Thursday, June 22, 2006

Fast comparators

Last week I went looking for ultrafast voltage comparators* for a new project.  I've never used these things before, and it's a bit intimidating to be turning radio waves directly into binary digital signals.  To my surprise, you can get >3 GHz devices pretty easily, and for not a lot of money.  That's fast enough that you could run a wire into your microwave oven and the comparator would watch the individual radio cycles going by.  For instance, the Analog Devices ADCMP572 has 35 picosecond rise and fall times, and only costs $7.

These parts even look pretty easy to use—just be real thorough with the usual high-speed design techniques.  The annoying part turns out to be processing the signal upstream of the comparator.  Nobody makes stable, predictable operational amplifiers that run at 5 GHz, and even if they did it would be sheer madness to try making it work.  No, I'm afraid it is going to be microwave radio amplifiers all the way, perhaps with a separate wideband transistor to help reshape the pulses.  At this rate I'm going to turn into a radio designer. 

*A comparator measures whether one voltage is greater than another.  Electrical engineers commonly use that function in their designs.  For instance, a comparator can tell you when the battery is completely charged.

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