Reduce time-base errors in sampling oscilloscopes
Engineers at NIST have developed a calibration technique that reduces oscilloscope time-base jitter.
By Martin Rowe, Senior Technical Editor -- Test & Measurement World, 6/1/2010 12:00:00 AM
Sampling oscilloscopes are often used to measure jitter in gigahertz-rate optical signals. With bit rates that high, errors in an oscilloscope's time base can add jitter to a viewed signal. Engineers at NIST (National Institute of Standards and Technology) have developed a calibration technique that reduces oscilloscope time-base jitter (Ref. 1). Experiments show that the technique reduced time-base jitter from 0.81 ps to 0.22 ps, a factor of nearly 4.
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The test setup in the figure uses a microwave signal generator set to 5 GHz. A PLL (phase-locked loop) locks a mode-locked EDFL (erbium-doped fiber laser), which produces optical pulses, to the generator's output.
A 10-MHz feedback signal from the EDFL returns to the PLL to maintain phase lock and to trigger the oscilloscope. The 1550-nm optical beam from the EDFL provides short optical pulses to the photodiode that is connected to the oscilloscope for measurement and calibration.
The signal generator's output also goes to a 90° phase splitter. It produces two sine waves in quadrature that become reference signals for two of the oscilloscope's channels.
The calibration technique uses software developed at NIST that corrects for random and systematic time-base errors. This technique also works on oscilloscopes with an optical input.
Reference
1. Jargon, J., et al., "Correcting Sampling Oscilloscope Timebase Errors With a Passively Mode-Locked Laser Phase Locked to a Microwave Oscillator." IEEE Transactions on Instrumentation and Measurement, April 2010. www.eeel.nist.gov/publications/get_pdf.cgi?pub_id=900876.
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