power consumption and you have a part that is ideal for the handset as well. Dynamic range and accuracy over temperature are outstanding and, not surprisingly, the part has design wins already. This is not the last rms detector to come from the Linear stable but it is going to be one huge money maker for the company.
The LT5581 is in production in DFN-8 and is priced at $2.29 in 1000-piece lots.
Data Sheet
|
|
|
|
|
