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 wirelessZONE Products for the week of December 3, 2007
Linear Technology Says…
LT5570: 2.7 GHz, 60 dB Rms RF Power Detector With 500ns Response
A new wide dynamic range mean-squared RF detector from Linear Technology sets a new level of accuracy and speed performance. The LT5570 provides accurate RMS (Root-Mean-Squared) power measurement of a 40MHz to 2.7GHz AC signal over 60dB dynamic range, even with a modulation crest-factor of up to 12 dB. It offers best-in-class measurement accuracy of +/- 0.5dB over its full dynamic range and over a temperature range of -40°C to +85°C. Moreover, the device allows exceptionally fast response with a full-scale rise time of 500 ns.
As nascent next-generation wireless standards such as mobile WiMAX and LTE (Long-Term Evolution) adopt more complex modulation schemes, combining OFDM (Orthogonal Frequency Division Multiplexing) and QAM (Quadrature Amplitude Modulation) to boost the data rate, it becomes increasingly difficult to accurately measure these high crest-factor signals. This problem is not just confined to wireless infrastructure, as many other wireless systems are similarly constrained by limited spectrum bandwidth. As a result, there is an ongoing need for higher order modulation to increase data rates. Cable networks, microwave datalinks, satellite communications, and military radios have similar needs, and the LT5570 is designed to meet these emerging challenges.
EN-Genius Says…
Everyone needs a little competition and the LT5570 is Linear’s answer to Analog Devices’ AD8362.
In a nutshell, the AD8362 offers 5 V operation; 20 mA quiescent; 3800 MHz bandwidth; 60 dB dynamic range; ±0.5 dB accuracy; 45 ns response time. The Linear LT5570 offers 5 V; 26.5 mA; 2700 MHz; 63 dB; ±0.5 dB; 500 ns (?)
However, the AD8362 numbers have been in some doubt, for me. When ADI released the AD8364 it described it as two closely-matched AD8362s in one package. However, the bandwidth tanked from 3800 MHz to 2700 MHz while the quiescent rather more than doubled -- from 20 mA to 70 mA, although there is added buffering and comparators for channel-to-channel balance -- and any mention of response time seems to have disappeared from the data sheet.
Obviously the numbers need to be compared at every frequency of operation, but the Linear part is a clear contender in the arena.
We can blame high crest factors on the designers (mostly digital engineers) of the complex modulation schemes that exist, and more that are on the horizon. It’s the continuing task of analog engineering to find solutions to these created problems. And it is going to be worse before it is better; with wider and wider bandwidths coming into vogue there will be strange effects noticed that few, even RF, engineers have worked on. For today, however, the existence of these sorts of true rms measurements is critical in system design and the power management of basestations -- a bigger and bigger problem by the day. Linear has priced the LT5570 to take a chunk of the market, and it will do so because of the numbers, and the packaging which is much more attractive than ADI’s SOP-16.
The LT5570 is in production in DFN-10 priced at $5.75 in 1000-piece lots.
Data Sheet
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